Afferent neuron · Lobeworks/17

An afferent neuron is a neuron that carries signals toward the central nervous system, from a sensory receptor to the spinal cord or brainstem, and its opposite, the efferent neuron, carries orders out of the central nervous system to muscles and glands.

Afferent neuron. An afferent neuron is a neuron that carries signals toward the central nervous system, from a sensory receptor to the spinal cord or brainstem, and its opposite, the efferent neuron, carries orders out of the central nervous system to muscles and glands.

A primary afferent is one long cell. Its body sits in a ganglion beside the cord, one branch runs out in a peripheral nerve to a receptor in the skin, a muscle or an organ, and the other enters the cord, where it passes the message on. At the receptor end a stimulus becomes a voltage, and the voltage becomes action potentials whose rate codes the stimulus and travel the whole length of the fibre. The fibres are classed by thickness and myelin, which set their conduction speed: Ia and Ib from muscle and tendon and Aβ from the touch receptors of the skin are thick and fast, Aδ thin and lightly myelinated, C bare and slow.

Afferent arrives, efferent exits. The words come from Latin ad ferre (to carry towards) and ex ferre (to carry out); a motor neuron is the classic efferent, and a peripheral nerve usually carries both in the same bundle.

The pattern of a message is set at the receptor. Whether an afferent reports change or persistence depends on its receptor's adaptation, and the fibre class says nothing about it: the same Aβ fibres serve phasic and tonic receptors.

The cord already edits the message. Before an afferent's signal climbs any further, presynaptic inhibition can turn down what its terminals release.

Afferent is the road in, efferent the road out.

The same nerve carries both, sensation up and commands down, on different fibres.

Questions: Does a peripheral nerve carry signals in or out? Both, on different fibres. Afferent fibres carry sensation in, from receptors in the skin, muscles and organs to the spinal cord; efferent fibres carry commands out, from motor neurons to the muscles and glands. A cut nerve therefore takes away feeling and movement together, which is why the two words, carried towards and carried out, are worth keeping apart. How can someone lose the sense of where the limbs are and still feel pain? Because the two travel on different fibres. Position and movement ride the thick myelinated fibres (Ia, II and Ib from muscles and tendons, Aβ from the skin); pain and temperature ride the thin Aδ and C fibres. A disease that destroys only the large fibres, as a viral illness did to Ian Waterman in 1971, takes position sense and touch and leaves pain and temperature. How can the spinal cord turn down one sensory input without silencing the neuron that receives it? By acting on the terminal instead of the receiving cell. GABAergic interneurons synapse onto the endings of a sensory fibre; opening GABA-A receptors there depolarises the terminal slightly and adds a conductance, so the spike that reaches the release sites is smaller and fewer vesicles are released. The receiving neuron keeps its other inputs at full strength, and descending pathways from the brain decide which terminals are turned down.