The Nervous System PDF

 In its broad aspects, the nervous system enables the animal to adjust itself or its parts to changes in the external or internal environment. The nervous system acts as a control system. There are mainly two cells (Neuron and glial cells) dealing with the complicated functions of th...

 In its broad aspects, the nervous system enables the animal to adjust itself or its parts to changes in the external or internal environment. The nervous system acts as a control system. There are mainly two cells (Neuron and glial cells) dealing with the complicated functions of the nervous system. The neuron (nerve cells) is the anatomic and physiologic unit of the nervous system. Glial cells (non-neuronal cells) that maintain homeostasis, form myelin, and provide support and protection for neurons in the central and peripheral nervous systems Human brain contains approximately 100 million neuron and a ten-fold number more of glial cells. Neuron  Cell body  Dendrites  Axon A nerve cell process is a dentrite if it conducts impulses toward the cell body and it is an axon if it conducts impulses away from the  Interneuron Sensory Synapse Neuron Synapse Motor Neuron Interneuron Synapse Motor Sensory Neuron Muscle Contracts Neuron Neurons and Synapses A neuron has only one axon but can have many dentrites. The dendrites provide the sites for receiving information from other neurons. They can be highly branched in order to provide a large surface area for communication with great numbers of axons.  The polarity of a neuron refers to the number of poles or processes that stem from its cell body. Mammalian neurons can be categorized;  Bipolar (one axon and one dentrite extending from the cell body, in the retina of eyesand olfactoric region in the nose)  Pseudounipolar (in which the axon and dentrite of a bipolar neuron have fused near the cell body, giving the appearance of only one process, most primary afferent neuron)  Multipolar (many branching dentrites and one axon extending from the cell body, most nuerons in the central nervous system)  The Axon (and its myelin covering, if present) is called a nerve fiber. The part of the cell membrane that covers the axon is known as the axolemma. In a myelinated axon, the axolemma is surrounded by a myelin sheath (neurilemma) that is interrupted at regularly spaced intervals by myelin-free gaps, called nodes (of Ranvier) A group of nerve cell bodies within the brain or spinal cord is referred to as a nucleus, and a group of nerve cell bodies outside the brain or spinal cord is called a ganglion. A bundle of parallel neuron fibers within the brain or spinal cord is known as either a tract  The Synapse provides continuity from one neuron to the next. There is no physical contact of neurons at the synapse. A space exists between the neurons, the synaptic gap, and impulses from one neuron to the next are transmitted by chemical means through this space. This is chemical synaptic transmission. Three notable characteristics of the synapse are;  One-way conduction (direction)  Facilitation (repeated impulses provide for easier subsequent transmission)  Greater fatigability than the neuron Neurons can not touch each other so the chemical inducers named neurotransmitters are released to the synapses. Neurotransmitters (pink vesicles) When the receptors receive neurotran smetters mean the message Synapse (gap) has been  Glial cells are the non neuronal cellular elements of the Central nervous system (CNS). They outnumber neurons by about 10-fold and make up about half of its volume. The dense packing of neurons and the more numerous glial cells cause nervous tissue to have less interstitial space than other tissues. Glial cells are metabolically quite active.  Glial cells are classified as Macroglia, microglia and ependymal cells. Macroglial cell are two types;  Oligodendrocytes (myelin sheath formation in CNS)  Astrocytes ( most prominent glial cells, their processes about blood vessels, synaptic structures and nerve cell bodies and processes ) The microglia (have a phagocytic function) CNS neuroglia PNS neuroglia  microglial astrocyte oligodendrocyte neuron and astrocyte http://www.emc.maricopa.edu/faculty/farabee/biobk/ BioBookNERV.html  Myelin Sheaths: Myelin is a white lipid (sphingomyelin) substance that forms a sheath around nerve fibers and serves as an electrical insulator. It is formed by oligodendrocytes in the CNS and by Schwann cells in the peripheral nervous system (PNS). Nerve fibers within the gray matter of the CNS are not myelinated; their white glistening appearance outside of the gray matter, as shown by the white matter and by peripheral nerves, is provided by the myelin that envelops the nerve fibers.  The Schwann cell cytoplasm (which contain the myelin) is wrapped around a nerve fiber many times, and the nucleus lies within the Schwann cell just beneath the neurolemma external to the myelin sheath. Interruptions of the myelin sheath that occur along the length of a fiber are called nodes of Ranvier.  In Ranvier nodes, the nerve fiber plasma membrane (axolemma) is directly exposed to extracellular fluid. The exposure is more intimate in the CNS. Whereas the sheathed portion of the nerve fiber is insulted, the nodes are insulated. Depolarization occurs at the nodes.  The The Nervous Nervous System System Central CentralNervous NervousSystem System(CNS) (CNS) Peripheral PeripheralNervous NervousSystem System(PNS) (PNS) Brain Brain Spinal SpinalCord Cord Motor MotorNeurons Neurons Sensory SensoryNeurons Neurons Somatic SomaticNervous NervousSystem Autonomic System AutonomicNervous NervousSystem System Sympathetic Sympathetic Parasympathetic Parasympathetic It is composed of brain and spinal cord Cerebrum brain Cerebellum Medulla Oblongata Spinal cord The CNS not only contains components of transmission, but the brain also provides for those functions that we associate with computers, such as memory, a central processing unit for problem-solving, and input-output capability (sensations resulting from sensory input).  Central Nervous System Brain Cerebrum (paired cerebral hemisphers) Cerebellum Brain stem  Central Nervous System Cerebral Hemispheres: Each hemisphere is composed of a covering of gray matter, but the cerebral cortex; a central mass of white matter, the medullary substance (made up of nerve fibers); and the basal nuclei. The cerebral cortex has the following characteristics: Acquired late in vertebrate evolution Concerned with those nervous reactions that result in consciousness Regarded as the seat of the highest type of nervous correlation Marked by a high degree of educability (especially in humans) Possesses a motor area; a.Impulses from these areas in one hemisphere cause muscle movements on the opposite (contralateral) side of the body, b.Size of motor area and number and complexity of skeletal muscle movements of which an animal i s capable are directly related Contains sensory areas, or centers, into which sensory fibers discharge.  Central Nervous System The sensory areas are: 1. the somesthetic or body sense area, which receives impulses from the skin concerned with touch, warmth, cold and pain localization; impulses concerned with taste; and impulses from muscles, tendons, and joints. 2. the visual area (sight), 3. the auditory area (hearing) and 4. the olfactory area (smell). The white matter is composed of myelinated nerve fibers situated beneath the cerebral cortex. These include; a.association fibers (connection between the different parts of the cortex), b.commissural fibers (connect the two hemispheres), c.projection fibers (connect the cerebral cortex with other parts of the brain and spinal cord).  Central Nervous System The basal nuclei lie deep within the cerebral hemispheres. They are composed of separate, large pools of neurons organized for the control of complex semivoluntary movements, such as walking and running. In birds the cerebral cortex is poorly developed, but the basal nuclei are highly developed. Because of this contrast, the basal nuclei perform nearly all of the motor functions, even the voluntary movements, in much the same manner as the motor area of the human cortex controls voluntary movements. In the cat, and to a lesser extent in the dog, removel of of the cerebral cortex prevents many sophisticated motor functions. Because of the basal nuclei, however, this does not  Central Nervous System Cerebellum: The cerebellum is not concerned with consciousness or sensation, as is the cerebral cortex. The cerebral cortex is not organized to mobilize the opposing force. The cerebellum, however, can make automatic adjustments to prevent the distortion of inertia and momentum. To accomplish this the cerebellum receives impulses; 1) from the proprioceptive receptors (located in the internal mass of the body) found in all joints, muscles and pressure areas (e.g. foot pads), 2) from the equilibrium apparatus of the inner ear, 3) from the visual cortex, and 4) directly from the motor cortex of all motor impulses being sent to muscles. Whereas the motor area of a cerebral hemisphere exerts its effect on the opposite (contralateral) side of the body, the effect of one side of the cerebellum is exerted on the same (ipsilateral) side of the body. The cerebellum act as a “collecting house” for all information regarding the instantaneous physical status of the body.  Central Nervous System Brain Stem: The brain stem is composed of the interbrain cranially followed caudally (in order) by the midbrain, pons, and medulla oblongata. The cerebral hemispheres and cerebellum arise from the brain stem In addition to the many fiber tracts that ascend and descend between the spinal cord and the cerebrum and cerebellum, the brain stem is the origin of all the cranial nerves except for the optic, olfactory, and acoustic nerves (special nerves). The cells of origin for the latter lie outside the skull.  Central Nervous System  From below upward, the interbrain is comprised of the hypothalamus, thalamus, and epithalamus. The Hypothalamus contains the hypophysis or pituitary gland, which is an endocrine organ. Associated with the hypothalamus is a complex sensing and neurosecretory function. Also, the hypothalamus assumes a major role in the integration of functions carried out by the autonomic nervous system. For these functions, the and middle portions contain parasympathetic components and the posterior portion contains sympathetic components. The Thalamus contains many nuclei and is truly a relay center. Impulses from all areas of the body are transmitted to the thalamus for transfer to the cerebral cortex. Other nuclei in the thalamus are associated with the relay of impulses within the brain. The Epithalamus, contains an olfactory (smell) correlation center and the pineal gland. The latter is a neurosecretory organ that regulates gonadal hormones and certain daily rhythms.  Central Nervous System Spinal Cord: The caudal continuation of the medulla. Receives sensory afferent fibers by way of the dorsal roots of the spinal nerves and gives off efferent motor fibers to the ventral roots of the spinal nerves. The centrally located gray matter (resembles a capital H and called gray H) consists primarily of nerve cell bodies and their processes. The peripherally arranged white matter, which has a white appearance because of its myelin ensheatment, is composed of many distinct tracts. The tracts connect the brain stem and higher centers with the spinal nerves. Different sensory and motor tracts are segregated in the cord.  Proprioceptive impulses from muscles, tendons, and joints have well-defined tracts, as do sensory impulses for pain, temprature, and touch. Similarly, impulses associated with motor functions descend in definite tracts. Many of the tracts are named according to the structures they connect. For example, the ventral spinocerebellar tract carries impulses from the spinal cord to the cerebellum. The cells of origin for sensory impulses to the brain or the other parts of the spinal cord are located in the dorsal horns of the gray H, and the cells of origin of motor impulses to the spinal nerves are located in the ventral horns of the gray H The cells of origin of the autonomic motor impulses arising from the spinal cord are the lateral masses of the ventral horns (intermediate location) of the gray H. As the spinal cord descends and proceeds caudally, its cross-sectional area decreases. Accordingly, the Cross section of the spinal cord of the dog. terminal part of the spinal cord, Located within the gray matter are ;1) nerve cell meninges, and nerves is called cauda bodiesfor sensory neurons, 2)somatic motor equina. neurons, 3) autonomic motor neurons.  The peripheral nervous system consists of the spinal nerves and  Peripheral Nervous System Spinal Nerves: As well as cranial nerves, are referred to as somatic nerves because of their association with voluntary control of muscles. Autonomic nerves are referred to as visceral nerves because they are involved with involuntary functions such as control of smooth muscle, cardiac muscle, and glands.  The spinal nerves are those that arise from the spinal cord and emerge from the vertebrae. In the dog, for example, 7 cervical, 13 thoracic, 7 lumbar, 3 sacral and an average of 20 caudal vertebrae. With the exception of the cervical and caudal nerves, there is a pair of spinal nerves (one right and one left) that emerges behind the vertebrae of the same of the same serial number and name. A spinal nerve is composed of a dorsal and ventral root and its branches. It carries afferent (sensory) impulses from the periphery toward the spinal cord. Peripheral Nervous System The ventral root, emerges from the ventral portion of the spinal cord. It carries efferent (motor) impulses from the spinal cord to striated muscle fibers. Near the intervertebral foramen, the dorsal root joins with the ventral root to form the main part of the spinal nerve. The spinal nerve proper is classified as a mixed nerve because it contains both sensory and motor fibers. After the spinal nerve emerges from the intervertebral foramen, it divides into a dorsal branch and a ventral branch; these supply innervation to the structures dorsal and ventral to the transverse processes of the vertebrae, respectively. Bir spinal sinir ve onun dalları, kökler, spinal ilik ve vertebra ile ilgili  Peripheral Nervous System Cranial Nerves: There are 12 pairs of cranial nerves, with a right and left nerve comprising each pair..  The cranial nerves usually supply innervation to structures in the head and neck. The vagus nerve is an exception.  In addition to its sensory and motor supply to the pharynx and larynx, it also supplies parasympathetic fibers to visceral structures in the thorax and abdomen.  These nerves have no dorsal or ventral roots and emerge through foramina in the skull. Origin and major distribution of cranial nervesin thee dog. CRANIAL NERVES  Autonomic Nervous System (ANS) The ANS, also known as the involuntary, vegetative, or visceral nervous system, is essential for maintaining normal organ function (homeostasis), adapting to environmental change (e.g., body temperature), and responding to stresses (e.g., excitement). The ANS has sympathetic, and enteric subdivisions: 1.The sympathetic nervous system (SNS); is associated with the body’s response to stress; 2. The parasympathetic nervous system (PSNS); is associated with homeostatic functions in the absence of stress; and 3.The enteric nervous system (ENS); is associated with regulation of the gastrointestinal (digestive) system. The ENS functions mostly by itself but can be modulated by both SNS and PSNS.  ANS innervation extends to smooth muscle, cardiac muscle, and glands. Most organs receive both sympathetic and parasympathetic innervations.  This system differs from the somatic system with the type of working involuntarily.  Somatic system and ANS are not the systems work independently each other, opposite that they work together. 40 Motor Neurons of Autonom and Somatic Nervous Systems dr. sefa gültürk 41  Autonom Nervous System Sympathetic Parasympathet  It prepares the ic body to react Calms the the stress in case of body, after emergency. emergency Reaction of situation “fight, fright, or passes it helps flight” the body to Catabolic return to the normal situation. 42 Sympathetic System: “Fight or Flight” Eye Awakens  (threshold of RF decreases ) Pupilla  Lung Blood flow to the skeletal s muscle  Diameter of respiratory tracts Heart and frequency of respiration Frequency of heart and blood pressure  Level of plasma glucose and free fatty acid  (energy) Digestion Blood flow to the skin Digestion 43  Parasympathetic System Eye relaxation Lungs Pupilla  Frequency of heart and blood pressure  Diameter of respiratory Heart tracts and frequency of respiration  Blood flow to the gastrointestinal system  Storage of glucose as glycogen Digestion 44 ACTIONS OF AUTONOMIC STIMULATION  Autonom Nervous System (ANS) Central Components; The central component resides in the central proccessing that results from data received (e.g., blood pressure, organ distention) from afferent (inflowing) neurons. The central processing also relies on a number of blood signals such as temperature, pH, glucose concentration, and others. The central proccessing for the reflex integration and modulation of body conditions by the ANS occurs in spinal cord sites and a number of brain locations. The latter is best represented by the  Autonom Nervous System (ANS) Peripheral Components; The cells of origin for the sympathetic nerves are located in the intermediolateral horn of the thoracic and lumbar segments of the spinal cord, and the cells of origin of the parasympathetic nerves are located in the brain stem and sacral segments of the spinal cord, hence, the terms for for their origin is noted as craniosacral for the parasympathetics, as opposed to thoracolumbar for the sympathetics. For both sympathetic and parasympathetic activity, two neurons (in series) are associated with the transmission of impulses from the cells of origin in the spinal cord or brain to the effector organ (glands or mucle). The cells of origin for the second neuron are located in ganglia. The first neuron is called preganglionic, and the second neuron is called postganglionic.  Nöronlar Pregangliyonic (B type fibres) – Cell bodies are in CNS – Miyelinated axons go to the ganglion of Autonom Nervous System Postgangliyonic (C type fibres) – Cell bodies are in the ganglions of Autonom Nervous System – Unmyelinated axons go the effector areas 48  Autonomic Nervous System (ANS) Differences between Autonomic and Somatic nerves; In the case of cutting or damaging a somatic nerve, the muscle works with this nerve become inefficient and smaller. In the case of cutting or damaging an autonomic nerve, the organ works with this nerve is not affected seriously. It is possible the organ does not work at the beginning, and then it works properly. There is no any atrophy in the organ. In somatic nerve inhibition happens in the centre. In autonomic nerve inhibition happens in both centre and periphery. In case of stimulating n.vagus, the ability of signal producing in sinus nodule getting less and the heart may stop at the end.  Autonomic Nervous System (ANS) Differences between Autonomic and Somatic nerves; The body of somatic nerves localised in central system. Axons of these nerves go to the organ directly without stopping in any ganglia. In autonomic nerves, axons definitely stop in ganglia. The axon placed before ganglia are myelinated and the axon placed after is unmyelinated. The diameters of somatic nerves are quite thick and signal speed is quite fast (120 m/sec). The diameters of autonomic nerves are thin and the speeds of signals are slow (2-14 m/sec). Somatic Somatic fibres Neurons and Plexus Voluntary control It controls sceletal muscle Induces the movements High impuls Otonom frequency Visceral fibres Neurons, ganglia and Plexus Involuntary control It controls smooth mucle, glands, and heart muscle Induces the movements or suppress dr. sefa gültürk 51  Autonomic Nervous System (ANS) Sympathetic Efferent Distribution  The preganglionic neuron for a sympathetic nerve traverses the ventral root of a thoracic or lumbar spinal nerve, enters the spinal nerve proper, and soon branches from it to enter a vertebral ganglion of the sympathetic trunk, a bilateral chain of ganglia ventral to the vertebrae with a ganglion on each side of each vertebrae.  It either synapses in a ganglion of the same vertebral segment, or it can continue over a considerable distance to another vertebral ganglion where it synapses. The synapse might not occur in a vertebral ganglion at all, however, but might continue to some paired ganglia that are ventral to the sympathetic trunk; these are called prevertebral or colleteral ganglia.  Autonomic Nervous System (ANS) The prevertebral ganglia are fewer in number and are named; The cranial cervical – ganglion with distribution to smooth muscle and glands of the head The middle cervical ganglion – heart and lungs, The servikotorasic ganglion – arteria in the neck and thorax, The celiac ganglia - stomach, liver, pancreas, kidney and adrenal, The cranial mesenteric ganglion- small intestine and upper colon The caudal mesenteric ganglia lower colon and neck of bladder The postganglionic neuron leaves the vertebral or  Autonomic Nervous System (ANS) Sympatheic system has got the structure which controls lots of work in organism. It is observed that in case of abolishing this system experimentally, animals can survive. In cats, aborts and dead births were seen often, and also lactation was not carried out properly. These kind of animals cannot adapt to environment. The low blood pressure cannot increase and the glucose level can not be adjusted. In case of facing with the dog, they did not have piloerection and their glucose level was not increased. It is understood that the dogs are more resistant to live without sympathetic system although some negative signs they show.  III = oculamotor nerve: Edinger- Westphal nucleus VII = facial nerve, n. salivarius sup. IX = glossopharyngeal nerve, n. salivarius inf. X = vaagal nerve, dorsal vagal nucleus, n. Sympathetic nerve ambiguus system toraco- lumbar system –T1 – L3 (or L2) segments Parasympathetic nerve system = cranio-sacral system –III, VII, IX and X. Cranial nerves and S2-S4 segments. dr. sefa gültürk 56  Autonomic Nervous System (ANS) Parasympatheic Efferent Distribution  The preganglionic neurons of the parasympathetic division are distributed to ganglia near the effector organs before they synapse with the postganglionic neuron.  Accordingly, the preganglionic neuron are relatively longer and the postganglionic fibers are relatively shorter.  Most of the parasympathetic ganglia are microscopic and are an intimate component of the tissue they innervate.  The parasympathetic preganglionic fibers that  Autonomic Nervous System (ANS)  The first three (III, VII and IX) supply regions of the head, and the last, cranial nerve X (the vagus nerve), supplies the heart and lungs in the thorax and nearly all of the abdominal viscera. The vagus nerve has sometimes been called the vagabond nerve because of its extensive wanderings.  The parasympathetic preganglionic fibers that arise from nerve cell bodies in the sacral portion of the spinal cord supply the last part of the digestive tract and most of the urogenital system. These fibers emerge from the ventral branches of their respective segments and are distributed to the ganglia near the effector organs supplied by the pelvic nerve.  Autonomic Nervous System (ANS) Autonomic reflexes Autonomic function is based on reflex activity, and these reflexes control such functions as blood pressure, heart rate, and the activity of the digestive and urogenital systems. Autonomic reflexes involves;  afferent transmission of sensory information from effector organs to to the CNS,  information processing, and  return of a motor response to the effector organs.  Autonomic Nervous System (ANS) Autonomic afferents are not designated as sympathetic or parasympathetic (i.e., they transmit information regardless of which division of the ANS), and most travel to the CNS via SNS and PSNS nerves. Their cell bodies are in Dorsal root ganglion and cranial nuclei. Some afferents (e.g., blood vessels in skeletal muscle) travel in spinal and cranial nerves. Most of the autonomic functions do not reach the conscious level. However, some afferent information carried by autonomic sensory neurons does reach conscious levels. This may be normal or pathologic. Normal would include feelings of fullness of the bladder or rectum and pathologic might include gallbladder pain or angina pectoris.  NEUROL CONTROL In animals, there are mainly two control systems. One is hormonal (also present in plants). Neural control is specific only for animals. The basic unit is neuron. Detecting internal and external changes happens by some facilities of the neurons. 1.Irritability: There are special nerve cells and creatures which have ability to react any stimulus in multicellular organisms. For example, those in innervestibulum give reaction to sound waves and receptors present on the skin sensitive to hot and cold and only react to the releated stimulus. 2.Conductivity: The impulse occured in receptor after stimulus is conducted to the special centres by nerves. 3.Correlation: The stimulus are evaluated in the special centres and decided about reaction of animals. After doing last evaluation, the impulses are sent to the effector organs from the centres (conductivity).  Dendrit Muscle e Axon Cell cell body body A typical motor neuron synaps Motor Neurons They carry the signals from brain and medulla spinalis to the muscles and glands End of axon Branching of the axons to the  The Nerve impuls and its transmission  Communication among neurons and with the cells of their control is accomplished by the transmission of a nerve impulse.  A nerve impulse originates in response to a stimulus of an electrical, chemical thermal, or mechanical nature that has been received by the cell membrane of a neuron.  The stimulus, elicits a wave of depolarization and repolarization that spreads along the axolemma, away from the site where the stimulus was received, which results in the transmission of the nerve impulse.  The Nerve impuls and its transmission Mechanisms of Transmission The word potential is used in regard to nerve cells as it is in the study of electricity, in which it refers to relative electric charges between two points in a field or circuit. For the neuron, this is referred to as a transmembrane potential, and the two points are the inside and outside of the confines of the cell membrane. All cells of the body have a transmembrane potential, but the neurons are unique in being able to alter this potential to produce an impulse. The charged transmembrane potential is a local phenomenon close to the cell membrane and does not refer to a charge inside and outside the cell, which is electrically neutral. A measured potential is relatively small, however, and its units are in millivolts rather than volts.  Resting Membrane Potential In a resting neuron, the potential between the two sides of the membrane is called the resting potential. The resting membrane potential results from the unequal distribution of sodium ions (Na+) and potassium ions (K+) on the outside and inside of the neuron. The active transport of Na+ to the outside, coupled with the transport of K+ into the neuron (the Na+ − K+ ATPase pump), keeps the concentration of Na+ low on the inside of the membrane. The outward active transport of Na+ occurs at a faster rate than the inward Resting Membrane Potential  Depolarization, Repolarization and the Nerve Impulse  Chemical or physical stimulation of a neuron increases the permeability of the membrane for Na+ at the point of stimulation and, because there is a high concentration of Na+ on the outside of the membrane in the extracellular fluid, Na+ rushes inward.  This reverses the membrane potential at the point of stimulation, so the membrane now becomes positive on the inside and negative on the outside; this is depolarization.  The inflow of Na+ soon stops and the permeability of the membrane for K+ increases; the K+ then flows outward because it has a higher concentration inside the neuron than outside. The outflow of K+ reestablishes the resting membrane potential at the point of stimulation; this is repolarization.  When a microregion of a nerve fiber is stimulated and subsequently depolarized, a current flow occurs from the point of depolarization to the adjoining microregions.. Current flow occurs because a positive charge now exists inside the membrane at the point of initial depolarization; because of the negative charge inside the membrane, beyond the point of stimulation, the positive charges (ions) flow toward the negatively charged portion. In addition, the outer aspect of the fiber membrane (which has become negatively charged at the point of depolarization) attracts positive ions to it from the charged membrane farther ahead. Because of these two events, the interior of the fiber just beyond the depolarized region becomes somewhat more positively charged and the exterior of the fiber just beyond the depolarized region becomes less positively charged. The passage of current out through the membrane, just beyond the site where depolarization has occurred, causes this region of the membrane to become depolarized in turn (because current flow increases permeability to Na+), just as the membrane did at the site of the stimulus.  Action Potential Action potentials are changes in the resting membrane potential that are actively propagated along the membrane of the cell. The application of a stimulus to a nerve cell membrane diminishes the resting membrane potential (zero direction). When the membrane potential reaches a critical value (usually 10 to 15 mV less than the resting level of −70 mV), an action potential occurs. The membrane potential at which an action potential is produced is referred to as threshold. The nerve fiber cannot be stimulated again until repolarization is nearly complete; this is known as the refractory period. If the stimulus is strong enough to initiate an action potential, the entire fiber will fire. This is known as the all or- none principle for nerve fibers. There is no such thing as a weak impulse.  Action potential The resting membrane potential (Vm ) has been measured as about, -70mV (Vm=VR). This reverses the membrane potential at the point of stimulation, so the membrane now becomes positive on the inside and negative on the outside; this is depolarization. The inflow of Na+ soon stops and the permeability of the membrane for K+ increases; the K+ then flows outward because it has a higher concentration inside the neuron than outside. The outflow of K+ reestablishes the resting membrane potential at the point of stimulation; this is repolarization.. After that, Vm decreases even below the resting potential. This last period is a recovery period of action potential (hyperpolarization).  Excitatory and inhibitory postsynaptic potentials Graded potentials modulate the postsynaptic neuron by shifting the resting membrane potential toward or away from the threshold potential. Shifting the membrane potential toward more positive is called depolarization and a depolarizing graded potential is referred to as an excitatory postsynaptic potential (EPSP). For example, acetylcholine and glutamate induce depolarizing graded potentials by opening ligand‐gated Na+ channels, triggering an influx of Na+. Subsequent shifting of the membrane potential toward more negative is called hyperpolarization. A hyperpolarizing graded potential is called an inhibitory postsynaptic potential (IPSP) and synapses that induce IPSPs are called inhibitory Postsynaptic potentials generated at the postsynaptic cell body and dendrites. (A) Neurotransmitters, for example acetylcholine (ACh) and glutamate, induce excitatory postsynaptic potentials (EPSPs) by opening ligand‐gated Na+ channels, triggering an influx of Na+. EPSPs drive the membrane potential toward threshold voltage. (B) The neurotransmitters glycine and GABA induce inhibitory postsynaptic potentials (IPSPs) by binding to ligand‐gated Cl− channels that trigger an influx of Cl− ions. Saltatory conduction: In myelinated fibers the depolarization and repolarization processes are the same, but the action potentials occur from one node of Ranvier to the next instead of over the entire area of the membrane. This process of impulse transmission is referred to as saltatory conduction (saltation refers to an abrupt movement, such as dancing or leaping). The axolemma is in intimate association with the extracellular fluid at the nodes of Ranvier, and the remainder of the membrane is relatively insulated from the extracellular fluid. Two functions are served by saltatory conduction. First, impulse transmission is accelerated; Second, less membrane is depolarized and repolarized, hence reducing the energy requirement for “recharging” the membrane. Transmission Velocity: The larger the diameter of the fiber and the thicker the myelin sheath, the faster the transmission of the impulse (The fastest transmission is about 100 m/s, and the slowest is about 0.5 m/s.). Large myelinated fibers can transmit about 2,500 impulses/s, contrasted with about 250 impulses/s for small unmyelinated fibers.  Neurotransmitters A nerve impulse causes an effect at a synapse or at the structure being innervated. Axons terminate by branching; the branches terminate with a structure known as a presynaptic terminal bulb at the synapse and with other similar, modified structures at the organs innervated. These terminations have vesicles containing chemical substances that are liberated when the impulse arrives. The chemical substance then diffuses to the membrane of the postsynaptic neuron or structure and influences the permeability of the membrane for sodium ions. Peripheral Neurotransmitters The neurotransmitters of the somatic peripheral nervous system are excitatory in nature— that is, they increase the permeability of the affected membrane for sodium ions. This substance is acetylcholine (ACh) for the somatic spinal and cranial nerves. ACh is also the preganglionic and postganglionic terminal neurotransmitter for the parasympathetic division of the autonomic nervous system. This division of the autonomic nervous system is, therefore, sometimes referred to as the cholinergic system. The preganglionic terminal neurotransmitter of the sympathetic division is also ACh, but the postganglionic terminal secretion is usually norepinephrine. Another name for norepinephrine is noradrenaline, so the sympathetic division is often referred to as the adrenergic system. 82 Central Neurotransmitters In the central nervous system there are not only excitatory but also inhibitory transmitters. In addition to ACh and norepinephrine, which are present in peripheral neurons, other excitatory transmitters are found in the central nervous system. At least two inhibitory transmitters are recognized within the brain and spinal cord, gamma-aminobutyric acid (GABA) and glycine, which is a simple amino acid. A decrease in the Final Common Pathway Somatic lower motor neurons (LMNs) are motor neurons of the spinal cord and brain stem nuclei that innervate skeletal muscle effectors. Upper motor neurons (UMNs) are located in the brain and have fibers that descend to and modify the activity of LMNs. Usually, the branches of many axons (some UMNs), perhaps 2,000 or so, will impinge on the dendritic zone of an LMN, which then, depending on the algebraic sum of the inhibiting and facilitatory input, will either fi re or not fire. Thus, the LMN serves as the final common pathway (and the last site for integration) for all output to striated skeletal muscle Lower motor neuron going to striated muscle. This represents the final common pathway. To fire, a greater amount of excitatory (E) neurotransmitter must be released than inhibitory (I) neurotransmitter. Dashed lines represent axons  Neuron Placement Within the central nervous system are several schemes of neuron placement (circuits) that allow for different patterns of activity. Converging Circuit A circuit in which several neurons impinge on one neuron is known as a converging circuit. It allows impulses from many different sources to cause some response or provide a sensation.  Neuron Placement Diverging Circuit A diverging circuit is one in which the axon branches of one neuron impinge on two or more neurons, and each of these in turn impinge on two or more neurons. This type of circuit allows for amplification of impulses and is found in the control of skeletal muscles.  Neuron Placement Reverberating Circuit A reverberating circuit is one in which each neuron in a series sends a branch back to the beginning neuron so that a volley of impulses is received at the final neuron. This type is associated with rhythmic activities, and the volley continues until the synapse fatigues or some other mechanism of unknown type stops the reverberating circuit.  Neuron Placement Parallel Circuit A parallel circuit contains a number of neurons in a series, with each neuron supplying a branch to the final neuron. Because there is a delay of transmission at the synapse, a volley of stimuli reaches the final neuron. Unlike the reverberating circuit, the impulses then stop. This type provide reinforcement to a single stimulus.  Neuron Placement Simple Circuits Many complex neuron connections are possible, but neuron connections also can be direct and simple. The neurons associated with the special senses might involve no more than two neurons for their projection to the cerebral cortex. A minimum of three neurons is required to transmit a nerve impulse from the periphery by way of a spinal nerve to the cerebral cortex. The three neuron circuit is the classic circuit for conscious sensations.  Reflexes A reflex is defined as an automatic Reflex Arc or unconscious response of an effector organ (muscle or gland) to an appropriate stimulus. The components involved for a reflex to occur make up what is known as a reflex arc and consist of: 1) a receptor, 2) an afferent  Characteristics of the Reflex Summation: EPSPEvent and IPSP created by the stimuli in the postsynaptic neuron may not be strong enough to have a sufficient effect. If stimuli with the same intensity lower than the threshold value are given consecutively from one point or many points, the EPSP created by each of them accumulates and exceeds the stimulation threshold and causes stimulation. This event is called summation. Facilitation: Some of the motor neurons may have more than one afferent nerve wires. If several of these nerve wires are simultaneously stimulated with a weak stimulus, the initiation of stimulation as a result of simultaneous  Characteristics of the Reflex Event Afterdischarge: Although the effect of the stimulus applied from the outside is no longer effective, stimulation is sent from the center of the reflex towards the periphery for a while. The continuation of the stimulation in synapses even though the stimulus has disappeared is called after-discharge or post- discharge (continuation of activity in intermediate neurons for a while). Inhibition: The muscles that undertake stretching and bending in an extremity coexist. These are called antagonist muscles. While the tension muscle contracts, the flexor muscle is inhibited. This condition is called reciprocal innervation of antagonist muscles. Prevention of a reflex by stimulation from another afferent path to the reflex center is called inhibition.  Reflex Types There are two types of reflexes; 1. Unconditional Reflexes:These types of reflexes belonging to all the medulla spinalis and brain-stem reflexes, respiratory, digestive and cardio-vascular systems, which are congenital, unchanging and unique to every animal species, are called unconditional reflexes. 2. Conditional Reflexes: They are reflexes that a living thing will learn later in its life. Their occurrence depends on a number of conditions. The experiments Pavlov conducted on dogs in establishing such reflexes are very interesting. These reflexes are also called acquired reflexes. Acquired reflexes can be created naturally or artificially. Putting a piece of meat in a puppy's mouth stimulates the inherited salivation reflex. When the older puppy sees or  Spinal Reflexes Flexsor reflex Periton reflex Extensor pushing Cremaster reflex reflex Patella reflex Cross extensor Rectum and reflex bladder emptying Itching reflex reflex Stepping reflex Spino-genital Withers reflex in reflex horse Vasomotor and sweating reflex  Spinal Reflex The reflex is elicited by striking the middle patellar ligament. This ligament, located at the knee, is the tendon of insertion for the quadriceps femoris and transmits its action to extend the tibia. Striking the middle patellar ligament stretches the quadriceps muscle,  Spinal Reflex A reflex can involve parts of the brain and autonomic nervous system, but the simplest reflex is the myotatic (stretch) spinal reflex. An example of a spinal reflex is the knee jerk reflex. The stretch reflex. Stretch of muscle stimulates the muscle spindle. The impulse travels to the spinal cord by way of an afferent neuron. Transmission of the impulse to an efferent neuron may be direct or by way of an interneuron as shown. Stimulation of an efferent neuron to striated muscle counteracts stretch by causing contraction. Muscle spindles, in addition to being involved in reflexes, also provide sensory input to cerebral and cortical levels  Spinal Reflex An impulse is transmitted by way of the dorsal root of the appropriate spinal nerve to the applicable motor neuron in the ventral horn of the gray matter, and thence to muscle fibers of the quadriceps muscle, causing it to contract. The purpose of the reflex is to oppose stretch of the muscle. Absence of the knee jerk reflex can help to confirm suspicion of damage or injury to the spinal cord or any of the five components of the reflex arc.  Spinal Reflex Spinal reflexes can also be rather complex, in which the central connections of the reflex extend over several segments and also extend contralaterally as well as ipsilaterally.  Spinal Reflex The crossed extensor response is an example of a complex spinal reflex. This is shown when there is painful stimulation of the skin or subcutaneous tissues and muscle. The response is flexor muscle contraction and inhibition of extensor muscles so that the part stimulated is flexed and withdrawn from the  Somatic and Visceral If the effector organs Reflexes are composed of striated muscle, the reflex is somatic. If the effector organs are either smooth or cardiac muscle, or glands, the reflex is visceral. Visceral reflexes regulate visceral functions and are transmitted by the autonomic nervous system (by visceral afferent fibers and preganglionic and postganglionic efferent fibers of the sympathetic or parasympathetic division).  Reflex Centers Reflex centers are located throughout the central nervous system. They are involved with the integration of more complex reflexes. The simplest reflexes are those associated with the spinal cord, and the more complex are carried out through reflex centers in the brain. Some of these centers are located in the pons and medulla oblongata and include reflex centers for the control of  Reflex Centers The cerebellum contains most of the reflex centers associated with locomotion and posture. The hypothalamus is the main integration and regulation center for the autonomic nervous system, e.g., contains reflex centers associated with temperature regulation. The midbrain contains visual and auditory reflexes, which can bring about constriction or dilatation of the  Postural Reflexes and Reactions The postural reflexes and reactions aid in maintaining an upright position. Responses that involve the cerebral cortex are more properly called reactions than reflexes. Muscle tonus (tone) is that state of muscle tension that enables an animal to assume and remain in the erect attitude. The stretch reflex, previously described, is the fundamental element of muscle tone. The spinal cord of domestic animals constitutes a greater proportion of the CNS (brain and spinal cord) than in humans. This reflects the fact that more of the CNS activity in animals is accomplished by reflex than by cerebral activity. There is approximately 10 times more spinal cord activity in dogs than in humans. Postural Reflexes and Reactions The following are examples of postural reflexes and reactions: 1. Standing reflex—pushing down on the back of a dog causes muscle movements that compensate for and resist the displacement. 2. Attitudinal reflexes—displacement of one part of the body is followed by postural changes in other parts (e.g., lifting the head of a horse is followed by postural changes in the rear quarters so that a new attitude is assumed). 3. Righting reflex—dropping an inverted cat is followed by its landing in the upright position. 4. Hopping reaction—pushing a supported dog with three limbs elevated results in a placement correction of the intact leg to act as a rigid pillar.  Cerebrospinal Fluid The cerebrospinal fluid is thin and watery; it is derived from blood plasma by a secretion process. Except for a few lymphocytes, the normal cellular elements of the blood are absent. In cases of injury or inflammation of the meninges, the number of cellular elements of blood can increase. The principal function of the cerebrospinal fluid is provision of a watery cushion for the brain and  Cerebrospinal Fluid The “lymphatic” function (see previous section) serves the brain and spinal cord for the return of protein that leaks from the capillaries. When blood volume in the brain increases, the volume of cerebrospinal fluid decreases, thereby keeping the volume of cranial contents constant. Determining the pressure of the cerebrospinal fluid can be helpful (e.g., in  CENTRAL NERVOUS SYSTEM METABOLISM The CNS receives its energy principally from carbohydrates, of which glucose is an important source. Unlike many tissues of the body, which require insulin for facilitated diffusion of glucose across cell membranes, the CNS receives glucose by simple diffusion, and insulin is not required. This is advantageous for the animal when insulin is lacking or in short supply because it enables CNS function to continue when other systems fail. The relatively high rate of metabolism of the CNS compared with that of other tissues can be shown by noting its oxygen consumption. Although the CNS constitutes only 2% of  CENTRAL NERVOUS SYSTEM METABOLISM Blood-Brain Barrier Many substances in the blood do not readily enter the cells of the CNS, a limitation referred to as the blood-brain barrier. The capillaries of the CNS have tight junctions between their endothelial cells rather than slit pores, which limit the diffusion of substances from capillaries. Lipid-soluble substances, however, such as oxygen and carbon dioxide, readily diffuse.  Transport for most substances is provided for by the CNS cells known as astrocytes (a glial cell), which are interposed between the capillaries and CNS cells. Astrocytes are selective regarding the materials they transport —hence, the blood-brain barrier. Some areas of the hypothalamus, as well as other portions of the brain that serve as chemoreceptor areas, lack a blood- brain barrier.  CENTRAL NERVOUS SYSTEM METABOLISM Blood Requirement The CNS must have a continuous supply of blood for normal functioning. Other tissues can be deprived of a blood supply for extended periods and recover to normal function when blood supply resumes. Five to 10 minutes of little or no blood to the brain injures higher brain cells (in the cerebrum) so that no recovery occurs. Respiratory and cardiovascular centers (in the medulla oblongata) are more resistant to hypoxia (deficient oxygen), and revival has occurred after 10 minutes without blood. The tolerance of an adult brain to hypoxia is much lower than the tolerance of a newborn brain.