A sensory neuron may have several terminal branches peripherally that enlarge the receptive area and innervate multiple receptors. As a sensory action potential which originated in one of the terminal branches propagates af-ferently, or orthodromically, it may also enter some other branches of that same axon and then conduct ef-ferently, or antidromically, for short distances. The distal ends of the sensory axons may release neurotransmit-ters in response to the antidromic action potentials. The process of action potential spread can result in a more wide-ranging reaction than that produced by the initial stimulus. If the sensory neuron innervates blood vessels or sweat glands, the response can produce reddening of the skin as a result of vasodilation, local sweating as a result of sweat gland activation, or pain as a result of the action of the released neurotransmitter. This process is called a local axon reflex (see Fig. 6.6). It differs from the usual reflex pathway in that a synapse with an efferent neuron in the spinal cord or peripheral ganglion is not required to produce a response. The neurotransmitter producing this local reflex is likely the same as that released
Local axon reflex
Autonomic effectors: Smooth muscle Cardiac muscle Glands
Mechanoreceptors Chemoreceptors y-Nociceptors
Collateral axon branch
Ventral root collateral branches of the same neuron. The antidromic action potentials may provoke release of the same neurotransmitters, like substance P or glutamate, from the nerve endings as would be released at the synapse in the spinal cord. Local axon reflexes may perpetuate pain, activate sweat glands, or cause vasomotor actions.
Collateral axon branch
Sensory components of autonomic function. Left, Sensory action potentials from mechanical, chemical, and nociceptive receptors that propagate to the spinal cord can trigger ANS reflexes. Right, Local axon reflexes occur when an orthodromic action potential from a sensory nerve ending propagates antidromically into collateral branches of the same neuron. The antidromic action potentials may provoke release of the same neurotransmitters, like substance P or glutamate, from the nerve endings as would be released at the synapse in the spinal cord. Local axon reflexes may perpetuate pain, activate sweat glands, or cause vasomotor actions.
at the synapse in the spinal cord—substance P or glutamate for sensory neurons or ACh and NE at the target tissues for autonomic neurons. Local axon reflexes in noci-ceptive nerve endings that become persistently activated after local trauma can produce dramatic clinical manifestations (see Clinical Focus Box 6.2).
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