The Peripheral Nervous System:Efferent Division

Chapter 7

Objectives

PNS: Efferent Division

• System by which CNS communicates and controls muscles and glands

• Autonomic and somatic systems

Overview of Nervous System

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Autonomic Nervous System

• System of motor neurons that innervates smooth and cardiac muscle and glands

• Involuntary nervous system

• General visceral motor system

Characteristics• Effectors

– Cardiac and smooth muscle– Glands

• Efferent Pathway– Involves two neurons

• Ganglia– Preganglionic neuron > preganglionic axon > ganglionic neuron

> postganglionic axon > effector

• Neurotransmitter– Norepinephrine, acetylcholine– Excitatory, inhibitory– All preganglionic neurons release acetylcholine

Divisions of ANS• Sympathetic

– Fight or flight– Norepinephrine– Cholinergic fibers

• Parasympathetic– Rest and digest– Acetylcholine– Adrenergic fibers

• Dual innervation– Counterbalance– Tonic activity

• Ongoing activity of both divisions

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Parasympathetic Division

• Anatomically simpler

• Craniosacral origin– Cranial nerves III, VII, IX, X

• Oculomotor• Facial• Glossopharyngeal• vagus

– Splanchnic nerves S2 - S4

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Sympathetic Division

• Anatomically more complex– Innervates more organs

• Thoracolumbar origins– T1 – L2

• Sympathetic chain (A)– Paravertebral ganglia

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Paravertebral Ganglia

• Synapse within same chain ganglion

• Ascend or descend then synapse in chain ganglion

• Pass through chain ganglion without synapsing– Collateral ganglia

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Receptor Types

• Initiate tissue specific response

• Cholinergic receptors– Acetylcholine receptors

• Nicotinic– Opens nonspecific ion channels in postganglionic cell– Action potential results– Excitatory

• Muscarinic– Activate secondary messengers via G proteins– Cardiac and smooth muscles, exocrine and few endocrine glands– Excitatory of inhibitory

Receptor Types• Adrenergic receptors– Norepinephrine and epinephrine receptors

• Alpha1 – Most sympathetic tissues– Activates IP3/Ca2+ secondary messenger pathway– Excitatory

• Alpha2 – Digestive organs– Inhibitory (inhibits cAMP)

• Beta1– Heart– Activates cAMP– Excitatory

• Beta2 – Smooth muscle of small vessels– Activates cAMP– inhibitory

Somatic Nervous System

• Motor neurons supply skeletal muscle

• Single neuron with cell bodies in ventral horn of spinal cord

• Release acetylcholine– Only excitatory

Neuromuscular Junction• Junction of motor axon with a muscle cell– Only one junction per muscle cell

• Motor end plate– Specialized area on muscle cell for synapse– High concentration of cholinergic receptors– Open nonspecific ion channels

• Na+ rushes in producing an end-plate potential

• Action potential on muscle cell– EPP opens ion channels in adjacent membrane– Threshold is reached and an action potential propagates

away from the motor end plate

Acetylcholinesterase

• Enzyme that inactivate Ach– Ensures that muscles only contract when needed

• Please look at chemicals and disease associated with the neuromuscular junction