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Chapter 38 The Nervous System Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 38 At a Glance 38.1 What Are the Structures and Functions of Nerve Cells? 38.2 How Do Neurons Produce and Transmit Information? 38.3 How Do Nervous Systems Process Information? 38.4 How Are Nervous Systems Organized? 38.5 What Are the Structures and Functions of the Human Nervous System? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? The nervous system has two principal cell types: – Neurons, often called nerve cells, which receive, process, and transmit information – Glia, which assist neuronal function by: –Providing nutrients –Regulating the composition of the extracellular fluid in the brain and spinal cord –Modulating communication between neurons –Speeding up the movement of electrical signals within neurons Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? The functions of a neuron are localized in separate parts of the cell – A neuron must perform four functions: –Receive information from the environment –Process the information and produce electrical signals –Conduct electrical signals over distances to a junction where it meets another cell –Transmit information to other neurons, muscles, or glands Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? The functions of a neuron are localized in separate parts of the cell (continued) – Typical neurons have four distinct parts that carry out these four functions: –Dendrites –A cell body –An axon –Synaptic terminals Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? Dendrites respond to stimuli – Dendrites are branched tendrils protruding from the cell body that perform the “receive information” function – Their branches provide a large surface area for receiving signals, either from the environment or from other neurons – Dendrites of sensory neurons respond to specific stimuli, such as pressure, odor, light, body temperature, blood pH, or the position of a joint – Dendrites of neurons in the brain and spinal cord usually respond to chemicals, called neurotransmitters, that are released by other neurons Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? The cell body processes signals from the dendrites – Electrical signals travel down the dendrites and converge on the neuron’s cell body, which integrates incoming information, performing the “process information” function – If incoming signals are positive enough, a large, rapid electrical signal called an action potential is produced – The cell body also contains other organelles such as the nucleus, endoplasmic reticulum, and Golgi apparatus that are typical of other cells, synthesizing complex molecules and coordinating the cell’s metabolism Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? The axon conducts action potentials along long distances – In a typical neuron, a long, thin strand called an axon extends outward from the cell body and conducts action potentials from the cell body to synaptic terminals at the axon’s end –Single axons may stretch from our spinal cord to our toes, a distance of about 3 feet –Axons are typically bundled together into nerves, much like wires are bundled within an electrical cable Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? At synapses, signals are transmitted from one cell to another – The site where a neuron communicates with another cell is called a synapse – A typical synapse consists of: –The synaptic terminal, which is a swelling at the end of an axon of the “sending” neuron –A dendrite or cell body of a “receiving” neuron, muscle, or gland cell –A small gap separating the two cells Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.1 What Are the Structures and Functions of Nerve Cells? At synapses, signals are transmitted from one cell to another (continued) – Most synaptic terminals contain neurotransmitters that are released in response to an action potential reaching the terminal – The plasma membrane of the receiving neuron bears receptors that bind the neurotransmitters and stimulate a response in this cell – Therefore, at a synapse, the output of the first cell becomes the input to the second cell Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. A Neuron 1 Synaptic terminals: Transmit signals from other neurons 2 Dendrites: Receive signals from other neurons 3 Cell body: Integrates signals; coordinates the neuron’s metabolic activities neurotransmitters dendrite synaptic terminal receptors synapse 4 An action potential starts here 5 Axon: Conducts the action potential 6 Synaptic terminals: Transmit signals to other neurons 7 Dendrites (of other neurons): Receive signals Fig. 38-1 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Information is carried within a neuron by electrical signals and is transmitted between neurons by neurotransmitters released from one neuron and received by a second neuron – An unstimulated, inactive neuron maintains a constant electrical voltage difference, or potential, across its plasma membrane, called a resting potential –The voltage inside the cell is always negative and ranges from about –40 to –90 millivolts (mV) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? If the membrane potential becomes less negative, it reaches a level called threshold and triggers an action potential – During an action potential, the membrane potential rises rapidly to +50 mV inside the cell, then returns to resting potential – The action potential signal flows down the axon to the synaptic terminals with no change in voltage from the cell body to the synaptic terminals Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Electrical Events During an Action Potential 4 action potential threshold resting potential 3 1 2 less more negative negative time (milliseconds) Biology: Life on Earth, 9e 5 Fig. 38-2 Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Myelin speeds up the conduction of action potentials – The thicker an axon, the faster the action potential moves – In addition to variable thickness, neurons can increase the rate of action potential conduction by covering portions of the axon with a fatty insulation called myelin – Myelin is formed by glial cells that wrap themselves around the axon, leaving naked nodes inbetween the segments of myelin – In myelinated neurons, action potentials “jump” rapidly from node to node, traveling at a rate of 10 to 330 feet per second Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. A Myelinated Axon An action potential jumps from node to node, greatly speeding up conduction down the axon Schwann cell node myelin myelin sheath axon axon Fig. 38-3 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Neurons use chemicals to communicate with one another at synapses – Although there are electrical synapses where electrical activity can pass directly from neuron to neuron through gap junctions connecting the insides of the cells, as occurs in heart tissue, most synapses use chemicals to transmit information from one neuron to another – A synapse is where the synaptic terminal of one neuron meets the dendrite of another Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Neurons use chemicals to communicate with one another at synapses (continued) – The two neurons do not actually touch at a synapse – A tiny gap, the synaptic cleft, separates the first, or presynaptic neuron, from the second, or postsynaptic neuron – The presynaptic neuron sends neurotransmitter chemicals across the gap to the postsynaptic neuron – There are many types of neurotransmitters – A synaptic terminal contains scores of vesicles, each full of neurotransmitter molecules – When an action potential is initiated, it travels down an axon until it reaches its synaptic terminal Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Table 38-1 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Neurons use chemicals to communicate with one another at synapses (continued) – The inside of the terminal becomes positively charged and triggers a cascade of changes that cause some of the vesicles to release neurotransmitters into the synapatic cleft – The outer surface of the plasma membrane of the postsynaptic neuron is packed with receptor proteins that are specialized to bind the neurotransmitter released by the presynaptic neuron – The neurotransmitter molecules diffuse across the gap and bind to these receptors Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: The Synapse Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Structure and Function of the Synapse 1 An action potential is initiated 2 The action potential reaches the synaptic terminal of the presynaptic neuron synaptic vesicle 3 The positive charge of the action potential causes the synaptic vesicles to release neurotransmitters 4 Neurotransmitters bind to receptors on the postsynaptic neuron synaptic terminal of presynaptic neuron dendrite of postsynaptic neuron neurotransmitters synaptic cleft 6 Neurotransmitters are taken back into the synaptic terminal, are degraded, or diffuse out of the synaptic cleft neurotransmitter 5 Neurotransmitter binding causes ion channels to open, and ions flow in or out receptor Biology: Life on Earth, 9e ions Fig. 38-4 Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Synapses produce excitatory or inhibitory postsynaptic potentials – At most synapses, the binding of neurotransmitter molecules to receptors on a postsynaptic neuron opens ion channels in the neuron’s plasma membrane – Depending on which channels are associated with the receptors, ions such as Na+, K+, Ca2+, or Cl– may move through these channels causing a small, brief change in voltage, called a postsynaptic potential or PSP Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Electrical Signals in Neurons Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Synapses produce excitatory or inhibitory postsynaptic potentials (continued) – If the postsynaptic neuron becomes more negative, its resting potential moves farther away from threshold, reducing the likelihood of firing an action potential – This change in voltage is called an inhibitory postsynaptic potential (IPSP) – If the postsynaptic neuron becomes less negative, then its resting potential will move closer to threshold, and it will be more likely to fire an action potential – This voltage change is called an excitatory postsynaptic potential (EPSP) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Neurotransmitter action is usually brief – Some neurotransmitters—like acetylcholine, the transmitter that stimulates skeletal muscle cells—are rapidly broken down by enzymes in the synaptic cleft – Many other neurotransmitters are transported back into the presynaptic neuron Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.2 How Do Neurons Produce and Transmit Information? Summation of postsynaptic potentials determines the activity of a neuron – The dendrites and cell body of a single neuron often receive EPSPs and IPSPs from the synaptic terminals of thousands of presynaptic neurons – The voltages of all the PSPs that reach the postsynaptic cell body at about the same time are added up, a process called integration – If the excitatory and inhibitory postsynaptic potentials, when added together, raise the electrical potential inside the neuron above threshold, the postsynaptic cell will produce an action potential Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? A nervous system must be able to perform four operations: – Determine the type of stimulus – Determine and signal the intensity of a stimulus – Integrate information from many sources – Initiate and direct appropriate responses Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? The nature of a stimulus is determined by connections between the senses and the brain – All nervous systems interpret what a stimulus is by monitoring which neurons are firing action potentials – For example, the brain interprets action potentials that occur in the axons of the eye and travel to the visual areas of the brain as the sensation of light – Therefore, you distinguish the sound of music from the taste of coffee, or the bitterness of coffee from the sweetness of sugar, because these different stimuli result in action potentials in different axons that connect to different areas of the brain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? The intensity of a stimulus is coded by the frequency of action potentials – Because all action potentials are the same size and duration, no information about the strength or intensity of a stimulus can be encoded in a single action potential Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? The intensity of a stimulus is coded by the frequency of action potentials (continued) – Intensity is coded in two other ways: – First, the intensity can be signaled by the frequency of action potentials in a single neuron—the more intense the stimulus, the faster the neuron fires action potentials – Second, most nervous systems have many neurons that can respond to the same input – Stronger stimuli excite more of these neurons, whereas weaker stimuli excite fewer neurons that fire at the same time – A gentle touch may cause a single touch receptor in the skin to fire action potentials very slowly; a hard poke may cause several touch receptors to fire, some very rapidly Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Signaling Stimulus Intensity sensory neuron 1 sensory neuron 1 sensory neuron 2 Sensory neuron 1 fires slowly; sensory neuron 2 is silent sensory neuron 2 (a) Gentle touch sensory neuron 1 Sensory neurons 1 and 2 both fire sensory neuron 1 (b) Hard poke Biology: Life on Earth, 9e sensory neuron 2 sensory neuron 2 time Fig. 38-5 Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? The nervous system processes information from many sources – The brain is continually bombarded by sensory stimuli from both inside and outside the body – The brain must evaluate these inputs, determine which ones are important, and decide how to respond – A large number of neurons may funnel their signals to fewer neurons –For example, many sensory neurons may converge onto a small number of brain cells Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? The nervous system processes information from many sources (continued) – Some of the brain cells act as “decision-making” cells, adding up the postsynaptic potentials that result from the synaptic activity of the sensory neurons – Depending on their relative strength (and other internal factors, such as hormones or metabolic activity), they produce appropriate outputs Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? The nervous system produces outputs to muscles and glands – Action potentials from the decision-making neurons may travel to other parts of the brain, to the spinal cord, or to the sympathetic and parasympathetic nervous system – Ultimately, the output of the nervous system will stimulate activity in the muscles or glands that produce behaviors Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.3 How Do Nervous Systems Process Information? The nervous system produces outputs to muscles and glands (continued) – The same principles of connectivity and intensity coding for sensory inputs are used for the brain’s outputs – Which muscles or glands are activated is determined by their connections to the brain or spinal cord – For example, the neurons that activate biceps muscles are different than those that activate muscles of the face – How hard a muscle contracts is determined by how many neurons connect to it and how fast those neurons fire action potentials Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.4 How Are Nervous Systems Organized? Most behaviors are controlled by pathways composed of four elements: – Sensory neurons respond to a stimulus, either internal or external to the body – Interneurons receive signals from sensory neurons, hormones, or neurons that store memories; based on this input, interneurons often activate motor neurons – Motor neurons receive information from sensory neurons or interneurons and activate muscles or glands – Effectors, usually muscles or glands, perform the response directed by the nervous system Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.4 How Are Nervous Systems Organized? Simple behaviors, such as reflexes, may be controlled by activity in as few as two or three neurons—a sensory neuron, a motor neuron, and an interneuron in between, usually stimulating a single muscle – In humans, simple reflexes such as the knee-jerk or pain-withdrawal reflexes are produced by neurons in the spinal cord Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.4 How Are Nervous Systems Organized? Complex behaviors are organized by interconnected neural pathways in which several types of sensory input converge on a set of interneurons – By integrating the postsynaptic potentials from multiple sources, the interneurons “decide” what to do and stimulate motor neurons to direct the appropriate activity in muscles and glands – Hundreds, or even millions of neurons, mostly in the brain, may be required to perform complex actions such as playing the piano Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.4 How Are Nervous Systems Organized? Complex nervous systems are centralized – In the animal kingdom, there are only two nervous system designs: –A diffuse nervous system, such as that of cnidarians (Hydra, jellyfish, and their relatives) –A centralized nervous system, found in more complex organisms Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.4 How Are Nervous Systems Organized? Complex nervous systems are centralized (continued) – Radially symmetrical cnidarians have no “front end,” so there has been no evolutionary pressure to concentrate the senses in one place – Cnidarian nervous systems are composed of a network of neurons, often called a nerve net, woven through the animal’s tissues, with a cluster of neurons, called a ganglion, but nothing like a real brain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Nervous System Organization ring of ganglia diffuse network of neurons (a) Hydra Biology: Life on Earth, 9e Fig. 38-6a Copyright © 2011 Pearson Education Inc. 38.4 How Are Nervous Systems Organized? Complex nervous systems are centralized (continued) – Almost all other animals are bilaterally symmetrical, with definite head and tail ends –Because the head is usually the first part of the body to encounter food, danger, and potential mates, it is advantageous to have sense organs concentrated there –Sizable ganglia evolved that integrate the information gathered by the senses and direct appropriate actions Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.4 How Are Nervous Systems Organized? Complex nervous systems are centralized (continued) – Over evolutionary time, the major sense organs of animals with complex nervous systems became localized in the head, and the ganglia became centralized into a brain – This process, called cephalization, reached a peak in the vertebrates where all nervous system cell bodies are localized in the brain and spinal cord Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Nervous System Organization brain nerve cords (b) Flatworm cerebral ganglia (brain) (c) Octopus Fig. 38-6b, c Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The nervous system of all mammals, including humans, can be divided into two parts: – The central nervous system (CNS), consisting of the brain and spinal cord – The peripheral nervous system (PNS), consisting of neurons that lie outside the CNS and the axons that connect these neurons with the CNS – The cell bodies of neurons of the PNS are often located in ganglia alongside the spinal cord or in ganglia near target organs, such as ganglia in the head and neck that control the salivary glands Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Organization and Functions of the Vertebrate Nervous System Fig. 38-7 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The peripheral nervous system links the central nervous system with the rest of the body – Nerves of the PNS: – Connect the brain and spinal cord with muscles, glands, sensory organs, and the digestive, respiratory, urinary, reproductive, and circulatory systems – Contain axons of sensory neurons, bringing sensory information to the central nervous system from all parts of the body – These nerves also contain the axons of motor neurons that carry signals from the central nervous system to the glands and muscles Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The motor portion of the peripheral nervous system consists of two parts: – The somatic nervous system – The autonomic nervous system Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The somatic nervous system controls voluntary movement – Motor neurons of the somatic nervous system form synapses with skeletal muscles and control voluntary movement –Lifting a cup of coffee or adjusting your iPod are both controlled by the somatic nervous system – The cell bodies of somatic motor neurons are located in the spinal cord, and their axons go directly to the muscles they control Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The autonomic nervous system controls involuntary actions – Motor neurons of the autonomic nervous system innervate the heart, smooth muscles, and glands, and produce involuntary actions –It is controlled by the hypothalamus, medulla, and pons—parts of the brain –It consists of two divisions that innervate the same organs, but with opposing actions: –The sympathetic division –The parasympathetic division Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The neurons of the sympathetic division release the neurotransmitter norepinephrine onto their target organs, preparing the body for stressful or energetic actions – During these activities, it directs some of the blood supply from the digestive tract to the muscles of the arms and legs – The heart rate accelerates, the pupils of the eyes open wider, and the air passages in the lungs expand Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The neurons of the parasympathetic division release acetylcholine onto their target organs – The parasympathetic division controls maintenance activities that can be carried out at leisure, often called “rest and digest” – Under parasympathetic control, the digestive tract becomes active, the heart rate slows, and air passages in the lungs constrict, because the body requires less blood flow and less oxygen Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Autonomous Nervous System PARASYMPATHETIC DIVISION SYMPATHETIC DIVISION eye constricts pupil stimulates salivation and tears dilates pupil salivary and lacrimal glands inhibits salivation and tearing cranial cranial constricts airways cervical lungs relaxes airways reduces heartbeat cervical heart stimulates pancreas to release insulin and digestive enzymes thoracic increases heartbeat stimulates glucose production and release liver stimulates secretion of epinephrine and norepinephrine from adrenal medulla pancreas kidney stomach stimulates digestion spleen lumbar dilates blood vessels in gut kidney small intestine thoracic inhibits digestion lumbar large intestine rectum urinary bladder sacral sacral relaxes bladder stimulates bladder to contract sympathetic ganglia uterus stimulates sexual arousal Biology: Life on Earth, 9e external genitalia stimulates orgasm Fig. 38-8 Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The central nervous system consists of the spinal cord and brain – The CNS receives and processes sensory information, generates thoughts, and directs responses – The brain and spinal cord are protected from physical damage in three ways: – The skull surrounds the brain, and a chain of vertebrae protect the spinal cord – The triple connective tissue layer of meninges lies between the bone and spinal cord – Between the mininges layers is the cerebrospinal fluid that cushions the brain and spinal cord, and nourishes the cells of the CNS Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The central nervous system consists of the spinal cord and brain – The brain is also protected from potentially damaging chemicals by the blood–brain barrier, a capillary system that is far less permeable than capillaries in the rest of the body and selectively transports needed materials into the brain while keeping many dangerous substances out – Generally, the blood–brain barrier keeps watersoluble substances from diffusing from the blood into the brain, but many lipid-soluble substances can still diffuse across the capillary walls Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The spinal cord controls many reflexes and conducts information to and from the brain – The spinal cord extends from the base of the brain to the lower back – Nerves carrying axons of sensory neurons emerge from the dorsal part of the spinal cord, and nerves carrying axons of motor neurons emerge from the ventral part – These nerves merge to form the spinal nerves that innervate most of the body Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The spinal cord (continued) – Because of their resemblance to the roots of a tree that merge into a single trunk, these branches are called the dorsal and ventral roots of the spinal nerves – Swellings on each dorsal root, called the dorsal root ganglia, contain the cell bodies of sensory neurons – In the center of the spinal cord is a butterfly-shaped area of gray matter – This consists of the cell bodies of motor neurons that control voluntary muscles and the autonomic nervous system, plus interneurons that communicate with the brain and other parts of the spinal cord Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The spinal cord (continued) – The gray matter is surrounded by white matter, which contains myelin-coated axons of neurons that extend up or down the spinal cord –These axons carry sensory signals from internal organs, muscles, and the skin up to the brain –Axons also extend downward from the brain, carrying signals that direct the motor portions of the peripheral nervous system Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The spinal cord (continued) – If the spinal cord is severed, body parts that are innervated by motor and sensory neurons located below the injury are paralyzed and feel numb, though the motor and sensory neurons, the spinal nerves, and the muscles remain intact Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Spinal Cord white matter contains myelinated axons gray matter contains the cell bodies of motor neurons and interneurons dorsal root contains the axons of sensory neurons dorsal root ganglion contains the cell bodies of sensory neurons spinal nerve ventral root contains the axons of motor neurons Fig. 38-9 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The neuronal circuits for many reflexes reside in the spinal cord – The simplest type of behavior is the reflex, a largely involuntary movement of a body part in response to a stimulus – In vertebrates, many reflexes are produced by the spinal cord and peripheral neurons, and do not use the brain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The pain-withdrawal reflex involves neurons of both the central and peripheral nervous systems – If you lean your hand on a tack, the tissue damage activates pain sensory neurons – Action potentials in the axons of these pain sensory neurons travel up the spinal nerve and enter the spinal cord through a dorsal root – Within the gray matter of the cord, the pain sensory neuron stimulates an interneuron, which stimulates a motor neuron Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The pain-withdrawal reflex (continued) – Action potentials in the axon of the motor neuron leave the spinal cord through a ventral root and travel in a spinal nerve to a skeletal muscle – The action potential stimulates the muscle, which contracts, so you withdraw your hand away from the tack Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Reflex Arcs Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Pain-withdrawal Reflex 2 The signal is transmitted by the pain sensory neuron to the spinal cord 1 A painful stimulus activates a pain sensory neuron dorsal root interneuron sensory neuron spinal cord motor neuron stimulus 5 The effector muscle causes a withdrawal response ventral root 3 The signal is transmitted to an interneuron and then to a motor neuron 4 The motor neuron stimulates the effector muscle Fig. 38-10 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? Many spinal cord interneurons also have axons that extend up to the brain – Action potentials in these axons inform the brain about pricked hands and may trigger more complex behaviors, such as shrieks and learning about the dangers of thumbtacks – The brain, in turn, sends action potentials down axons in the spinal cord white matter to interneurons and motor neurons in the gray matter, which modify spinal reflexes – With enough training, or motivation, you can suppress the pain-withdrawal reflex Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? Some complex actions are coordinated within the spinal cord – The wiring for some fairly complex activities also resides within the spinal cord – All the neurons needed for basic movements of walking and running are contained in the spinal cord – The advantage of the semi-independent arrangement between brain and spinal cord probably increases speed and coordination, because messages do not have to travel up to the brain and back down again just to swing a leg forward while walking – The brain’s role in these semi-automatic behaviors is to initiate, guide, and modify spinal motor neuron activity Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The brain consists of many parts that perform specific functions – All vertebrate brains consist of three major parts: – The hindbrain – The midbrain – The forebrain – Scientists believe that in the earliest vertebrates, these three anatomical divisions were also functional divisions: – The hindbrain governed breathing and heart rate – The midbrain controlled vision – The forebrain dealt with the sense of smell Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. A Comparison of Vertebrate Brains thalamus optic lobe cerebellum cerebrum medulla midbrain forebrain midbrain hindbrain cerebrum cerebellum (a) Embryonic vertebrate brain midbrain cerebellum cerebrum (d) Horse brain (b) Shark brain cerebrum midbrain cerebrum cerebellum midbrain (inside (c) Goose brain Biology: Life on Earth, 9e (e) Human brain cerebellum Fig. 38-11 Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The brain consists of many parts that perform specific functions (continued) – In nonmammalian vertebrates, the three divisions remain prominent – However, in mammals—particularly humans— the brain regions are significantly modified – Some have been reduced in size; others, especially the forebrain, are greatly enlarged Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The hindbrain consists of the medulla, pons, and cerebellum – In both structure and function, the medulla is very much like an enlarged extension of the spinal cord – Like the spinal cord, the medulla has neuron cell bodies at its center, surrounded by a layer of myelincovered axons – It controls several automatic functions, such as breathing, heart rate, blood pressure, and swallowing – Certain neurons in the pons, located just above the medulla, appear to influence transitions between sleep and wakefulness; other pons neurons affect the rate and pattern of breathing Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The cerebellum is crucial for coordinating movements of the body – It receives information both from command centers in the forebrain that control movement and from position sensors in muscles and joints – By comparing information from these two sources, the cerebellum guides smooth, accurate motions and body position – The cerebellum is also involved in motor learning as a result of practice of a repeated activity Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The midbrain contains clusters of neurons that contribute to movement, arousal, and emotion – The midbrain is quite small in humans, and contains an auditory relay center and clusters of neurons that control reflex movements of the eyes –For example, if you are sitting in class and someone races through the door, centers in your midbrain are alerted and direct your gaze to the new, and potentially interesting or threatening, visual stimulus Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The midbrain (continued) – The midbrain also contains neurons that produce the transmitter dopamine – One cluster of neurons, called the substantia nigra, helps control movement – Another cluster is an essential part of the “reward circuit” that is responsible for pleasurable sensations and, unfortunately, addiction Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The midbrain (continued) – Finally, the midbrain contains a portion of the reticular formation that consists of dozens of interconnected clusters of neurons in the medulla, pons, and midbrain, which neurons send axons to the forebrain – These neurons receive input from virtually every sense, from every part of the body, and from many areas of the brain as well – The reticular formation plays a role in sleep and wakefulness, emotion, muscle tone, and some movements and reflexes – It filters sensory inputs before they reach the conscious regions of the brain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The midbrain (continued) – Activities of the reticular formation allow you to read and concentrate in the presence of a variety of distracting stimuli, such as music or smells – An example is the mother who wakens upon hearing the faint cry of her infant, but sleeps through loud traffic noise outside her window Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The forebrain includes the thalamus, hypothalamus, and cerebrum – The thalamus is a complex relay station that channels all sensory information, except olfaction, from all parts of the body to the cerebral cortex – Signals traveling from the spinal cord, cerebellum, medulla, pons, and reticular formation also pass through the thalamus – The hypothalamus contains many clusters of neurons that release hormones into the blood or control the release of hormones from the pituitary gland Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The forebrain includes the thalamus, hypothalamus, and cerebrum (continued) – Other regions of the hypothalamus direct the activities of the autonomic nervous system – The hypothalamus, through its hormone production and neural connections, maintains homeostasis by influencing body temperature, food intake, water balance, heart rate, blood pressure, the menstrual cycle, and circadian rhythms Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The cerebrum consists of two cerebral hemispheres – Each hemisphere is composed of an outer cerebral cortex, several clusters of neurons beneath the cortex near the thalamus, and bundles of axons that both interconnect the two hemispheres and connect the hemispheres with the midbrain and hindbrain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? Structures in the interior of the cerebrum – Clusters of neurons in the amygdala produce sensations of pleasure, fear, or sexual arousal when stimulated – The hippocampus plays an important role in the formation of long-term memory, particularly of places, and is thus required for learning – All vertebrates have a hippocampus part of their brain – Birds, such as jays and nutcrackers, remember where they store seeds during the winter and have a larger hippocampus than other birds Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The basal ganglia consist of structures deep within the cerebrum, as well as substantia nigra in the midbrain – These structures are important in the overall control of movement – The basal ganglia appear to be essential to the decision to initiate a particular movement and to suppress other movements – In Parkinson’s disease, the substantia nigra degenerates, and affected people have a hard time starting a movement – In Huntington’s disease, basal ganglia in the cerebrum degenerate, and affected people make involuntary, undirected, flailing movements Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Human Brain meninges FOREBRAIN (within dashed blue line) cerebral cortex skull corpus callosum cerebral cortex (gray matter) hypothalamus thalamus pituitary gland myelinated axons (white matter) corpus callosum thalamus basal ganglia MIDBRAIN cerebellum pons HINDBRAIN hypothalamus medulla spinal cord (a) A lateral section of the human brain hippocampus substantia nigra (b) A cross-section of the brain Fig. 38-12 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The limbic system is a diverse group of structures – These structures include the hypothalamus, the amygdala, and the hippocampus, as well as nearby regions of the cerebral cortex, located in a ring between the thalamus and cerebral cortex – The limbic system helps to produce emotions and emotional behaviors, including fear, rage, calm, hunger, thirst, pleasure, and sexual responses – Other brain regions are also involved in emotions, including other parts of the cerebral cortex, the midbrain, the hindbrain, and even the spinal cord Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Limbic System limbic region of cortex cerebral cortex corpus callosum thalamus olfactory bulb hypothalamus Biology: Life on Earth, 9e amygdala hippocampus Fig. 38-13 Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The cerebral cortex – The cerebral cortex is the thin outer layer of each cerebral hemisphere, with billions of neurons packed in a highly organized way into a sheet just a few millimeters thick – The cortex is folded into convolutions, which are raised, wrinkled ridges that increase its surface area to over two square yards – Neurons in the cortex receive sensory information, process it, direct voluntary movements, create memories, and allow us to be creative and even envision the future – The cortexes in each hemisphere communicate with each other through a large band of axons, the corpus callosum Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The cerebral cortex is divided into four anatomical regions: frontal, parietal, occipital, and temporal – It can also be divided into functional areas: –Primary areas are regions where signals originating in sensory organs such as the eyes and ears are received and converted into subjective impressions –Nearby association areas interpret the sounds as speech or music, and the visual stimuli as recognizable objects or words Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Cerebral Cortex Frontal Lobe primary motor area premotor area leg trunk arm higher intellectual functions primary sensory area Parietal Lobe hand sensory association area face speech motor area memory tongue primary auditory auditory association area area: language comprehension Occipital Lobe visual association area primary visual area Temporal Lobe Fig. 38-14 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The cerebral cortex (continued) – Primary sensory areas in the parietal lobe interpret sensations of touch that originate in all parts of the body – In an adjacent region of the frontal lobe, primary motor areas command movements in corresponding areas of the body by stimulating the motor neurons in the spinal cord that innervate muscles, allowing you to walk – Like the primary sensory area, the primary motor area has adjacent association areas, including the motor association area which directs the motor area to produce movements Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The cerebral cortex (continued) – Behind the bones of the forehead lies the association areas of the frontal lobe –They are important in complex reasoning functions such as short-term memory, decision making, predicting the consequences of actions, controlling aggression, planning for the future, and working for delayed rewards Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The cerebral cortex (continued) – Damage to the cortex from trauma, stroke, or a tumor results in specific deficits such as problems with speech, difficulty reading, or the inability to sense or move specific parts of the body – Most brain cells of adults cannot be replaced, so if a brain region is destroyed, these deficits may be permanent – Training sometimes allows undamaged regions of the cortex to take over and restore some of the lost functions Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: The Human Brain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? How do neuroscientists learn about the functions of brain regions? – The functions of different parts of the brain were discovered by examining the behaviors and abilities of people who suffered brain injuries, often in accidents or wars – In 1848, Phineas Gage was setting an explosive charge to clear rocks from a railroad line under construction when the gunpowder triggered prematurely Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? How do neuroscientists learn about the functions of brain regions? (continued) – The blast blew a 13-pound steel rod through Gage’s skull, severely damaging both of his frontal lobes – Gage survived his wounds, but his personality changed radically – Before the accident, Gage was conscientious, industrious, and well-liked; after his recovery, he became impetuous, profane, and incapable of working toward a goal Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. A Revealing Accident Fig. 38-15 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? Studies of other people with brain injuries have revealed that many parts of the brain are highly specialized – One patient with damage to the left frontal lobe was unable to name fruits and vegetables, although he could name everything else – Other victims of brain damage are unable to recognize faces, suggesting that the brain has regions specialized to recognize categories of things Modern neuroscience has powerful techniques for visualizing brain structure and activity, offering insights into the functioning of the human brain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The “left brain” and the “right brain” are specialized for different functions – The structural brain symmetry does not extend to brain function – In the 1950s, Roger Sperry of the California Institute of Technology studied people whose hemispheres had been separated by cutting the corpus callosum to prevent the spread of epilepsy from one hemisphere to the other – Severing the corpus callosum prevented the two hemispheres from communicating with each other Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The “left brain” and the “right brain” are specialized for different functions (continued) – Sperry made use of the fact that axons from the eyes, which are not severed by the surgery, follow a pathway that causes the left half of each visual field to be “seen” by the high hemisphere and the right half to be seen by the left hemisphere – Sperry used a device that projected different images onto the left and right visual fields and thus sent different signals to each hemisphere – When Sperry projected an image of a nude figure onto only the left visual field, the subjects would blush and smile, but would claim to have seen nothing – The same figure projected onto the right visual field was readily described verbally Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Specialization of the Cerebral Hemispheres Left HEART LEFT HEMISPHERE 1. Controls right side of body 2. Input from right visual field, right ear, left nostril 3. Centers for language, speech, reading, mathematics, logic Right RIGHT HEMISPHERE 1. Controls left side of body 2. Input from left visual field, left ear, right nostril 3. Centers for spatial perception, music, creativity, recognition of faces and emotions retina optic nerve optic chiasma corpus callosum visual cortex Biology: Life on Earth, 9e Fig. 38-16 Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? The “left brain” and the “right brain” are specialized for different functions (continued) – These and later experiments revealed that, in righthanded people, the left hemisphere is usually dominant in speech, reading, writing, language, comprehension, mathematical ability, and logical problem solving – The right side of the brain is superior to the left in musical skills, artistic ability, recognizing faces, spatial visualization, and the ability to recognize and express emotions – Recent experiments indicate that the left–right dichotomy is not as rigid as was once believed Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? Learning and memory involve biochemical and structural changes in specific parts of the brain – Learning has two phases: working memory and longterm memory – Remembering a telephone number long enough to dial it is working memory; if the number is called often enough, it becomes permanently remembered in longterm memory – The frontal and parietal lobes of the cerebral cortex, and some of the basal ganglia deep in the cerebrum, are the primary sites of working memory Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 38.5 What Are the Structures and Functions of the Human Nervous System? – Most working memory probably requires the repeated activity of a particular neural circuit in the brain, and as long as the circuit is active, the memory stays – In contrast, long-term memory seems to be structural and the result of persistent changes in the expression of certain genes – It may require the formation of new, long-lasting synaptic connections between specific neurons, or the long-term strengthening of existing, but weak, synapses – For many memories, converting working memory into long-term memory seems to involve the hippocampus, which is believed to process new memories and transfer them to the frontal and temporal lobes of the cerebral cortex for permanent storage Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc.