Peripheral and autonomic nervous system of vertebrates

Central nervous system: The central nervous system consists of the brain and spinal cord. The brain plays a central role in the control of most bodily functions, including awareness, movements, sensations, thoughts, speech, and memory. Some reflex movements can occur via spinal cord pathways without the participation of brain structures. The spinal cord is connected to a section of the brain called the brainstem and runs through the spinal canal.
Peripheral Nervous System: Nerve fibers that exit the brainstem and spinal cord become part of the peripheral nervous system. Cranial nerves exit the brainstem and function as peripheral nervous system mediators of many functions, including eye movements, facial strength and sensation, hearing, and taste.
The autonomic nervous system: The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.
The autonomic nervous system comprises two antagonistic sets of nerves, the sympathetic and parasympathetic nervous systems. The hypothalamus is the key brain site for central control of the autonomic nervous system, and the paraventricular nucleus is the key hypothalamic site for this control.
Divisions of Nervous System:
The vertebrate nervous system has three divisions:
(i) A central nervous system comprising the brain and spinal cord. Its function is to receive the stimulus from the receptors and transmit its response to the effectors. Thus, it coordinates all the functions of the body.
(ii) A peripheral nervous system consisting of cranial and spinal nerves arising from the brain and spinal cord respectively. It forms a connecting link between the receptors, central nervous system (CNS) and effectors.
(iii) An autonomic nervous system made of two ganglionated sympathetic nerves, ganglia in the head and viscera, and their connecting nerves. The autonomic nervous system is often regarded as a part of the peripheral nervous system because the two are connected. But all the three divisions of the nervous system are connected intimately both structurally and functionally.

1. Comparative Account of Peripheral and Autonomous nervous system in Vertebrates: Dr. P.B.Reddy M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM PG DEPARTMENT OF ZOOLOGY GOVERTNAMENT PG COLLEGE, RATLAM.M.P [email protected]

2. Central nervous system : The central nervous system consists of the brain and spinal cord. The brain plays a central role in the control of most bodily functions, including awareness, movements, sensations, thoughts, speech, and memory. Some reflex movements can occur via spinal cord pathways without the participation of brain structures. The spinal cord is connected to a section of the brain called the brainstem and runs through the spinal canal. Peripheral Nervous System: Nerve fibers that exit the brainstem and spinal cord become part of the peripheral nervous system. Cranial nerves exit the brainstem and function as peripheral nervous system mediators of many functions, including eye movements, facial strength and sensation, hearing, and taste. The autonomic nervous system: The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response. The autonomic nervous system comprises two antagonistic sets of nerves, the sympathetic and parasympathetic nervous systems. The hypothalamus is the key brain site for central control of the autonomic nervous system , and the paraventricular nucleus is the key hypothalamic site for this control.

3. Divisions of Nervous System: The vertebrate nervous system has three divisions: (i) A central nervous system comprising the brain and spinal cord. Its function is to receive the stimulus from the receptors and transmit its response to the effectors. Thus, it coordinates all the functions of the body. (ii) A peripheral nervous system consisting of cranial and spinal nerves arising from the brain and spinal cord respectively. It forms a connecting link between the receptors, central nervous system (CNS) and effectors. (iii) An autonomic nervous system made of two ganglionated sympathetic nerves, ganglia in the head and viscera, and their connecting nerves. The autonomic nervous system is often regarded as a part of the peripheral nervous system because the two are connected. But all the three divisions of the nervous system are connected intimately both structurally and functionally.

4. KEY POINTS (1) The general anatomical plan of the peripheral and ANS is noticeably similar in all vertebrate groups (except in cyclostomes). (2) The dominance of the brain in autonomic regulation was already present in the early evolution of vertebrates and this dominance of neural control by the brain increases with evolution. (3) Probably, this dominance of the control of autonomic functions by the brain is paralleled by an increasing complexity and functional differentiation of autonomic regulations, related particularly to the adaptation of the vertebrates to terrestrial life. (4) The increased complexity of regulation may be reflected, for example, in the cardiovascular regulation in the field of gravity, in the regulation of body core temperature, in the regulation of fluid homeostasis (volume regulation and osmoregulation), in the regulation of metabolisms (in relation to the terrestrial changes of climate), and in the regulation of body defense. (5) The molecular tools used by the peripheral autonomic neurons and their target cells probably are very similar or identical in all vertebrate groups (transmitters and their receptors, neuropeptides, intracellular pathways).

5. Nerve: A nerve is formed of a number of fibres (axons or dendrites) outside the central nervous system. These are bound together in a smaller bundle by white connective sheaths, called perineurium. A number of such smaller bundles of nerve fibres are externally surrounded by another coat of fibrous connective tissue, called epineurium, which has blood vessels. Functional Divisions of the Nervous System: The body functions of the nervous system are somatic or visceral. Somatic functions are carried out by the voluntary muscles, skeleton, and the skin and its derivatives. Visceral functions are performed by the digestive, circulatory, excretory, endocrine, and the urinogenital system. Both somatic and visceral functions are divided into two components each- (i) afferent or sensory, and (ii) efferent or motor.

6. The nervous system has four functional divisions: 1. Somatic Sensory: The somatic sensory fibres carry impulses from the skin, sense organs, voluntary muscles, and joints to the central nervous system. 2. Somatic Motor: The somatic motor dorsal root fibres carry impulses from the central nervous system to the voluntary muscles. 3. Visceral Sensory: The visceral sensory component carries sensations from the viscera and organs of taste and smell to the central nervous system. 4. Visceral Motor: The visceral motor component carries impulses from the central nervous system to the involuntary muscles, branchial muscles, and glands.

7. Association or Adjustor Neurons: These are located in the brain and spinal cord. They link the afferent and efferent fibres through synapses. The dorsal half of the central nervous system is sensory and the ventral half is motor, this is especially marked in the medulla oblongata and the spinal cord where the four functional components are arranged from the dorsal to the ventral side in the order, somatic sensory, visceral sensory, visceral motor, and somatic motor. Typically each peripheral nerve is also made of all four, functional components. This condition is typical of amniotes, but in anamniotes the visceral motor component has its fibres passing through both dorsal and ventral roots.

8. Peripheral Nervous System: The peripheral nervous system consists of nerves connected to or arising from the central nervous system, it has cranial and spinal nerves. All the nerves arise in pairs. Cranial nerves originate from brain and emerge through foramina of the skull. Except the first four pairs of cranial nerves, the rest arise from the medulla oblongata. There are ten pairs of cranial nerves in anamniota (cyclostomes, fishes and amphibians), and twelve pairs in amniota (reptiles, birds and mammals). There is a paired nervus terminalis or number zero nerve arising from the cerebral hemisphere in all vertebrates except birds, it goes to the organ of Jacobson. The cranial nerves do not show a clear metameric arrangement, yet they represent a regular series of segmental dorsal and ventral roots of the segments of the head, but the dorsal and ventral roots do not join.

9. Spinal Nerves: The spinal nerves arise from the spinal cord. They are also paired segmental structures emerging through inter vertebral foramina of the vertebral column. Their number corresponds approximately to the number of vertebrae present. Each spinal nerve is formed by two roots: (i) A dorsal root (sensory) attached to the dorsal horn of the gray matter. The dorsal root has always a ganglion containing nerve cell bodies of sensory fibres. (ii) A ventral root (motor) arising from the ventral horn of the gray matter. The nerve cell bodies of motor fibres are always located in the brain or spinal cord. The roots have different embryonic origin- the dorsal roots arise from neural crests, while the ventral roots arise from the gray matter of the spinal cord. In anamniota, the dorsal roots contain somatic sensory, visceral sensory and visceral motor fibres. In amniota the dorsal roots have only somatic sensory and visceral sensory fibres. The ventral roots have visceral motor and somatic motor fibres in all. Except in some cyclostomes, a dorsal and a ventral root of each side unite to form a spinal nerve. In cyclostomes, the sensory and motor roots do not join together to form a common trunk. In lampreys, they remain separate and emerge alternately from the spinal cord, whereas in hagfishes there may be an incomplete union.

10. In all other fishes the dorsal sensory root and ventral motor root unite with each other outside the vertebral column to form a common trunk. In amphibians dorsal and ventral roots of the spinal nerves unite in their passage through the inter vertebral foramen rather than outside as in most fishes or within the neural canal as in amniotes (reptiles, birds and mammals). In mammals, complicated plexuses, resulting from the intermixing of fibres from the ventral branches of spinal nerves are found. These are differentiated into cervical, brachial, lumbar and sacral plexuses. Each spinal nerve divides into three branches or rami, a dorsal ramus supplying the skin and muscles of the back, a ventral ramus going to body muscles and skin of ventral side, and a communicating ramus communicants or visceral ramus going to the viscera and a ganglion of the autonomic nervous system. A ramus communicans has two parts, a white ramus and a gray ramus. There are no gray rami in elasmobranchs. The white ramus has medullated visceral sensory and visceral motor fibres. The axons of visceral motor fibres go to autonomic ganglia and form synapses with non- medullated fibres of the gray ramus.

11. The medullated visceral motor fibres are called preganglionic fibres because they terminate in autonomic ganglia where they form synapses with non-medullated postganglionic fibres of the gray ramus. These fibres enter the spinal nerves and pass into the dorsal ventral rami to supply structures under involuntary control. In brief the white ramus of ramus communicans carries medullated visceral sensory fibres and medullated preganglionic motor fibres, the gray ramus carries only non-medullated postganglionic autonomic motor fibres .

12. Autonomic Nervous System: The autonomic nervous system has two main divisions: Sympathetic and Parasympathetic . The autonomic nervous system is so called because it is partly independent and not under voluntary control. It is also connected to spinal nerves and some prevertebral ganglion ramus, cranial nerves. Cranial and spinal somatic nerves mainly innervate the voluntary (skeletal) muscles of the animal. While the autonomic nerves and ganglia innervate the smooth or involuntary muscles of the viscera (alimentary canal, heart, etc.) and glands, and, thus, control the internal environment of the body. Little is known about the autonomic nervous system in cyclostomes although sympathetic ganglia are present. The digestive system and heart are supplied by the vagus nerve.

13. The sympathetic nervous system prepares the body for the “fight or flight” response during any potential danger. On the other hand, the parasympathetic nervous system inhibits the body from overworking and restores the body to a calm and composed state. 

14. In elasmobranchs, an irregular series of sympathetic ganglia lies along the body wall. Fibres from these ganglia connect both to the spinal cord and to the smooth muscles of the digestive and circulatory systems. In bony fishes autonomic nervous system is more advanced i.e., the sympathetic ganglia are arranged in a chain extending forward to the trigeminal nerve. In some groups the sympathetic chains may be reduced (dipnoans and caecilians), and have yet to be properly described in snakes.