Autonomic Nervous System Overview

Autonomic Nervous System

• The autonomic nervous system is a major control system for rapid regulation of body functions.
• It receives information from sensory organs and integrates it to determine the body's response.
• Anatomically divided into:
  • Central Nervous System (CNS):
    • Brain
    • Spinal Cord
  • Peripheral Nervous System (PNS):
    • Peripheral Ganglia
    • Peripheral Nerves

Divisions of the Nervous System

• Central Nervous System (CNS) - relays information from the body.
• Peripheral Nervous System (PNS) - consists of:
  • Somatic Division - carries information to the CNS from senses and from the CNS to skeletal muscles.
  • Autonomic Division- regulates internal environment, carrying information from CNS to organs, blood vessels and glands.
    • Sympathetic- arouses the body.
    • Parasympathetic- calms the body after arousal.

The Neuron

• The neuron is the functional unit of the nervous system.
• Composed of:
  • Cell body- surrounded by cell membrane, contains the nucleus with many cytoplasmic organelles.
  • Dendrites- receive impulses to the cell body.
  • Axon- carries impulses from the cell body, may be myelinated or non-myelinated
• A nerve is composed of many nerve fibers

Synapses

• Synapses are the sites where the axon of one neuron terminates on another neuron (dendrites, soma or axon).
• Synaptic cleft- space between neurons where chemical transmitters are released (no direct communication).
• Function- transmission of impulses (signals) from one nerve cell to another.

Autonomic Ganglia

• A ganglion is a collection of neurons outside the central nervous system.
• Contains mother neurons of postganglionic nerve fibers.
• Function: act as distribution centers
• Preganglionic fibers synapse with 8-9 ganglionic neurons, diffusing autonomic signals.
• Ratio of preganglionic to postganglionic fibers is 1:8 or 1:9

Types of Autonomic Ganglia

• Paravertebral sympathetic chain- located on both sides of the vertebral column.
• Collateral ganglia- Located midway between the spinal cord and viscera, stemming from the abdominal aorta.
• Terminal ganglia - located near or in the visceral organ.
• Adrenal medulla- modified sympathetic ganglion where postganglionic cells release catecholamines (adrenaline and noradrenaline) directly to the bloodstream.

Autonomic Innervation

• Autonomic innervation of both systems consists of myelinated preganglionic fibers that synapse with the cell body of a non-myelinated postganglionic neuron, terminating in a synapse with organ receptors.

Transmission in the ANS

• A. Electrical Conduction - impulse propagation along intact nerve fibers via membrane potential changes from ionic movements (Sodium influx and Potassium outflux), leading to depolarization.
• B. Synaptic Chemical Transmission - communication occurs at synapses where nerve fibers have discontinuities. It involves specific chemical transmitters released from the pre-synaptic site, binding to receptors on the postsynaptic site.

Chemical Transmission at Autonomic Junctions

• Acetylcholine (ACh) and Norepinephrine (NE) are the main chemical transmitters.
• Autonomic nerve fibers are categorized as cholinergic or adrenergic, based on the transmitters they utilize.
• Acetylcholine activates cholinergic receptors.
• Norepinephrine activates adrenergic receptors.

Types of Chemical Transmitters

• Parasympathetic: Acetylcholine
• Sympathetic: Norepinephrine; Epinephrine (adrenaline), only present in adrenal medulla.

General Steps of Chemical Transmission

1. Synthesis of the transmitter.
2. Storage of the transmitter.
3. Release of the transmitter.
4. Binding with postsynaptic receptors.
5. Termination of transmission action.

Cholinergic Synapse Events

• Action potential depolarizes synaptic knob.
• Calcium ions enter.
• Acetylcholine (ACh) is released.
• ACh binds to receptors causing depolarization.
• ACh is broken down by acetylcholinesterase into acetate and choline.
• Choline is reabsorbed.

Acetylcholine (ACh)

• ACh is the transmitter at all autonomic ganglia (sympathetic and parasympathetic).
• All postganglionic parasympathetic fibers.
• Postganglionic sympathetic fibers to sweat glands, some vasodilator fibers, nerve endings supplying adrenal medulla.

Cholinergic Transmission Steps

1. Synthesis: Choline + acetyl CoA -> ACh (by choline acetyltransferase).
2. Storage & Release: Stored in vesicles, released by calcium-dependent exocytosis.
3. Termination: Hydrolyzed into choline and acetate by acetylcholinesterase.

Norepinephrine (NE)

• NE is the primary transmitter at most postganglionic sympathetic nerve endings (except for sweat glands and some vasodilator fibers).

Adrenergic Transmission Steps

1. Synthesis: Tyrosine -> Dopa -> Dopamine -> Norepinephrine (by tyrosine hydroxylase, decarboxylase, dopamine-ß-hydroxylase).
2. Storage & Release: Stored in vesicles, released by calcium-dependent exocytosis.
3. Termination: Reuptake into nerve terminals (uptake 1), extra-neuronal uptake (uptake 2). Enzymatic degradation (by COMT, MAO). Diffusion from synaptic cleft.

Receptors of Transmission

• A. Cholinergic Receptors: - Nicotinic: Found in ganglia and adrenal medulla. - Muscarinic: Found in effector organs innervated by parasympathetic fibers.
• B. Adrenergic Receptors: - Alpha (α1 and α2) and Beta (β1, β2, and β3): Found on effector organs innervated by sympathetic fibers.
• C. Non-Adrenergic Non-Cholinergic (NANC): Diverse neurotransmitters not directly related to only ACh or NE.

Explore the intricacies of the autonomic nervous system, which regulates vital body functions and responses. This quiz covers the divisions of the nervous system, including the central and peripheral systems, as well as the function of neurons. Test your knowledge on how sensory information influences bodily control mechanisms.