Physiology

Efferent Division

  • Efferent neurons carry commands from the central nervous system (CNS) to muscles and glands in the body.

Two Subdivisions

Somatic Motor Neurons

  • Controls skeletal muscles.

  • Mostly voluntary movements.

Autonomic Neurons

  • Controls smooth muscle, cardiac muscle, and many glands.

  • Mostly involuntary functions.

    • Sympathetic Division:

      • Associated with 'Fight or Flight' responses; increases heart rate.

    • Parasympathetic Division:

      • Associated with 'Rest and Digest' activities; decreases heart rate.

Control Mechanisms

Antagonistic Control

  • One autonomic branch is excitatory while the other is inhibitory to achieve the same goal.

Cooperative Control

  • Different branches work on different tissues to coordinate a unified response.

Divergence

  • One preganglionic neuron may synapse with multiple postganglionic neurons (ranging from 4 to 32 in some cases).

Autonomic Pathways

  • Involves two neuron chains:

    • Preganglionic Neuron:

      • First neuron in the chain with cell body located in the CNS.

      • Projects from CNS to an autonomic ganglion outside the CNS.

    • Postganglionic Neuron:

      • Second neuron in the chain with cell body located in autonomic ganglion.

      • Projects from the autonomic ganglion to target tissue.

      • Synapses with target cell.

Sympathetic Nervous System

  • Paravertebral Ganglia: Located on each side of the vertebrae.

  • Prevertebral Ganglia: Located around major branches of the abdominal aorta.

  • Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (ACh) onto nicotinic cholinergic receptors on postganglionic cells.

ANS Preganglionic Neurons

  • Release ACh onto Nicotinic Cholinergic receptors (nAChR) on postganglionic neurons.

Postganglionic Neurons

  • Exceptions to the Rule:

    • Sweat Glands:

      • Instead of norepinephrine (NE), they release ACh onto muscarinic receptors.

    • Sympathetic Neurons:

      • Mostly release NE onto adrenergic receptors on target tissue.

    • Parasympathetic Neurons:

      • Release ACh onto muscarinic cholinergic receptors (mAChR) on target tissue.

Key Receptors

Receptor Type

Neurotransmitter

Location

Function

Nicotinic (nAChR)

ACh

Autonomic ganglia

Stimulates postganglionic neurons for both divisions

Muscarinic (mAChR)

ACh

Target organs

Regulates heart rate, digestion, gland secretion, and sweat glands

Adrenergic

NE

Sympathetic target organs

Increases heart rate, dilates pupils (α + β receptors), constricts blood vessels (α) or relaxes airways (β)

Autonomic Synapses

Neuroeffector Junctions

  • Do not have precise receptor locations; neurotransmitters diffuse into interstitial fluid.

Communication Mechanism

  1. Autonomic varicosities release neurotransmitters over a large surface area of target cells, allowing for less directed control compared to somatic motor neurons.

  2. Single postganglionic neurons can affect a larger area instead of targeting one specific site.

Comparison of Autonomic and Somatic Synapses

Feature

Autonomic Synapses

Somatic Synapses

Target Cell

Smooth muscle, cardiac muscle, glands

Skeletal muscle

Control

Diffuse over a large area

Directed (at specific synaptic cleft)

Control Type

Involuntary, long-lasting

Voluntary, short, immediate

Autonomic Receptors

  • G protein-coupled receptors influence various physiological responses.

Sympathetic and Adrenergic Receptors

Alpha Receptors

  • React strongly to NE; cause muscle contraction or secretion (exocytosis).

Beta Receptors

  • Have different affinities for epinephrine:

    • β1 Receptors: Respond equally to epinephrine and NE.

    • β2 Receptors: More sensitive to epinephrine than NE; cause smooth relaxation or decreased secretion.

    • β3 Receptors: More sensitive to norepinephrine.

Somatic Motor Division

  • Enables voluntary reactions to environmental events.

  • Pathway: One neuron pathway from CNS to target muscle.

Neuromuscular Junction (NMJ)

  • Synapse where a somatic neuron communicates with a skeletal muscle fiber to trigger contractions.

Steps in NMJ Function:

  1. The motor end plate receives signals from the neuron.

  2. The synaptic cleft contains AChE, which breaks down ACh to stop continuous muscle activation.

  3. Nicotinic ACh receptors (nAChR) on motor end plates belong to NM subtype.

  4. Chemically-gated ion channels have two binding sites for ACh; both must be occupied to open the channel.

  5. Opening nAChR allows sodium (Na+) to enter the muscle cell, leading to depolarization and triggering an action potential that causes contraction.

  6. The nAChR at NMJ is always excitatory, leading to muscle contractions when ACh binds.