The nervous system is divided into:
- Central (includes brain and spinal cord)
- Peripheral Nervous System, which is again subdivided into:
- – Somatic Nervous Systems
- – Autonomic Nervous System
SOMATIC NERVOUS SYSTEM
The somatic nervous system innervates and controls the motor function of the body. It supplied to skeletal muscles. The nerve fibers are myelinated, there is no peripheral plexus formation. The neurotransmitter released at periphery is Acetyl choline. If we cut the nerve, there occur paralysis and atrophy in supplied muscles.
AUTONOMIC NERVOUS SYSTEM
- The autonomic nervous system (ANS) is not under voluntary control and therefore was so named by Langley (Autos=self, nomos=governing—in Greek).
- The ANS innervates and supplied to all the visceras, i.e. the heart, the smooth muscles, the glands and controls the functions of these organs.
- Like somatic nervous system ANS also consists of:
- – Autonomic afferents: Which carrying visceral pain, cardiovascular and respiratory reflexes to the central connections via dorsal root ganglion of spinal nerve and sensory ganglion of cranial nerve.
- – Autonomic central: The highest centres for autonomic nervous system are present in the hypothalamus, medulla and spinal cord. Hypothalamus coordinates the autonomic activity.
- – The postero-lateral nuclei regulate sympathetic activity while.
- – Anterio-medial nuclei regulate parasympathetic activity.
- – Autonomic efferent: The ANS efferent consists of two major divisions–the sympathetic and the parasympathetic.
- Most of the visceras have both sympathetic and parasympathetic innervations.
- The two divisions, i.e. sympathetic and parasympathetic have opposing effects and normally their effects are in a state of equilibrium.
- The prime function of parasympathetic system is mainly to participates in tissue building reactions, i.e. it control the basal functions of the body like heart rate (72/min) and respiratory rate (14/min). While that of the sympathetic system is to help the person to adjust stressful situation and prepare the body for (FFF) fight, fright and flight reactions (responses).
- Man can still survive without sympathetic system but not without parasympathetic.
Autonomic Innervations
- The autonomic afferents are carried in visceral nerves through nonmyelinated fibres. For example, the parasympathetic afferents are carried by the 9th and 10th cranial nerves.
- The autonomic efferent innervations consists of:
- A myelinated preganglionic fibre which synapses with
- A nonmyelinated postganglionic fibre. The postganglionic fibre in turn forms a junction with the receptors of the organs supplied by it.
- The junction between the pre-and postganglionic fibers is called a ganglion.
- The junction between the postganglionic fibers and the receptors is the neuroeffector junction, i.e. neuromuscular junction.
- The traveling of an impulse along the nerve fiber is known as conduction of action potential, while its passage across a synapse is known as transmission (neurotransmission).
Fig. 2A.1.1: The autonomic nervous system. M = Medullary, C = Cervical, T = Thoracic, L = Lumber, S = Sacral
The autonomic efferent is divided into sympathetic and parasympathetic divisions (Fig. 2A.1.1).
- The parasympathetic efferents are carried through the craniosacral (3, 7, 9 and 10th, S2 – S4) outflow. Its distribution is limited to head, neck and trunk only. The ganglia is located on or close to the organ while the neurotransmitter released is Acetyl choline, which is rapidly hydrolyzed locally.
- The sympathetic efferents extend from 1st thoracic to 2nd or 3rd lumbar segments (T1-L3) of the spinal cord. The sympathetic ganglia are found at 3 sites: paravertebral, prevertebral and terminal. Postganglionic fibers arising from sympathetic ganglia innervate the head, neck and viscera of the thorax and abdomen. The major neurotransmitter is Nor-adrenaline while minor one is Acetyl choline.
- Adrenal medulla is also considered as sympathetic ganglia and differs from other sympathetic ganglia in that the principal catecholamine that is released is adrenaline.
SYNAPSE
It is a junction between two neurons where one neuron conducts the impulse in chemical form to the other, so that the impulse is transmitted from one neuron to the other.
NEUROHUMORAL TRANSMISSION
Neurohumoral transmission means nerves transmit their message across synapse (junction between two neurons) (Fig. 2A.1.2) and neuro-effector junction (junction between nerve and organ) by the release of chemical (humoral) messenger.69
Fig. 2A.1.3: General outlay of autonomic nervous system
N = Nicotinic receptors, M = Muscarinic receptors, α, β = Adrenergic receptors, ACh = Acetylcholine, NE = Nor-epinephrine, D = Dopamine
The steps in neurohumoral transmission are:
- Impulse conduction: Generation and propagation of action potential in axon.
- Transmitter release: Release of specific neurotransmitter from the terminal button on approach of action potential.
- Transmitter action on postjunctional membrane: The released neurotransmitter binds with the postjunctional receptors and produces: EPSP (Excitatory post synaptic potential) or IPSP (Inhibitory pot syneptic potential)
- Postjunctional activity: EPSP generates a propagated postjunctional AP while IPSP stabilizes the postjunctional membrane.
- Termination of transmitter action: The neurotransmitter action get terminated by immediate hydrolysis of transmitter or uptake of transmitter by surrounding tissue or nerve tissue.
Neurotransmitters of the autonomic nervous system (ANS): For the transmission of an impulse across a synapse, a neurohumoral transmitter substance is released into the synaptic cleft. In the ANS:
- The principal transmitters are acetychloine and noradrelaline.
- Preganglionic neurons are cholinergic, ganglionic transmisson occurs via nicotinic ACh receptors (though excitatory muscarinic ACh receptors are also present on postganglionic cells). See Fig. 2A.1.3.
- Postganglionic parasympathetic neurons are cholinergic, acting on muscarinic receptors in target organs.
- Postganglionic sympathetic neurons are mainly noradrenergic, though a few are cholinergic (e.g sweat glands).
- Transmitters other than noradrenaline and acetylcholine (NANC transmitters) are also used extensively in the autonomic nervous system. The main ones are nitric oxide and VIP (parasympathetic), ATP and NPY (sympathetic). Others, such as 5-HT, GABA and dopamine, also pay a role (Fig. 2A.1.4).
- Co-transmission is a general phenomenon.
