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Fundamentals of the Nervous System and Nervous Tissue Chapter 12 Introduction The nervous system is the master controlling and communicating system of the body It is responsible for all behavior Along with the endocrine system it is responsible for regulating and maintaining body homeostasis Cells of the nervous system communicate by means of electrical signals Nervous System Functions The nervous system has three overlapping functions Gathering of sensory input Integration or interpretation of sensory input Causation of a response or motor output Introduction Sensory input Integration The nervous system has millions of sensory receptors to monitor both internal and external change It processes and interprets the sensory input and makes decisions about what should be done at each moment Motor output Causes a response by activating effector organs (muscles and glands) Organization of the Nervous System Organization There is only one nervous system; however, for convenience the nervous system is divided into two parts The central nervous system • Brain and spinal cord • Integrative and control centers The peripheral nervous system • Spinal and cranial nerves • Communication lines between the CNS and the rest of the body Organization Basic divisions of the nervous system Central Nervous Systems Peripheral Nervous System Organization The peripheral nervous system has two fundamental subdivisions Sensory (afferent) division • Somatic and visceral sensory nerve fibers • Consists of nerve fibers carrying impulses to the central nervous system Motor (efferent) division • Motor nerve fibers • Conducts impulses from the CNS to effectors – (glands and muscles) Organization Organization The motor division of the peripheral nervous system has two main subdivisions The somatic nervous system • Voluntary (somatic motor) • Conducts impulses from the CNS to skeletal muscle Branchial motor • Motor innervation of pharyngeal arch muscles The autonomic nervous system (ANS) • Involuntary • Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Organization Innervation of Visceral Organs Organization The autonomic nervous system has two principle subdivisions Sympathetic division • Mobilizes body systems during emergency situations Parasympathetic division • Conserves energy • Promotes non-emergency functions The two subdivisions bring about opposite effects on the same visceral organs What one subdivision stimulates, the other inhibits Peripheral Nervous System Visceral organs are served by motor fibers of the autonomic nervous system and by visceral sensory fibers The somata (limbs and body wall) are served by motor fibers of the somatic nervous system and by sensory somatic sensory fibers Arrows indicate the direction of impulses Histology of the Nervous Tissue Nervous tissue is highly cellular Less that 20% of the CNS is extracellular space Cells are densely packed and tightly intertwined Nervous tissue is made up of two cell types Neurons • Excitable cells that transmit electrical signals Support cells • Smaller cells that surround and wrap the delicate neurons These same cells are found within CNS and PNS Supporting Cells All neurons associate closely with nonnervous support cells of which there are 6 types Support cells of the CNS • • • • Astrocytes Microglial Ependymal Oligodendrocyte Support cells of the PNS • Schwann cells • Satellite cells Supporting Cells While each support cell has a unique specific function, in general these cells provide a supportive scaffolding for neurons In addition, they all cover nonsynaptic parts of the neurons thereby insulating the neurons and keeping the electrical activities of adjacent neurons from interfering with each other Clinical Insight The importance of support cells insulating nerve fibers is illustrated in the disorder call tic douloureux (doo loo-roo) In this condition the support cells around the sensory nerve fibers of the trigeminal nerve degenerate and are lost Impulses that carry touch sensations proceed to influence and stimulate the uninsulated pain fibers in the same nerve Supporting Cells in the CNS The supporting cells of the CNS are collectively called neuroglia or simply, glial cells Neuroglia usually refer to the CNS but some authors include the PNS Supporting Cells in the CNS Like neurons, glial cells have branching processes and a central cell body Neuroglia can be distinguished from neurons by their much smaller size and darker staining nuclei They outnumber neurons in the CNS by a ratio of 10 to 1 Make up half of the mass of the brain Astrocytes Star shaped Most abundant type of glial cell Radiating projections cling to neurons and capillaries, bracing the neurons to their blood supply Astrocytes play a role in exchanges of ions between capillaries and neurons Astrocytes Astrocytes take up and release ions to control the environment around neurons Concentrations of ions must be kept within narrow limits for nerve impulses to be generated & conducted Astrocytes recapture and recycle potassium ions and released neurotransmitters Astrocytes Astrocytes contact both the neuron and the capillary in order to sense when the neuron are highly active and releasing large amounts of neurotransmitters (glutamate) Astrocytes then extract blood sugar from the capillaries they contact to obtain the energy they need to fuel the process of glutamate uptake Microglial Smallest and least abundant type of neuroglial cell The ovid cells have relatively long “thorny” processes Their branches touch nearby neurons to monitor health of the neuron Microglial These are small ovid cells with relatively long “thorny” processes Microglial derive from blood cells and migrate to the CNS during embryonic and fetal development Microglial These cells are phagocytes, the marcophages of the CNS Microglial move to and then engulf microorganisms and injured or dead neurons Microglial When invading microorganisms are present or damaged neurons have died, the microglial transforms into a special type of macrophage that protects the CNS by phagocytizing the microorganisms or neuronal debris Important because cells of the immune system can enter CNS Ependymal Range in shape from squamous to columnar and many are cilated Line the central cavities of the brain and spinal cord Form a fairly permeable barrier between cerebrospinal fluid of those cavities and the cells of the CNS Beating cilia circulates cerebrospinal fluid Oligodendrocytes Fewer branches than astrocytes Cells wrap their cytoplasmic extensions tightly around the thicker neurons in the CNS Produce insulating coverings called myelin sheaths Supporting Cells of the PNS There are two supporting cells in the PNS Satellite cells Schwann cells These cells are similar in type and differ mainly in location Satellite Cells Somewhat flattened satellite cells surround cell bodies within ganglia Thought to play some role in controlling the chemical environment of neurons with which they are associated, but function is largely unknown Schwann Cells Surround and form myelin sheaths around the larger nerve fibers in PNS Similar to the oligodendrocytes of CNS Schwann cells are vital to peripheral nerve fiber regeneration Neurons Neurons are the structural units of the nervous system Neurons are highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another Neuron Characteristics Extreme longevity Amitotic Live and function optimally for a lifetime As neurons assume their role in the nervous system they lose their ability to divide Neurons cannot be replaced if destroyed High metabolic rate Require continuous and abundant supplies of oxygen and glucose Homeostatic deviations often first appear in nervous tissue which has specific needs Neurons The plasma membrane of neurons is the site of electrical signaling, and it plays a crucial role in most cell to cell interaction Most neurons have three functional components in common A receptive component A conducting component A secretion or output component Each component is associated with a particular region of a neuron’s anatomy Neuron structure Typically large, complex cells, they all have the following structures Cell body • • • • Nuclei Chromatophilic (Nissl) bodies Neurofibrils Axon hillock Cell processes • Dendrites • Axon • Myelin sheath or neurilemma Neuron structure Cell Body Nuclei Chromatophilic (Nissl) bodies Neurofibrils Axon hillock Neuron Processes Dendrites Axons Myelin sheaths Axon terminals Neuron structure The cell body consists of a large, spherical nucleus with a prominent nucleolus surrounded by cytoplasm The cell ranges from 5 to 140m in diameter The cell body is the biosynthetic center of the neuron Neuron structure The cell body contains the usual organelles with the exception of centrioles (not needed in amitotic cells) The rough endoplasmic reticulum or Nissl bodies is the protein and membrane making machinery of the cell The cell body is the focal point for neuron growth in development Neuron structure Neurofibrils are bundles of intermediate filaments (neurofilaments) that run in a network between the chromatophilic bodies Neurofibrils keep the cell from being pulled apart when it is subjected to tensile stresses Neuron structure In most neurons, the plasma membrane of the cell body acts as a receptive surface that receives signals from other neurons Neuron Cell Bodies Most neuron cell bodies are located with the CNS where they are protected by the bones of the skull and vertebral column Clusters of cell bodies in the CNS are called nuclei The relatively rare collection of cell bodies in the PNS are called ganglia Neuron Processes Cytoplasmic extension called processes extend from the cell body of all neurons The CNS contain both neuron cell bodies and their processes The PNS consists chiefly of processes Motor neuron Neuron Processes Bundles of neuron processes in the CNS are called tracts Bundles of neuron processes in the PNS are called nerves Motor neuron Dendrites Dendrites are short, tapering diffusely branching extensions Motor neurons have hundreds of dendrites clustering close to the cell body Dendrites are receptive cites and provide an enormous surface area for the reception of signals In many areas of the brain the finer dendrites are highly specialized for information collection Dendrites Dendritic spines represent areas of close contact with other neurons Dendrites convey information toward the cell body These electrical signals are not nerve impulses but are short distance signals call graded potentials Axons Each neuron has a single axon The axon arises from the cone shaped axon hillock It narrows to form a slender process that stays uniform in diameter the rest of its length Length varies; short or absent to 3 feet in length Axons Each axon is called a nerve fiber Axons are impulse generators and conductors that transmit nerve impulses away from the cell body Axons Chromatophilic bodies and the Golgi apparatus are absent from the axon and the axon hillock The axons also lack ribosomes and all organelles involved in protein synthesis so they must receive their proteins from the cell body Axons Neurofilaments, actin microfilaments, and microtubules are especially evident in axons, where they provide structural strength Axons Neurofilaments are cytoskeleton elements that also aid in the transport of substances to and from the cell body as the axonal cytoplasm is continually recycled and renewed This movement of substances along axons is called axonal transport Axons Axons branch less extensively that dendrites Each neuron has only one axon but may possess a collateral branch All axons branches profusely at its terminal end to form more than 10,000 telodendria or terminal branches Axons The axon terminals contact other neurons to form specialized cell junctions called synapses A nerve impulse is conducted along the axon to the axon terminals where it causes a release of chemicals called neurotransmitters Axons Neurotransmitters are release into the extracellular space called a synaptic cleft The neurotransmitters excite or inhibit the neurons with which axon is in close contact Because each neuron typically receives signals from and sends to scores of other neurons, it “carries on conversations” with many different neurons at the same time Axons Axon diameter varies considerably among the different neurons of the body Axons with larger diameters conduct impulses faster than those of smaller diameters because of the basic laws of physics: The resistance to the passage of an electrical current decreases as the diameter of any “cable” increases Synapses The site at which neurons communicate is called a synapse, a cell junction that mediates the transfer of information from one neuron to the next Synapses Because signals pass across most synapses in one direction only, synapses determine the direction of information flow throughout the nervous system Synapses The neuron the conducts impulses toward a synapse is called the presynaptic neuron Synapses The neuron that conducts impulses away from the synapse is called the postsynaptic neuron Synapses Most neurons function as presynaptic (information sending) and postsynaptic (information receiving neurons In essence they get information from some neurons and dispatch it to others Synapses Most synapses occur between the axon terminals of one neuron and the dendrites of another axons These are called axodendritic synapses Synapses Less common, and far less understood, are synapses between two axons (axoaxonic), between two dendrites (dendrodendritic) or between a dendrite and a cell body (dendosomatic) Synapses Structurally synapses are elaborate cell junctions At the typical axodendritic synapse the presynaptic axon terminal contain synaptic vesicles Synapses Synaptic vesicles are membrane bound sacs filled with molecular neurotransmitters These molecules transmit signals across the synapse Synapses Mitochondria are abundant in the axon terminal as the secretion of neurotransmitters requires a great deal of energy Synapses At the synapse, the plasma membranes of the two neurons are separated by a synaptic cleft On the under surfaces of the opposing cell membranes are dense materials; the pre- and postsynaptic densities Synapses When an impulse travels along the axon of the presynaptic neuron, it signals the synaptic vesicles to fuse with the presynaptic membrane at the presynaptic density The released neurotransmitter molecules diffuse across the synaptic cleft and bind to the postsynaptic membrane at the post synaptic density Synapse The binding of the two membranes changes the membrane charge on the postsynaptic neuron, influencing the generation of a nerve impulse or action potential in that neuron Signals Carried by Neurons In review, plasma membranes of neurons conduct electrical signals and that synapses relay the signals from neuron to neuron Signals Carried by Neurons In a resting (unstimulated) neuron, the membrane is polarized which means that the inner cytoplasmic side is negatively charged with respect to its outer, extracellular side Signals Carried by Neurons When a neuron is stimulated the permeability of the plasma membrane changes at the site of the stimulus, allowing positive ions to rush in. As a result, the inner face of the membrane becomes less negative or depolarized Signals Carried by Neurons Any part of the neuron depolarizes if stimulated, but at the axon alone this can result in the triggering of a nerve impulse or action potential Signals Carried by Neurons When a nerve impulse or action potential develops the membrane is not only depolarized , but its polarity is completely reversed so it becomes negative externally and positive internally Signals Carried by Neurons Once begun, the nerve impulse travels rapidly down the entire length of the axon without decreasing in strength Signals Carried by Neurons After the impulse has passed the membrane repolarizes itself Graded Potential In humans, natural stimuli are not applied directly to axons, but to dendrites and the cell body which constitute the receptive zone of the neuron When the membrane of this receptive zone is stimulated it does not undergo a polarity reversal Instead it undergoes a local depolarization in which the inner surface of the membrane merely becomes less negative Graded Potential This local depolarization is called a graded potential which spreads from the receptive zone to the axon hillock (trigger zone) decreasing in strength as it travels If this depolarizing signal is strong enough when it reaches the initial segment of the axon, it acts as the trigger that initiates an action potential in the axon Signals from the receptive zone determine if the axon will fire an impulse Synaptic Potential Most neurons in the body do not receive stimuli directly from the environment but are stimulated only by signals received at synapses from other neurons Synaptic input influences impulse generation through either excitatory or inhibitory synapses Synaptic Potential In excitatory synapses, neurotransmitters released by presynaptic neurons alter the permeability of the postsysnaptic membrane to certain ions, this depolarizes the postsynapatic membrane and drives the postsynaptic neuron toward impulse generation Synaptic Potential Inhibitory synapses cause the external surface of the postsynaptic membrane to become even more positive, thereby reducing the ability of the postsynaptic neuron to generate an action potential Thousands of excitatory and inhibitory synapses act on every neuron, competing to determine whether or not that neuron will generate an impulse Neural Integration The organization of the nervous system is hierarchical The parts of the system must be integrated into a smoothly functioning whole Neuronal pools represent some of the basic patterns of communication with other parts of the nervous system Neuronal Pools Neuronal pools are functional groups of neurons that process and integrate incoming information from other sources and transmit it forward One incoming presynaptic fiber synapses with Several different neurons in the pool. When Incoming fiber is excited it will excite some Postsynaptic neurons and facilitate others. Neuronal Pools Neurons most likely to generate impulses are those most closely associated with the incoming fiber because they receive the bulk of the synaptic contacts These neurons are in the discharge zone Discharge Zone Neuronal Pools Neurons farther away from the center are not excited to threshold by the incoming fiber, but are facilitated and can easily brought to threshold by stimuli from another source The periphery of the pool is the facilitated zone Facilitated zone Neuronal Pools Note: The illustrations presented are a gross oversimplification of an actual neuron pool Most neuron pools consist of thousands of neurons and include inhibitory as well as excitatory neurons Classification of Neurons Neurons can be classified structurally or functionally Both classifications are described in the text According to the structural classification system there are three types of neurons; Multipolar Bipolar Unipolar Structural Classification Multipolar - many processes extend from cell body, all dendrites except one axon Bipolar - Two processes extend from cell, one a fused dendrite, the other an axon Unipolar - One process extends from the cell body and forms the peripheral and central process of the axon Multipolar Neurons Multipolar neurons have more than two processes Most common type in humans Major neuron of the CNS Most have many dendrites and one axon, some neurons lack an axon Bipolar Neurons Bipolar neurons are rare in the human body Found only in special sense organs where they function as receptor cells Examples include those found in the retina of the eye, inner ear, and in the olfactory mucosa They are primarily sensory neurons Unipolar Neuron Unipolar neurons have a single process that emerges from the cell body The central process (axon) is more proximal to the CNS and the peripheral is closer to the PNS Unipolar neurons are chiefly found in the ganglia of the peripheral nervous system Function as sensory neurons Functional Classification The functional classification scheme groups neurons according to the direction in which the nerve impulse travels relative to the CNS Based on this criterion there are three neurons Sensory neurons Motor neurons Interneurons Functional Classification Sensory Neurons Neurons that transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS are called sensory or afferent neurons Virtually all primary sensory neurons of the body are unipolar Sensory Neurons Sensory neurons have their ganglia outside of the CNS The single (unipolar) process is divided into the central process and the peripherial process Sensory Neuron The central process is clearly an axon because it carries a nerve impulse and carries that impulse away from the cell body which meet the criteria which define an axon The peripheral by contrast carries nerve impulses toward the cell body which suggests that it is a dendrite However, the basic convention is that the central process and the peripheral process are parts of a unipolar neuron Motor Neurons Neurons that carry impulses away from the CNS to effector organs (muscles and glands) are called motor or efferent neurons Upper motor neurons are in the brain Lower motor neurons are in PNS Motor Neurons Motor neurons are multipolar and their cell bodies are located in the CNS (except autonomic) Motor neurons form junctions with effector cells, signaling muscle to contract or glands to secrete Interneuron or Association Neuron These neurons lie between the motor and sensory neurons These neurons are found in pathways where integration occurs Confined to CNS Make up 99% of the neurons of the body and are the principle neuron of the CNS Interneuron Neurons Almost all interneurons are multipolar Interneurons show great diversity in the size and branching patterns of their processes Interneurons The Pyramidal cell is the large neuron found in the primary motor cortex of the cerebrum The Purkinje cell from the cerebellum Interneurons Stellate cells of the cerebellum Interneurons Granule cells of the cerebellum Interneurons Basket cells of the cerebellum Myelin Sheaths Myelin sheaths are segmented structures, each composed of the lipoprotein myelin The sheaths surround the thicker axons of the body Myelin Sheaths Myelin sheaths form an insulating layer that… Prevents the leakage of electrical current from the axon Increases the speed of impulse conduction Makes impulse propagation more energy efficient Myelin Sheath Myelin increases the speed of transmission of nerve impulses Myelinated axons transmit nerve impulses rapidly; 150 meters/second Unmyelinated axons transmit quite slowly; 1 meter/second Myelin Sheaths Each segment of myelin consists of the plasma membrane of the supporting cell rolled in concentric layers around the axon Myelin Sheaths - PNS The myelin sheaths in the PNS are formed by Schwann cells Myelin develops during the fetal period and the first year or so of postnatal life Myelin Sheaths - PNS In forming the cells indent to receive the axon and then wrap themselves around the axon repeatedly in a jellyroll fashion Initially loose, the wrapping eventually squeeze the cytoplasm outward between cell membrane layers Myelin Sheaths - PNS The nucleus and most of the cytoplasm end up just external to the myelin layers Myelin Processes - PNS Myelin sheaths are associated only with axons and their collaterals as these are impulse conducting fibers and need insulation Dendrites which carry only graded potentials are always unmyelinated Myelin Sheaths - PNS When the wrapping process is complete many concentric layers wrap the axon Plasma membranes of myelinating cells have less protein which makes them good electrical insulators Myelin Sheaths - PNS Because the adjacent Schwann cells do not touch one another there are gaps in the myelin sheath These gaps, called nodes of Ranvier, occur at regular intervals about 1 mm apart Myelin Sheaths - PNS Since the axon is only exposed at these nodes nerve impulses are forced to jump from one node to the next which greatly increases the rate of impulse conduction Myelin Sheaths - PNS Schwann cells that surround but do not coil around peripheral fibers are considered unmyelinated A single Schwann cell can partly enclose 15 or more axons Each ends occupying a separate tubular recess CNS Axons Oligodendrocytes form the CNS myelin sheaths In contast to Schwann cells, oligodendrocytes can form the sheaths of as many as 60 processes at one time Nodes are spaced more widely than in PNS Axons can be myelinated or unmyelinated CNS Axons Regions of the brain containing dense collections of myelinated fibers are referred to as white matter and are primarily fiber tracts Gray matter contains mostly nerve cell bodies and unmyelinated fibers Types of Circuits Individual neurons in a neuron pool send and receive information and synaptic contacts may cause either excitation or inhibition The patterns of synaptic connections in neuronal pools are called circuits and they determine the functional capabilities of each type of circuit There are four basic types of circuits Diverging, converging, reverberating, and parallel discharge circuits Diverging Circuits In diverging circuits one incoming fiber triggers responses in ever-increasing numbers of neurons farther and farther along in the circuit Diverging circuits are often called amplifying circuits because they amplify the response Diverging Circuits These circuits are common in both sensory and motor systems Input from a single receptor may be relayed up the spinal cord to several different brain regions Impulses from the brain can activate a hundred neurons and thousands of muscle fibers Converging Circuits The pattern of converging circuits is opposite to that of diverging circuits Common in both motor and sensory pathways In these circuits, the pool receives inputs from several presynaptic neurons, and the circuit as a whole has a funneling or concentrating effect Converging Circuits Incoming stimuli may converge from many different areas or from the same source, which results in strong stimulation or inhibition Reverberating (oscillating) Circuits In reverberating circuits the incoming signal travels through a chain of neurons, each Reverberating of which makes circuit collateral synapses with neurons in the previous part of the pathway As a result of this positive feedback, the impulses reverberate through the circuit again and again Reverberating (oscillating) Circuits Reverberating circuits give a continuous signal until one neuron in the circuit is inhibited and fails to fire These circuits are involved in the control of rhythmic activities such as the sleep-wake cycle and breathing The circuits may oscillate for seconds, hours, or years Parallel After-Discharge Circuits The incoming fiber stimulates several neurons arranged in parallel arrays that eventually stimulate a common output cell Impulses reach the output cell at different times, creating a burst of impulses called an after discharge that may last 15 ms after initial input ends Parallel After-Discharge Circuits This circuit has no positive feedback and once all the neurons have fired, circuit activity ends These circuit may be involved with complex problem solving activities Patterns of Neural Processing Processing of inputs in the various circuits is both serial and parallel In serial processing, the input travels along a single pathway to a specific destination In parallel processing, the input travels along several different pathways to be integrated in different CNS regions Each pattern has its advantages The brain derives its power from its ability to process in parallel Serial Processing In serial processing the whole system works in a predictable all-or-nothing manner One neurons stimulates the next in sequence, producing a specific, anticipated response Reflexes are examples of serial processing but there are others Parallel Processing In parallel processing inputs are segregated into many different pathways Information delivered by each pathway is dealt with simultaneously by different parts of neural circuitry During parallel processing several aspects of the stimulus are processed Barking dog The same stimulus can hold common or unique meaning to different individuals Parallel Processing Parallel processing is not repetitious because the circuits do different things with more information Each parallel path is decoded in relation to all the others to produce a total picture of the stimulus Parallel Processing Even simple reflex arcs do not operate in complete isolation As an arc moves through an association neuron this activates parallel processing of the same input at higher brain levels The reflex arc may cause you to pull away from a negative stimulus while parallel processing of the stimulus initiates problem solving about what need to be done Parallel Processing Parallel processing is extremely important for higher level mental functioning An integrated look at the whole problem allows for faster processing Parallel processing allows you to store a large amount of information in a small volume This allows logic systems to work much faster Reflexes Reflexes are rapid, automatic responses to stimuli, in which a particular stimulus always causes the same motor response Reflex activity is stereotyped and dependable Some your are born with and some you acquire as a consequence of interacting with your environment Reflex Arcs Reflex arcs are simple chains of neurons that explain our simplest, reflective behaviors and determine the basic structural plan of the nervous system Reflex arcs are responsible for reflexes, which are defined as rapid, automatic motor responses to stimuli Reflex Arcs Reflexes that involve the contraction of skeletal muscle are referred to as somatic reflexes Reflexes that involve the contraction of smooth muscle, cardiac muscle, or glands are referred to as visceral reflexes Serial Processing: A Reflex Arc Reflexes occurs over neural pathways called reflex arcs that contain five essential components Receptor Sensory neuron CNS integration center Motor neuron Effector Reflex Arcs The receptor, sensory neuron, motor neuron, and effector are all relatively straightforward components When considering the integration center associated with reflex arcs, it is important to understand that the number of synapses involved can vary The simplest reflex arcs involve only one synapse in the CNS while others involve multiple synapses and interneurons Reflex Arcs At the top is a reflex arc, at the left is a monosynaptic reflex and on the right is a poly synaptic reflex Reflex Arcs The monosynaptic reflex has only one synapse and no interneuron, while the polysynaptic has multiple synapses and an interneuron Reflex Arcs - Monosynaptic This is the simple knee-jerk reflex The impact of the hammer on the patellar tendon stretches the quadriceps muscles Reflex Arcs - Monosynaptic Stretching activates a sensory neuron that directly activates a motor neuron in the spinal cord, which then signals the quadriceps muscle to contract This contraction counteracts the original stretching caused by the hammer Reflex Arcs - Monosynaptic Many skeletal muscles of the body can be activated by monosynaptic stretch reflexes These reflexes help maintain equilibrium and upright posture In these postural muscles, sensory neurons sense the stretching of muscles that occurs when the body begins to sway Motor neurons activate muscles that adjust the body’s position to prevent a fall Reflex Arcs - Monosynaptic Because stretch reflexes contain just one synapse monosynaptic reflexes are the fastest of all reflexes They are used in the body to maintain balance and equilibrium where speed of adjustment is essential to keep from falling Reflex Arcs - Polysynaptic Polysynaptic reflexes are the more common reflexes in the body In these reflexes, one or more interneurons are part of a reflex pathway between the sensory and motor neurons Reflex Arcs - Polysynaptic Most of the simple reflex arcs in the body contain a single interneuron and therefore have a total of three neurons Since there are two synapses joining the three neurons they are referred to as polysynaptic Reflex Arcs - Polysynaptic Withdrawal reflexes by which we pull away from danger are three-neuron reflexes Pricking a finger with a tack initiates an impulse in the sensory neuron, which activates the interneuron in the CNS Reflex Arcs - Polysynaptic The interneuron signals the motor neuron to contract the muscle that withdraws the hand from the negative stimulus Reflex Arcs - Polysynaptic The three neuron reflex arc are of special importance in the science of neuroanatomy Three neuron reflex arcs reveal the fundamental design of the entire nervous system Design of the Nervous System Three neuron reflex arcs from the basis of the structural plan of the nervous system Design of the Nervous System Note that the cell bodies of the sensory neurons lie outside the CNS in sensory ganglia and that their central processes enter the dorsal aspect of the cord Design of the Nervous System In the CNS the cell bodies of most interneurons lie dorsal to those of the motor neurons and the long axons exit the ventral aspect of the spinal cord Design of the Nervous System The nerves of the PNS consist of the motor axons plus the long peripheral process of the sensory neurons Design of the Nervous System These motor and sensory nerve fibers extend throughout the body to reach the peripheral effectors and receptors Design of the Nervous System Even though reflex arcs determine its basic organization, the human nervous system is obviously more complex than a series of simple reflex arcs To appreciate its complexity, we must expand our conception of interneurons Interneurons include not only the intermediate neurons of reflex arcs, but also all the neurons that are entirely confined within the CNS Design of the Nervous System The complexity of the CNS arises from the organization of the vast numbers of interneurons in the spinal cord and brain into complex neural circuits that process information The complexity of the CNS results from long chains of interneurons that are interposed between each sensory and motor neuron Design of the Nervous System Although tremendously oversimplified, the information depicted is a useful way to conceptualize the organization of neurons in the CNS Design of the Nervous System The CNS has distinct regions of gray and white matter that reflect the arrangement of its neurons The gray matter is a gray colored zone that surrounds the hollow cavity of the CNS It is H-shaped in the spinal cord, where its dorsal half contains cell bodies of interneurons and its ventral half contains cell bodies of motor neurons Design of the Nervous System Gray matter is a site where neuron cell bodies are clustered Specifically, gray matter is a mixture of neuron cell bodies, dendrites, and short unmyelinated axons Design of the Nervous System White matter which contains no neuron cell bodies but millions of axons Its white color comes from the myelin sheaths around many of the axons Most of these axons ascend from the spinal cord to the brain or descend from the brain to the spinal cord, allowing these two regions of the CNS to communicate with each other Design of the Nervous System White matter consists of axons running between different parts of the CNS Within the white matter, axons traveling to similar destinations form axon bundles called tracts Nervous Tissue Development During the embryonic period, which spans 8 weeks, the embryo goes from zygote to blastocyst, to two layer embryo, to three layer embryo The embryo upon reaching three layers begins to form the neural tube from which will differentiate the brain and spinal cord Nervous Tissue Development The nervous system develops from the dorsal section of the ectoderm, which invaginates to form the neural tube and the neural crest Nervous System Development The walls of the neural tube begin as a layer of neuroepithelial cells become the CNS These cells divide, migrate externally, and become neuroblasts (future neurons) which never again divide Nervous System Development These cells divide, migrate externally, and become neuroblasts (future neurons) which never again divide They cluster as future interneurons and motor neurons Nervous System Development Just external to the neuroepithelium, the neuroblasts cluster into alar and basal plates Nervous System Development Dorsally, the neurons of the alar plate become interneurons Ventrally, the neuroblasts of the basal plate become motor neurons and sprout axons that grow out to the effector organs Nervous System Development Axons that sprout from the young interneurons form the white matter by growing outward the length of the CNS These events occur in both the spinal cord and the brain Nervous System Development Most of the events described take place in the second month of development, but neurons continue to form rapidly until the about the sixth month At the sixth month neuron formation slows markedly, although it may continue at a reduced rate into childhood Nervous System Development Just before neuron formation slows, the neuroepithelium begins to produce astrocytes and oligiodendrocytes The earliest of these glial cells extend outward from the neuroepithelium and provide pathways along which young neurons migrate to reach their final destination As the division of its cells slows, the neuroepithelium becomes the ependymal layer Nervous System Development Sensory neurons do not arise from the neural tube but from the neural crest This explains why the cell bodies of the sensory neurons lie outside the CNS Sensory neurons also stop dividing during the fetal period Nervous System Development Sensory neurons cell bodies develop outside the CNS in the neural crest Sensory neurons also stop dividing during the fetal period Nervous System Development Neuroscientists are actively investigating how forming neurons “hook up” with each other during development As the growing axons elongate at growth cones, they are attached by chemical signals from other neurons called neurotrophins At the same time, the receiving dendites send out thin, extensions to reach the approaching axons to form synapses Nervous System Development Which synaptic connections are made, and which persist, are determined by two factors; The amount of neurotrophin initially received The degree to which a synapse is used after being established Nervous System Development Neurons that make “bad” connections are signaled to die via apoptosis Of the neurons formed during the embryonic period, about two-thirds die before birth This initial overproduction of neurons ensures that all necessary neural connections will be made and that mistaken connections will be eliminated