L34 Types & Function of ANS Ganglia PDF

34 Types & Function of ANS ganglia ILOs By the end of this lecture, students will be able to 1. Interpret the functions of the autonomic ganglia in relation to their location 2. Deduce the mechanism of neurotransmitter and receptors in relation to autonomic ganglia’s function...

34 Types & Function of ANS ganglia ILOs By the end of this lecture, students will be able to 1. Interpret the functions of the autonomic ganglia in relation to their location 2. Deduce the mechanism of neurotransmitter and receptors in relation to autonomic ganglia’s function 3. Relate the location of autonomic ganglia to the extent of spread of autonomic. Autonomic ganglion: The autonomic ganglion is an aggregation of cell bodies of neurons outside the CNS. (Figure 1). There are two major groups of autonomic ganglia: (1) sympathetic ganglia, which are components of the sympathetic division of the ANS, and (2) parasympathetic ganglia, which are components of the parasympathetic division of the ANS. Figure 1; autonomic ganglion Organization of the autonomic nervous system 1. Preganglionic neurons have their cell bodies in the CNS and synapse in autonomic ganglia. Preganglionic neurons of the sympathetic nervous system originate in spinal cord segments T1–L3 or the thoracolumbar region. Preganglionic neurons of the parasympathetic nervous system originate in the nuclei of cranial nerves and in spinal cord segments S2–S4 or the craniosacral region. 2. Synapses between neurons are made in the autonomic ganglia. Parasympathetic ganglia are located in or near the effector organs. Sympathetic ganglia are located in the paravertebral chain. 3. Postganglionic neurons of both divisions have their cell bodies in the autonomic ganglia and end on effector organs (e.g., heart, blood vessels, sweat glands). Page 1 of 6  4. Adrenal medulla is a specialized ganglion of the sympathetic nervous system, where the preganglionic fibers synapse directly on chromaffin cells in the adrenal medulla. The chromaffin cells secrete epinephrine (80%) and norepinephrine (20%) into the circulation (Figure 2). Figure 2: Organization of the autonomic nervous system Types of autonomic ganglia: According to their sites; they are classified into: 1. Paravertebral (lateral) ganglia: There are two lateral chain on each side of vertebral column forming two chains known as sympathetic chains (the lateral ganglia are the sites of relay of preganglionic sympathetic fibres to head and neck, thorax and some abdominal viscera) (Figure 3a) 2. Collateral ganglia: Lie between the sympathetic chain and the organ of supply, relay of preganglionic sympathetic fibres to the abdominal viscera and pelvis (Figure 3a) Page 2 of 6 3. Terminal ganglia: They are present near (or on the surface) of the innervated organs. They are the sites of relay of the preganglionic parasympathetic fibres. (Figure 3b) a b Figure 3; (a)Autonomic Nervous System: Sympathetic division showing the lateral ( sympathetic chain) and collateral ganglia (b) Parasympathetic division showing the terminal ganglia Functions of autonomic ganglia: 1. They act as discharging centres; because each preganglionic axon diverge to many postganglionic neuron. In the sympathetic system, the preganglionic fibres relay in the lateral or collateral ganglia and each preganglionic fibre activates many postganglionic neurons. A single sympathetic preganglionic fibre may synapse with 20 or more postganglionic neurons This allows for wide spread distribution of nerve impulses producing generalized sympathetic effects. This pattern of projection is an example of divergence and helps explain why many sympathetic responses affect almost the entire body simultaneously. on the other hand, in the parasympathetic system, the preganglionic fibres relay in ganglia present close to or even embedded in the effector organs (terminal ganglia) and each preganglionic fibre activates only four or five postsynaptic neurons, all of which supply a single visceral effector, allowing parasympathetic responses to be localized to a single effector, thus this distribution produces localized parasympathetic effects. Page 3 of 6 2. Site of relay: Autonomic ganglia are cell stations for relay of preganglionic fibres coming from CNS. 3. Site of Release of Chemical transmitter: Acetylcholine is the mediator liberated at all preganglionic endings (sympathetic and parasympathetic). It is responsible for transmission of nerve impulse from preganglionic to postganglionic neurons (synaptic transmission). Synaptic transmission and receptors in autonomic ganglion: Transmission of nerve impulse at the synaptic junction in the autonomic ganglia between preganglionic and postganglionic neurons, as well as, between postganglionic neurons and the autonomic effectors is chemically mediated i.e. it occurs through release of chemical substance (transmitter) from the stimulated nerve endings. In autonomic ganglia, preganglionic neurons terminals secrete acetylcholine thus are called cholinergic neurons. An action potential in the preganglionic neuron causes depolarization of its terminal. As a result of the depolarization, Ca2+ enters the presynaptic terminal, causing release of ACh into the synaptic cleft. ACh diffuses across the synaptic cleft and combines with ACh receptors on the postganglionic neural membrane, causing a change in its permeability to ions and, consequently, a change in its membrane potential that ends by the creation of an action potential Receptors in autonomic ganglia are cholinergic nicotinic receptors. Small doses of nicotine produce the same effects as stimulation of the central cholinergic fibres, so the action of acetylcholine at these sites is called nicotine like action of acetylcholine and the receptors are called nicotinic receptors. (Nicotine, a natural substance in tobacco leaves, is not a naturally occurring substance in humans and is not normally present in non smokers). Nicotinic receptors Are located in the autonomic ganglia (NN) of the sympathetic and parasympathetic nervous systems, at the neuromuscular junction (NM), and in the adrenal medulla (NN). The receptors at these locations are similar, but not identical. Page 4 of 6  Are activated by ACh or nicotine. Produce excitation. Are blocked by ganglionic blockers (e.g., hexamethonium) in the autonomic ganglia, but not at the neuromuscular junction. (It acts in autonomic ganglia by binding mostly to the nicotinic acetylcholine receptor, and not the acetylcholine binding site itself) Synthesis and fate of acetylcholine (figure 4) Acetylcholine is synthesized in the terminal endings of cholinergic nerve fibres. Choline acetyl Acetyl CoA+ choline ------------------------ ► Acetylcholine. Once acetylcholine is secreted most Transferase of it is hydrolysed into acetate and choline by the enzyme choline esterase present in the tissues supplied by cholinergic nerve fibres. The value of this enzyme is to keep the action of acetylcholine localized at the site of release and continue for a short time, otherwise it may diffuse to the blood causing generalized effect. Figure 4: Synthesis and fate of acetylcholine Recycling of choline: Choline that is formed is in turn transported back into terminal nerve endings where is used again for synthesis of new Ach. Page 5 of 6 Page 6 of 6