Axonal Conduction

Conduction refers to the passage of an impulse along an axon or muscle fiber; transmission refers to the passage of an impulse across a synaptic or neuroeffector junction. Axonal conduction has its basis in transmembrane ionic gradients and in selectively permeable membrane channels. At rest, the interior of the typical mammalian axon is ~70 mV negative to the exterior. The resting potential is essentially a K+ Nernst potential based on the higher concentration of K+ [~40x] in the axoplasm versus the extracellular fluid and the relatively high permeability of the resting axonal membrane to K+, Na+, and Cl- are present in higher concentrations in the extracellular fluid than in the axoplasm, but the axonal membrane at rest is considerably less permeable to these ions; hence, their contribution to the resting potential is small. These ionic gradients are maintained by the Na+, K+-ATPase.

In response to depolarization to a threshold level, an action potential (AP) is initiated locally in the membrane. The AP consists of two phases. Following a small gating current resulting from

Responses of Effector Organs to Autonomic Nerve Impulses

Organ System

Sympathetic Effect"

Adrenergic Receptor Type6

Parasympathetic Effect"

Cholinergic Receptor Type6

Radial muscle, iris Sphincter muscle, iris Ciliary muscle Lacrimal glands

Heartc

Sinoatrial node Atria

Atrioventricular node His-Purkinje system Ventricle

Blood vessels

(Arteries and arterioles)d

Coronary

Skin and mucosa

Skeletal muscle

Cerebral

Pulmonary

Abdominal viscera

Salivary glands

Renal

(Veins)d

Contraction (mydriasis)++

Relaxation for far vision+ Secretion+

Increase in heart rate++

Increase in contractility and conduction velocity++ Increase in automaticity and conduction velocity++ Increase in automaticity and conduction velocity

Increase in contractility, conduction velocity, automaticity and rate of idioventricular pacemakers+++

Constriction+; dilatione++ Constriction+++ Constriction; dilation^-H-Constriction (slight) Constriction+; dilation Constriction +++; dilation + Constriction+++ Constriction++; dilation++ Constriction; dilation

Contraction (miosis)+++ Contraction for near vision+++ Secretion+++

Decrease in heart rate+++ Decrease in contractility++ and shortened AP duration Decrease in conduction velocity;

AV block+++ Little effect

Slight decrease in contractility

No innervation'1 No innervation' Dilation' (?) No innervation' No innervation' No innervation' Dilation'++ No innervation'

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