Efferent nerve fiber

Efferent nerve fibers refer to axonal projections that exit a particular region; as opposed to afferent projections that arrive at the region. These terms have a slightly different meaning in the context of the peripheral nervous system (PNS) and central nervous system (CNS). The efferent fiber is a long process projecting far from the neuron's body that carries nerve impulses away from the central nervous system toward the peripheral effector organs (mainly muscles and glands). A bundle of these fibers is called a motor nerve or an efferent nerve. The opposite direction of neural activity is afferent conduction,[1][2][3] which carries impulses by way of the afferent nerve fibers of sensory neurons.

In the nervous system there is a "closed loop" system of sensation, decision, and reactions. This process is carried out through the activity of sensory neurons, interneurons, and motor neurons.

In the CNS, afferent and efferent projections can be from the perspective of any given brain region. That is, each brain region has its own unique set of afferent and efferent projections. In the context of a given brain region, afferents are arriving fibers while efferents are exiting fibers.

Efferent nerve fiber
Afferent (PSF)
Details
SystemPeripheral nervous system
Identifiers
Latinneurofibrae efferentes
TAA14.2.00.018
THH2.00.06.1.00016
FMA76571
Anatomical terms of neuroanatomy

Structure

Motor nerve

Motoneuron
Myelinated GS efferent fiber leaving cell body of motor neuron to form a neuromuscular junction

The efferent nerve fibers of motor neurons are involved in muscle control, both skeletal and smooth muscle. The cell body of the motor neuron is connected to a single, long axon and several shorter dendrites projecting out of the cell body itself. This axon then forms a neuromuscular junction with the effectors. The cell body of the motor neuron is satellite-shaped. The motor neuron is present in the grey matter of the spinal cord and medulla oblongata, and forms an electrochemical pathway to the effector organ or muscle. Besides motor nerves, there are efferent sensory nerves that often serve to adjust the sensitivity of the signal relayed by the afferent sensory nerve.

Types

There are three types of efferent fibers: general somatic efferent fibers (GSE), general visceral efferent fibers (GVE) and special visceral efferent fibers (SVE).

Subtypes of general somatic efferent fibers include: alpha motor neurons (α) – these target extrafusal muscle fibers, and gamma motor neurons (γ) that target intrafusal muscle fibers. Beta motor neurons target both types of muscle fiber and there are two types known as static and dynamic.

Etymology and mnemonics

Both afferent and efferent come from French, evolved from Latin (the basis of many terms in medicine and biology) for the terms, respectively, ad ferens (Latin verb ferre: carry), meaning carrying into, and ex ferens, meaning carrying away (ad literally means to, and e = ex means from). Ad and ex give an easy mnemonic device for remembering the relationship between afferent and efferent: afferent connection arrives and an efferent connection exits.[4]

Afferent and efferent are connected to affect and effect through their common Latin roots: Afferent nerves affect the subject, whereas efferent nerves allow the subject to effect change.

References

  1. ^ Mader S. S. (2000): Human biology. McGraw-Hill, New York, ISBN 0-07-290584-0; ISBN 0-07-117940-2.
  2. ^ Hall J. E., Guyton A. C. (2006): Textbook of medical physiology, 11th edition. Elsevier Saunders, St. Louis, Mo, ISBN 0-7216-0240-1.
  3. ^ Warrell D. A., Cox T. M., Firth J. D. (2010): The Oxford Textbook of Medicine Archived 2012-03-21 at the Wayback Machine. (5th ed.). Oxford University Press
  4. ^ MedicalMnemonics.com: 3502 3463 367 115
Anatomical terms of neuroanatomy

This article describes anatomical terminology that is used to describe the central and peripheral nervous systems - including the brain, brainstem, spinal cord, and nerves.

Effector cell

An effector cell is any of various types of cell that actively responds to a stimulus and effects some change (brings it about).

Examples of effector cells include:

The muscle, gland or organ cell capable of responding to a stimulus at the terminal end of an efferent nerve fiber

Plasma cell, an effector B cell in the immune system

Effector T cells, T cells that actively respond to a stimulus

Cytokine-induced killer cells, strongly productive cytotoxic effector cells that are capable of lysing tumor cells

Microglia, a glial effector cell that reconstructs the Central nervous system after a bone marrow transplant

Fibroblast, a cell that is most commonly found within connective tissue

Mast cell, the primary effector cell involved in the development of asthma

Ganglion

A ganglion is a nerve cell cluster or a group of nerve cell bodies located in the autonomic nervous system and sensory system. Ganglia house the cell bodies of afferent nerves (input nerve fibers) and efferent nerves (output/motor nerve fibers), or axons.

A pseudoganglion looks like a ganglion, but only has nerve fibers and has no nerve cell bodies.

Nerve

A nerve is an enclosed, cable-like bundle of nerve fibres called axons, in the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system. Each axon within the nerve is an extension of an individual neuron, along with other supportive cells such as Schwann cells that coat the axons in myelin.

Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium.

In the central nervous system, the analogous structures are known as tracts.

Photic sneeze reflex

The photic sneeze reflex (also backronymed as, Autosomal Compelling Helio-Ophthalmic Outburst (ACHOO) syndrome and colloquially sun sneezing) is a condition that causes sneezing in response to numerous stimuli, such as looking at bright lights or periocular (surrounding the eyeball) injection. The condition affects 18-35% of the population in the United States, but its exact mechanism of action is not well understood.

Reflex arc

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. However, the brain will receive the sensory input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

There are two types: autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). However, autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord.During a somatic reflex, nerve signals travel along the following pathway:

Somatic receptors in the skin, muscles and tendons

Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem

An integrating center, the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse

Efferent nerve fibers carry motor nerve signals from the anterior horn to the muscles

Effector muscle innervated by the efferent nerve fiber carries out the response.A reflex arc, then, is the pathway followed by nerves which (a.) carry sensory information from the receptor to the spinal cord, and then (b) carry the response generated by the spinal cord to effector organ(s) during a reflex action.

Vocal cord paresis

Vocal fold paresis, also known as recurrent laryngeal nerve paralysis or vocal fold paralysis, is the medical term describing an injury to one or both recurrent laryngeal nerves (RLNs), which control all muscles of the larynx except for the cricothyroid muscle. The RLN is important for speaking, breathing and swallowing.The primary larynx-related functions of the mainly efferent nerve fiber RLN, include the transmission of nerve signals to the muscles responsible for regulation of the vocal folds' position and tension to enable vocalization, as well as the transmission of sensory nerve signals from the mucous membrane of the larynx to the brain.

A unilateral injury of the nerve typically results in hoarseness caused by a reduced mobility of one of the vocal folds. It may also cause minor shortages of breath as well as aspiration problems especially concerning liquids. A bilateral injury causes the vocal folds to impair the air flow resulting in breathing problems, stridor and snoring sounds, and fast physical exhaustion. This strongly depends on the median or paramedian position of the paralyzed vocal folds. Hoarseness rarely occurs in bilaterally paralyzed vocal folds.

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