Physiology 1, Fall 2008, LPC

Chapter 11 - Efferent Division: Autonomic and Somatic Motor Control

I-Autonomic Division (self-governing, visceral)

Fig 12-1

A. General
        Sympathetic, dominant in stressful situations, fight-or-flight

Parasympathetic, dominant in restful situations
           
Necessary for homeostasis

Fig 12-3. Centers in hypothalamus, pons, and medulla receive sensory input re: temperature, saltwater balance, blood pressure, etc, integrate it, and initiate proper response, sometimes emotions

Most internal organs under antagonistic control

Sympathetic vs parasympathetic
Some responses -> different results in different tissues
Few responses under tonic control (up, down only)

B. Sympathetic & Parasympathetic Pathways

1. Two Neurons-Fig 11-4

preganglionic neuron from CNS to ganglion
Postganglionic neuron from ganglion to target tissue
Interneurons and integration of information in ganglions

Most sympathetic pathways originate in the thoracic and lumbar regions of spinal cord. Ganglia primarily in two chains that run along either side of spinal column. Long nerves from ganglia to target.

Most parasympathetic pathways originate in brain stem or sacral region. Ganglia usually located either on or near target.
Vagus nerve (cranial nerve X) – fig 11-6. Controls most of the internal organs

2. Autonomic Control

Control smooth & cardiac muscle, glands and lymphoid & adipose tissues
Neuroeffector junction – fig 11-8 – postganglionic axon terminal has varicosities that contain neurotransmitter and lie on surface of target. Neurotransmitter is released into interstitial fluid, -> slower, more widespread response than if clusters of receptors in specific sites

C. Neurotransmitter synthesis and receptors

Both sympathetic and parasympathetic preganglionic neurons release acetylcholine onto nicotinic cholinergic receptor on postganglionic cells.

Autonomic neurotransmitters synthesized in axon varicosities, packaged into synaptic vesicles
Action potential -> opens Ca++ channels, Ca++ enters neuron, -> exocytosis. Fig 11-9

Fig 11-7
1. Sympathetic

a. Norepinephrine to adrenergic receptors – most
Two types of receptors – alpha & beta, subtypes
b. Catecholamines – from adrenal medulla – fig 11-10
preganglionic sympathetic neuron from spine to ganglion in adrenal medulla, epinephrine secreted directly into blood by axonless cell bodies

2. Parasympathetic

most postganglionic parasympathetic neurons secrete acetylcholine onto muscarinic cholinergic receptors on the receptor. Response determined by receptor subtype.

D. Summary of Sympathetic & Parasympathetic Branches

Fig 11-11 & Table 11-4, very good

1. Both sympathetic & parasympathetic pathways  - 2 neurons, except adrenal medulla where postganglionic neurons are neuroendocrine organ
2. Preganglionic autonomic neurons -> acetylcholine -> nicotinic receptors. Most sympathetic neurons -> norepinephrine onto adrenergic receptors.

Most parasympathetic neurons -> acetylcholine (Ach) -> muscarinic receptors.

3. Sympathetic pathways originate in thoracic & lumbar regions of spinal cord.

Most sympathetic ganglia close to spinal cord.
Parasympathetic pathways leave CNS at brain stem & sacral region of spinal cord
Parasympathetic ganglia are close to or in target tissue

4. Sympathetic –stress or emergencies.

Parasympathetic – rest and digest activities

II-Somatic Motor Division

Always excitatory.
Single neuron in CNS projects its axon to target, skeletal muscle.

1. Pathway

Cell body in ventral horn of spinal cord or in brain.
Neurons branch close to target. Each branch -> cluster of enlarged axon terminals on surface of skeletal muscle fiber = neuromuscular junction

2. Neuromuscular Junction

Fig 11-12
neuromuscular junction = axon terminal, synaptic cleft, postsynaptic membranes of skeletal muscle fiber
Neuromuscular junction has matrix to hold axon and muscle together.
Schwann cells insulate nerve, secrete factors which maintain neuron health.

Fig 11-13
Motor end plate

ACh vesicles released when voltage-gated Ca++ channels open, -> exocytosis.
ACh binds to nicotinic cholinergic receptors, -> Na+ & K+ movement in skeletal muscle fiber.
Acetylcholinesterase degrades Ach