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Biology A Guide to the Natural World Chapter 27 • Lecture Outline Communication and Control 1: The Nervous System Fifth Edition David Krogh © 2011 Pearson Education, Inc. 27.1 Structure of the Nervous System © 2011 Pearson Education, Inc. The Nervous System • The nervous system includes all the nervous tissue in the body plus the body’s sensory organs, such as the eyes and ears. © 2011 Pearson Education, Inc. The Nervous System • Nervous tissue is composed of two kinds of cells: • Neurons: transmit nervous system messages. • Glial cells: support neurons and modify their signaling. © 2011 Pearson Education, Inc. The Nervous System • The two major divisions of the human nervous system are: • The central nervous system (CNS), consisting of the brain and spinal cord. • The peripheral nervous system (PNS), which includes all the neural tissue outside the CNS plus the sensory organs. © 2011 Pearson Education, Inc. (b) How these two components interact (a) The nervous system has two components Central nervous system Central nervous system (CNS) brain information processing spinal cord Peripheral nervous system (PNS) sensory information travels in afferent division motor information travels in efferent division, which includes… somatic nervous system sensory receptors in eyes nose, etc. Peripheral nervous system autonomic nervous system sympathetic division parasympathetic division cardiac muscle, smooth muscle, glands skeletal muscle effectors © 2011 Pearson Education, Inc. Figure 27.1 Divisions of the Nervous System • The PNS has an afferent division, which brings sensory information to the CNS; and an efferent division, which carries action (motor) commands to the body’s “effectors”—muscles and glands. © 2011 Pearson Education, Inc. Divisions of the Nervous System • Within the PNS’s efferent division are two subsystems: • The somatic nervous system, which provides voluntary control over skeletal muscles. • The autonomic nervous system, which provides involuntary regulation of smooth muscle, cardiac muscle, and glands. © 2011 Pearson Education, Inc. Divisions of the Nervous System • The autonomic system is further divided into the sympathetic division, which generally has stimulatory effects; and the parasympathetic division, which generally facilitates routine maintenance activities. © 2011 Pearson Education, Inc. 27.2 Cells of the Nervous System © 2011 Pearson Education, Inc. Cells of the Nervous System • There are three types of neurons: • sensory neurons • motor neurons • interneurons © 2011 Pearson Education, Inc. Cells of the Nervous System • Sensory neurons sense conditions inside and outside the body and convey information about these conditions to neurons inside the CNS. • Motor neurons carry instructions from the CNS to such structures as muscles or glands. • Interneurons are located entirely within the CNS and which interconnect other neurons. © 2011 Pearson Education, Inc. Cells of the Nervous System (a) Three types of neurons sensory neuron afferent neuron interneuron neuron within central nervous system motor neuron efferent neuron effector (muscle) (b) Anatomy of a neuron axon synaptic terminals dendrites cell body © 2011 Pearson Education, Inc. Figure 27.2 Cells of the Nervous System • Each neuron multiple dendrites through which signals travel to the neuron cell body and a single axon that carries signals away from the cell body. © 2011 Pearson Education, Inc. Cells of the Nervous System • Glial cells produce fat-rich myelin, which can surround neuronal axons and which increases the speed of neural signals. © 2011 Pearson Education, Inc. (a) A myelinated axon myelin nodes glial cells glial cell nucleus myelin covering axon glial cell cytoplasm (b) Anatomy of a nerve nerve blood vessels connective tissue axons © 2011 Pearson Education, Inc. Figure 27.3 Cells of the Nervous System • A nerve is a bundle of axons in the PNS that transmits information to or from the CNS. © 2011 Pearson Education, Inc. 27.3 Nervous-System Signaling © 2011 Pearson Education, Inc. Nervous System Communication • Nervous system communication can be conceptualized as working through a twostep process: 1. signal movement down a neuron’s axon 2. signal movement from this axon to a second cell across a structure known as a synapse © 2011 Pearson Education, Inc. Nervous System Communication • An electrical charge difference, called a membrane potential, exists across the plasma membrane of neurons because the inside of the neuron is negatively charged relative to the outside. © 2011 Pearson Education, Inc. Nervous System Communication • This form of potential energy is used when special protein channels in the neuron’s membrane open up on stimulation, thereby allowing ions to flow into the neuron. © 2011 Pearson Education, Inc. Nervous System Communication • This influx of ions at an initial point on the axon triggers reactions that cause the adjacent portion of the axonal membrane to initiate the same influx of ions. • Thus, a conducted nerve impulse, called an action potential, moves down the entire axon in a set of linked reactions. © 2011 Pearson Education, Inc. sodium ion (a) Membrane potential Electrical energy is stored across the plasma membrane of a resting neuron. There are more negatively charged compounds just inside the membrane than outside of it. As a result, the inside of the cell is negatively charged relative to the outside. The charge difference creates a form of stored energy called a membrane potential. Protein channels (shown in green) that can allow the movement of electrically charged ions across the membrane remain closed in a resting cell, thus maintaining the membrane potential. Cell exterior is positively charged relative to interior outside cell resting membrane potential plasma membrane inside cell potassium ion (b) Action potential 1. Nerve signal transmission begins when, upon stimulation, some protein channels open up, allowing a movement of positively charged sodium ions (Na+) into the cell. For a brief time, the interior of the cell becomes positively charged at this location. On either side of this location, however, the interior of the cell remains negatively charged. Attracted by this negative charge, the Na+ ions move laterally in both directions from their point of entry. Cell interior becomes positively charged sodium ions potassium ions Cell interior becomes negatively charged again 2. The Na+ gates close and the gates for positively charged potassium ions (K+) open up, allowing a movement of K+ out of the cell. With this, there is once again a net positive charge outside the membrane. Meanwhile, the arrival of the charged Na+ ions “downstream” from their original point of entry triggers an influx of Na+ ions at the next Na+ channel. 3. Through repetition of this process, the nerve signal is then propagated one way along the axon. Given that Na+ ions move laterally in both directions from their original point of entry, why isn’t the signal propagated in both directions? Because once an Na+ channel has opened, it enters a brief period in which it cannot respond to any additional stimulus. Thus, each “upstream” Na+ channel remains briefly closed, while each “downstream” channel is opened in succession. action potential © 2011 Pearson Education, Inc. Figure 27.4 How Neurons Work Suggested Media Enhancement: How Neurons Work To access this animation go to folder C_Animations_and_Video_Files and open the BioFlix folder. © 2011 Pearson Education, Inc. How Synapses Work Suggested Media Enhancement: How Synapses Work To access this animation go to folder C_Animations_and_Video_Files and open the BioFlix folder. © 2011 Pearson Education, Inc. Nervous System Communication • A nerve signal moves from one neuron to another across a synapse. • This includes a “sending” neuron, a “receiving” cell, and a tiny gap between the two cells, called a synaptic cleft. © 2011 Pearson Education, Inc. Nervous System Communication • A chemical called a neurotransmitter diffuses across the synaptic cleft from the sending neuron to the receiving neuron. © 2011 Pearson Education, Inc. Nervous System Communication • It then binds with receptors on the receiving neuron, thus keeping the signal going. © 2011 Pearson Education, Inc. sending neuron receiving cell synaptic cleft synaptic terminal arrival of nerve impulse initiation of new impulse mitochondrion vesicles containing neurotransmitter molecules (such as acetylcholine) © 2011 Pearson Education, Inc. neurotransmitter receptors Figure 27.5 Nervous System Communication Animation 27.1: The Nervous System © 2011 Pearson Education, Inc. 27.4 The Spinal Cord © 2011 Pearson Education, Inc. The Spinal Cord • The spinal cord can receive input from sensory neurons and direct motor neurons in response, with no input from the brain. • The spinal cord also channels sensory impulses to the brain. © 2011 Pearson Education, Inc. The Spinal Cord • In cross section, the spinal cord has a darker, H-shaped central area, composed mostly of the cell bodies of neurons; and a lighter peripheral area, composed mostly of axons. • These two areas are the gray matter and white matter of the spinal cord, respectively. © 2011 Pearson Education, Inc. The Spinal Cord • The central canal of the spinal cord is filled with cerebrospinal fluid, which provides the spinal cord with nutrients. • Spinal nerves extend from the spinal cord to most areas of the body. © 2011 Pearson Education, Inc. (a) brain cervical spinal nerves thoracic spinal nerves lumbar spinal nerves (b) white matter gray matter sacral spinal nerves ventral root dorsal root ganglion central canal spinal nerve dorsal root tip of spinal cord © 2011 Pearson Education, Inc. Figure 27.6 The Spinal Cord • Spinal cord motor neurons have cell bodies that lie within the gray matter of the spinal cord. • The axons of these neurons leave the spinal cord through its ventral roots. © 2011 Pearson Education, Inc. The Spinal Cord • Sensory neurons, which transmit information to the spinal cord, have their cell bodies outside the spinal cord, in the dorsal root ganglia. © 2011 Pearson Education, Inc. The Spinal Cord • Dorsal and ventral roots come together, like fibers being joined in a single cable, to form a given spinal nerve. © 2011 Pearson Education, Inc. Reflexes • Reflexes are automatic nervous system responses, triggered by specific stimuli, that help us avoid danger or preserve a stable physical state. © 2011 Pearson Education, Inc. Reflexes • The neural wiring of a single reflex, called a reflex arc, begins with a sensory receptor, runs through the spinal cord to a motor neuron, and proceeds back out to an effector such as a muscle or gland. © 2011 Pearson Education, Inc. 1. Stimulus (tapping) arrives and receptor is activated. 2. The signal from the receptor reaches a sensory neuron cell body in the dorsal root ganglion. spinal cord afferent signal receptor reflex arc stimulus effector response motor efferent neuron signal 3. The signal arrives at a sensory neuron/motor neuron synapse in the spinal cord. Information processing takes place, prompting a signal to be sent through the motor neuron. 4. The motor neuron signal stimulates the effector (the quadriceps muscles) to contract. Note that CNS processing for this reaction was handled entirely in the spinal cord; the brain was not involved. © 2011 Pearson Education, Inc. Figure 27.7 27.5 The Autonomic Nervous System © 2011 Pearson Education, Inc. The Autonomic Nervous System • The sympathetic division of the autonomic nervous system is often called the fight-orflight system because it generally prepares the body to deal with emergencies. © 2011 Pearson Education, Inc. The Autonomic Nervous System • The parasympathetic division is often called the rest-and-digest system because it conserves energy and promotes digestive activities. • Most organs receive input from both systems. © 2011 Pearson Education, Inc. Parasympathetic division (rest and digest) Sympathetic division (fight or flight) constricts pupil dilates pupil stimulates salivation inhibits salivation cranial nerves slows heart accelerates heart cervical nerves facilitates breathing constricts breathing stimulates digestion thoracic nerves inhibits digestion stimulates gallbladder stimulates release of glucose lumbar nerves contracts bladder sacral nerves secretes adrenaline and noradrenaline relaxes bladder inhibits sex organs stimulates sex organs © 2011 Pearson Education, Inc. Figure 27.8 27.6 The Human Brain © 2011 Pearson Education, Inc. The Human Brain • There are six major regions in the adult brain: • • • • • • • cerebrum cerebellum thalamus hypothalamus midbrain pons medulla oblongata © 2011 Pearson Education, Inc. The Human Brain • The cerebrum also has a thin outer layer of gray matter, the cerebral cortex, that surrounds a much larger volume of cerebral white matter. • Differing portions of the cerebral cortex play a central role in processing sensory information and in carrying out nearly all of our conscious mental activities. © 2011 Pearson Education, Inc. The Human Brain • The cerebellum refines bodily movement and balance based on sensory inputs. • The thalamus receives much of the body’s sensory information and then transfers it to different regions of the cerebral cortex for processing. © 2011 Pearson Education, Inc. (a) cerebral cortex cerebrum cerebellum (b) thalamus hypothalamus pituitary gland brainstem midbrain pons medulla oblongata © 2011 Pearson Education, Inc. Figure 27.9 The Human Brain • The hypothalamus is critical to regulating drives and maintaining homeostasis, in part through its regulation of hormonal release. • The brainstem is a collective term containing: • The midbrain • Pons • Medulla oblongata © 2011 Pearson Education, Inc. The Human Brain • The midbrain helps maintain muscle tone and posture. • The pons serves primarily to relay messages between the cerebrum and the cerebellum. • The medulla oblongata helps regulate such involuntary functions as breathing and digestion. © 2011 Pearson Education, Inc. somatosensory cortex thalamus gustatory cortex auditory cortex olfactory cortex visual cortex vision hippocampus amygdala smell hypothalamus brainstem taste taste hearing touch senses touch senses © 2011 Pearson Education, Inc. Figure 27.10 27.7 Our Senses © 2011 Pearson Education, Inc. Our Senses • All human senses operate through cells called sensory receptors that respond to stimuli. © 2011 Pearson Education, Inc. Our Senses • The sensory receptors transform the responses to stimuli into electrical signals that travel through action potentials. © 2011 Pearson Education, Inc. Our Senses • Signals from every sense except smell are routed through the brain’s thalamus and then to specific areas of the cerebral cortex. © 2011 Pearson Education, Inc. 27.8 Touch © 2011 Pearson Education, Inc. Our Senses of Touch • Touch works through a variety of sensory receptors that distinguish among such qualities as light or heavy pressure and new or ongoing contact. • In some sensory cells, the stretching of their outer membrane prompts an influx of ions that results in the initiation of a nerve signal. © 2011 Pearson Education, Inc. (a) Touch receptors in the skin hair tactile receptor free nerve endings (pain, temperature) epidermis tactile receptor dermis hair follicle receptor tactile receptor Pacinian corpuscle (b) How one touch receptor works interior of nerve ending pressure stretch stretch stretch © 2011 Pearson Education, Inc. stretch stretch Figure 27.11 27.9 Smell © 2011 Pearson Education, Inc. Our Sense of Smell • Our sense of smell, or olfaction, works through a set of sensory receptors whose dendrites extend into the nasal passages. © 2011 Pearson Education, Inc. Our Sense of Smell • “Odorants,” which are molecules that have identifiable smells, bind with hair-like extensions of these dendrites, resulting in a nerve signal to the brain. • The higher processing centers of the brain distinguish odorants sensing unique groups of neurons that fire in connection with given odorants. © 2011 Pearson Education, Inc. Our Sense of Smell (b) (a) olfactory bulb to olfactory cortex, amygdala, and hypothalamus olfactory bulb olfactory tract supporting cells olfactory receptor cell olfactory epithelium odorants odorants mucous layer cilia © 2011 Pearson Education, Inc. Figure 27.12 27.10 Taste © 2011 Pearson Education, Inc. Our Sense of Taste • Our sense of taste works through a group of taste cells, located in taste buds near the surface of the tongue, which have receptors that bind to “tastants,” or molecules of food that elicit different tastes. © 2011 Pearson Education, Inc. Our Sense of Taste • A given taste cell can respond through any of four to six chemical signaling routes that correspond to the basic tastes of sweet, sour, salty, bitter, and the possible fifth and sixth tastes of umami and calcium. © 2011 Pearson Education, Inc. papilla taste buds connective tissue salivary glands muscle layer papillae taste bud taste pore microvilli taste connective dendrites cell tissue © 2011 Pearson Education, Inc. Figure 27.13 Our Sense of Taste • The neurons that receive input from taste cells vary in their response to different tastants. • The brain makes sense of the pattern of input it gets from these neurons, thus yielding the large number of tastes we experience. © 2011 Pearson Education, Inc. 27.11 Hearing © 2011 Pearson Education, Inc. Our Sense of Hearing • Our sense of hearing is based on the fact that vibrations result in “waves” of air molecules that are, by turns, more and less compressed than the ambient air around them. © 2011 Pearson Education, Inc. Our Sense of Hearing • These waves of compression bump up against our eardrums (or tympanic membranes), which in turn vibrate. • This initiates a chain of vibration that ends in the fluid-filled cochlea of the inner ear. • “Hair cells” in the cochlea have ion channels that open and close in response to this vibration, resulting in nerve signals to the brain. © 2011 Pearson Education, Inc. (a) Anatomy of the ear incus malleus stapes oval window tympanic membrane cochlea nerve ear canal outer ear middle ear inner ear (b) From air vibration to nerve signal 3. The vibration of the stapes focuses the sound-wave vibration on the membrane of the oval window. 2. The tympanic membrane vibrates the three bones of the middle ear; the malleus, incus and stapes. perception of sound 2 1. Sound waves enter through the ear canal and vibrate the tympanic membrane. 5 3 sound 4. The oval window’s vibrations cause fluid vibrations within the coiled, tubular cochlea (shown elongated here for illustrative purposes). 4 5. These fluid vibrations cause cells within the cochlea to release a neurotransmitter, which triggers a nerve signal to the brain. 1 (c) How fluid triggers nerve signal tectorial membrane tectorial membrane vestibular duct hair cells cochlear duct tympanic duct nerve basilar membrane 1. Seen in cross section, the cochlea has vestibular and tympanic ducts, in which fluid is vibrating. 2. This vibration shakes the basilar membrane, pushing hair cells on it up against the overlying tectorial membrane. © 2011 Pearson Education, Inc. nucleus 3. As the hair cells contact the tectorial membrane, cilia on them bend. This change in position causes “trap door” channels in the hair cells to open, which allows potassium ions (K+) to flow into them. 4. This influx triggers an influx of calcium ions (Ca2+) at the base of the hair cells, which in turn causes the cells to release a neurotransmitter. 5. The neurotransmitter is received by adjacent dendrites and a nerve signal is sent to the brain. Figure 27.14 27.12 Vision © 2011 Pearson Education, Inc. Our Sense of Vision • Light first enters the eye through the cornea and then passes through the lens on its way to the retina at the back of the eye. • Light is bent or refracted by the cornea and the lens in such a way that it ends up as a tiny, sharply focused image on the retina. © 2011 Pearson Education, Inc. vitreous body retina cornea iris pupil lens optic nerve © 2011 Pearson Education, Inc. Figure 27.15 Our Sense of Vision • Light signals are converted to nervous system signals by cells in the retina called photoreceptors, which come in two varieties: rods and cones. • Rods function in dim light but are not sensitive to color. • Cones function best in bright light but are sensitive to color. © 2011 Pearson Education, Inc. (a) Normal vision light rays converge on the retina (b) Farsighted vision light rays converge behind the retina (c) Nearsighted vision light rays converge in front of the retina © 2011 Pearson Education, Inc. Figure 27.16 Our Sense of Vision • These photoreceptors have pigments embedded in membranes within them. • When light strikes a pigment, it changes pigment shape in a way that prompts a cascade of chemical reactions that results in neurotransmitter release being inhibited between the rod or cone and its adjoining connecting cell. • This lack of release sends the signal, “Photoreceptor stimulated here.” © 2011 Pearson Education, Inc. Our Sense of Vision • Vision signals travel from photoreceptors through two sets of adjoining cells, the latter of which have axons that come together to form the body’s optic nerves. © 2011 Pearson Education, Inc. Our Sense of Vision • The brain does not passively record visual information. Rather, it constructs images as much as it records them. • The visual perception operates through a series of genetically based “rules” that allow us to quickly make sense of what we perceive. © 2011 Pearson Education, Inc.