Types & Subdivisions of Nervous System PDF

31 Types & Subdivisions and Communication in the nervous tissue ILOs By the end of this lecture, students will be able to 1. Describe the organization of the nervous system in relevance to its function. 2. Correlate the process of chemical transmission to function of synapse...

31 Types & Subdivisions and Communication in the nervous tissue ILOs By the end of this lecture, students will be able to 1. Describe the organization of the nervous system in relevance to its function. 2. Correlate the process of chemical transmission to function of synapses. 3. Explain the role of reflex arc in responding to a stimulus. 4. Compare between the somatic and autonomic reflex action. 5. Relate the reflex action to the body functions. Division of nervous system: (figure 1) Nervous system controls all the activities of the body. It is quicker than the other control system in the body namely, the endocrine system. The anatomical unit of the nervous system is the nerve cell or the neuron. The nervous system is divided into two parts namely: 1. Central nervous system 2. Peripheral nervous system 1) - Central nervous system (CNS): The central nervous system includes brain and spinal cord. It is formed of neurons and the supporting cells (neuroglia). The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS. 2) - Peripheral nervous system (PNS): The peripheral nervous system is formed by the neurons and their processes present in all regions of the body. It consists of all nervous tissue outside the CNS. Neurons are capable of generating and transmitting electrochemical impulses. PNS consists of cranial nerves arising from the brain (Twelve pairs of cranial nerves emerge from the brain) and spinal nerves arising from the spinal cord (thirty-one pairs of spinal nerves emerge from the spinal cord). Each nerve follows a defined path and serves a specific region of the body. The peripheral nervous system relays information to and from the central nervous system. PNS is again divided into two subdivisions: a. Somatic nervous system b. Autonomic nervous system Somatic nervous system: Page 1 of 9 The somatic nervous system includes the nerves supplying the skeletal muscles. Thus, it controls the voluntary movement of the body by acting on skeletal muscles. It is also called the voluntary nervous system. Autonomic nervous system: The autonomic nervous system is concerned with regulation of visceral or vegetative functions, such as heart rate, blood pressure, digestion, temperature regulation, and reproductive function. So it is otherwise called vegetative or involuntary nervous system. The autonomic nervous system consists of two divisions: a. Sympathetic division b. Parasympathetic division In general, the sympathetic division helps support exercise or emergency actions, the “fight or-flight” responses, and the parasympathetic division takes care of “rest-and-digest” activities Figure 1: division of nervous system Comparison of somatic and autonomic nervous system: (table 1) The somatic nervous system includes both sensory and motor neurons. Sensory neurons convey input from receptors for somatic senses (tactile, thermal, pain, and proprioceptive sensations) and from receptors for the special senses (sight, hearing, taste, smell, and equilibrium). All of these sensations normally are consciously perceived. In turn, somatic motor neurons innervate skeletal muscles, the effectors of the somatic nervous system, and produce both reflexive and voluntary Page 2 of 9  movements. When a somatic motor neuron stimulates the muscle, it contracts; the effect always is excitation. If somatic motor neurons cease to stimulate a muscle, the result is a paralyzed muscle. Autonomic Nervous System The main input to the ANS comes from autonomic (visceral) sensory neurons. Mostly, these neurons are associated with sensory receptors located in blood vessels, visceral organs, muscles, and the nervous system that monitor conditions in the internal environment. Comparison of Somatic and Autonomic Motor Neurons The axon of a single, myelinated somatic motor neuron extends from the central nervous system (CNS) all the way to the skeletal muscle fibers in its motor unit. By contrast, most autonomic motor pathways consist of two motor neurons in series, that is, one following the other. The first neuron (preganglionic neuron) has its cell body in the CNS; its myelinated axon extends from the CNS to an autonomic ganglion (a ganglion is a collection of neuronal cell bodies in the PNS.) The cell body of the second neuron (postganglionic neuron) is also in that same autonomic ganglion; its unmyelinated axon extends directly from the ganglion to the effector (smooth muscle, cardiac muscle, or a gland). All somatic motor neurons release only acetylcholine (ACh) as their neurotransmitter, but autonomic motor neurons release either ACh or norepinephrine (NE). Table 1: Comparison of the somatic and autonomic nervous system: Somatic nervous system Autonomic nervous system Sensory input From somatic and special senses Mainly from visceral sensory receptors Control of motor Voluntary control from cerebral cortex, Involuntary control from output with contribution from basal ganglia, hypothalamus, limbic system, cerebellum, brain stem and spinal cord brain stem and spinal cord; limited control from cerebral cortex Motor neuron One neuron pathway: somatic motor Usually two-neuron pathway: pathway neurons extending from CNS synapse preganglionic neuron extending directly with effector from CNS synapse with postganglionic neuron in autonomic ganglia, and postganglionic neuron extending from ganglion synapse with visceral effector. Neurotransmitters All somatic motor neurons release only All preganglionic neurons acetylcholine (ACh) (sympathetic & parasympathetic) Page 3 of 9  release ACh. Most sympathetic postganglionic neurons release noradrenaline (NA). all postganglionic parasympathetic neurons and few sympathetic neurons release ACh Effectors Skeletal muscles Smooth muscles, cardiac muscles and glands Responses Excitation (contraction of skeletal Excitation or inhibition muscles) (contraction or relaxation of muscles, increased or decreased secretions of glands) Classification of nerve fibers: A) According to distribution: Nerve fibers are classified into two types on the basis of distribution: 1. Somatic nerve fibers which supply skeletal muscles of the body 2. Autonomic nerve fibers which supply the various internal organs of the body B) According to function: Functionally nerve fibers are of two types: 1. Motor nerve fibers: The motor nerve fibers carry motor impulses from the central nervous system to different parts of the body. These nerve fibers are also called the efferent nerve fibers. 2. Sensory nerve fibers: The sensory nerve fibers carry sensory impulses from different parts of the body to central nervous system. These nerve fibers are also known as afferent nerve fibers. C) According to structure: Depending upon the structure, the nerve fibers are classified into: 1. Myelinated nerve fibers: which are covered by myelin sheath 2. Non-Myelinated nerve fibers: these fibers do not have myelin sheath D) According to diameter and conduction(table 2) The nerve fibers are classified into 3 major types on the basis of thickness of the nerve fiber and the conduction velocity. The velocity of impulse through the nerve fiber is directly proportional to the thickness of the fibers. The different types of nerve fibers are given in table 2. Except C fibers, all nerve fibers are myelinated. Page 4 of 9  Table 2: types of nerve fibers Type Diameter Velocity of conduction (m/second) A Alpha (type I) 12-24 µ 70-120 Beta (type II) 6-12 µ 30-70 Gamma 5-6 µ 15-30 Delta (type III) 2-5 µ 12-15 B 1-2 µ 3-10 C (type IV) ≤1.5 µ 0.5-2 Neuromuscular and synaptic transmission Impulses are transmitted over chemical or electrical synapses linking one neuron (presynaptic cell) with another neuron, muscle, or gland (postsynaptic cell). At chemical synapses, an impulse in the presynaptic axon causes secretion of a chemical that diffuses across the 30- nm-wide (approximately) synaptic cleft and binds to receptors on the surface of the postsynaptic cell. This triggers events that open or close channels in the membrane of the postsynaptic cell, mediating excitation or inhibition. At electrical synapses, the membranes of the presynaptic and postsynaptic neurons are close together, and gap junctions form low resistance bridges through which ions pass with relative ease from one neuron to the next. General characteristics of chemical synapses 1. An action potential in the presynaptic cell causes depolarization of the presynaptic terminal. 2. As a result of the depolarization, Ca2+ enters the presynaptic terminal, causing release of neurotransmitter into the synaptic cleft. 3. Neurotransmitter diffuses across the synaptic cleft and combines with receptors on the postsynaptic cell membrane, causing a change in its permeability to ions and, consequently, a change in its membrane potential. 4. Inhibitory neurotransmitters hyperpolarize the postsynaptic membrane: excitatory neuro- transmitters depolarize the postsynaptic membrane. Neuromuscular junction (Figure 2) Is the synapse between axons of motor neurons and skeletal muscle. The neurotransmitter released from the presynaptic terminal is Acetylcholine (Ach) , and the postsynaptic membrane contains a nicotinic receptor. 1. Depolarization of the presynaptic terminal and Ca 2+ uptake Action potentials are conducted down the motor neuron. Depolarization of the presynaptic terminal opens Ca2+ channels. When Ca2+ permeability increases, Ca2+ rushes into the presynaptic terminal down its electrochemical gradient. 2. Ca2+ uptake causes release of ACh into the synaptic cleft The synaptic vesicles containing the Ach fuse with the plasma membrane and empty their contents into the cleft by exocytosis. Page 5 of 9 3. Diffusion of ACh to the postsynaptic membrane (muscle end plate) and binding of ACh to Acetylcholine (cholinergic) receptors. The ACh receptor is also a Na+ and K+ ion channel. Binding of Ach to α subunits of the receptor causes a conformational change that opens the central core of the channe land increases its conductance to Na and K.These are examples of ligand-gated channels. 4. End plate potential (EPP) in the postsynaptic membrane Because the channels opened by ACh conduct both Na+ and K+ ions, the postsynaptic membrane potential is depolarized to a value halfway between the Na+ and K+ equilibrium potentials (approximately 0 mV). The produced end plate potential (EPP)is not an action potential, but simply a depolarization of the specialized muscle end plate. 5. Depolarization of adjacent muscle membrane to threshold Once the end plate region is depolarized, local currents cause depolarization and action potentials in the adjacent muscle tissue. Action potentials in the muscle are followed by contraction. Degradation of Ach The EPP is transient because Ach is degraded to acetylCoA and choline by acetylcholinesterase (AChE) on the muscle end plate. One-half of the choline is taken back into the presynaptic ending by Na+-choline cotransport and used to synthesize new ACh. Figure 2: Neuromuscular junction Reflex arc and reflex action The basic unit of integrated reflex activity is the reflex arc. This arc consists of a sense organ (receptor), an afferent neuron, one or more synapses within a central integrating station (center), an efferent neuron, and an effector( Figure 3). Receptor; a specialized structure sensitive to changes inside or outside the body. It converts different forms of energy into nerve impulses. Page 6 of 9  Afferent neuron; it carries nerve impulses from receptor to the CNS. Center; inside the CNS. Efferent neuron; carries the impulses from center to the effector organ. In mammals, the connection between afferent and efferent somatic neurons is generally in the brain or spinal cord.The simplest reflex arc is one with a single synapse between the afferent and efferent neurons. Such arcs are monosynaptic, and reflexes occurring in them are called monosynaptic reflexes. Reflex arcs in which one or more interneuron is interposed between the afferent and efferent neurons are called polysynaptic reflexes. There can be anywhere from two to hundreds of synapses in a polysynaptic reflex arc. Figure 3; Reflex arc Monosynaptic reflexes: The stretch reflex (Figure 4) When a skeletal muscle with an intact nerve supply is stretched, it contracts. This response is called the stretch reflex. The stimulus that initiates the reflex is stretch of the muscle, and the response is contraction of the muscle being stretched. The stretch reflex is the best known and studied monosynaptic reflex and is typified by the knee jerk reflex. Figure 4: Stretch reflex Page 7 of 9 Polysynaptic reflexes: Withdrawal reflex (Figure 5)The withdrawal reflex is a typical polysynaptic reflex that occurs in response to a usually painful stimulation of the skin or subcutaneous tissues and muscle. The response is flexor muscle contraction and inhibition of extensor muscles, so that the body part stimulated is flexed and withdrawn from the stimulus. Figure 5: Withdrawal reflex Autonomic Reflex action: Stretch receptors in the bladder wall initiate a reflex contraction. Fibers in the pelvic nerves are the afferent limb of the voiding reflex, and the parasympathetic fibers to the bladder that constitute the efferent limb also travel in these nerves. The reflex is integrated in the sacral portion of the spinal cord. Peristalsis is a reflex response that is initiated when the gut wall is stretched by the contents of the lumen, and it occurs in all parts of the gastrointestinal tract from the esophagus to the rectum. These are examples of autonomic reflexes where the effector organ is under involuntary control (smooth muscle or a gland) Figure 6: Autonomic reflex action Page 8 of 9 Difference between Somatic and Autonomic reflex arcs Page 9 of 9