An adrenergic nerve fibre is a

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

for which the

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...

is either

adrenaline Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands and ...

(epinephrine), noradrenaline or dopamine. These neurotransmitters are released at a location known as the

synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...

, which is a junction point between the

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

of one nerve cell and the

dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...

of another. The neurotransmitters are first released from the axon and then bind to the receptor site on the dendrite. Adrenergic nerve terminals are found in the secondary neurons of the

sympathetic nervous system The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of th ...

, one of two divisions of the

autonomic nervous system The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...

which is responsible for the

fight-or-flight response The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first des ...

. This system increases

heart rate Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...

, slows

digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...

, dilates pupils, and also controls the secretion of apocrine sweat glands in the dermal layer of skin, in addition to other responses.

Composition

The nerve fibre is a thread-like extension of a nerve cell that includes the axon which may or may not be encased in a myelinated sheath. The androgenic nerve fibre when myelinated increases the speed of transmission for an action potential across the length of the cell. The gaps in the sheath along the axon are called the

nodes of ranvier In neuroscience and anatomy, nodes of Ranvier ( ), also known as myelin-sheath gaps, occur along a myelinated axon where the axolemma is exposed to the extracellular space. Nodes of Ranvier are uninsulated and highly enriched in ion channels, al ...

Receptor site

Molecular level

There are several types of adrenergic receptors which are identified by their differing sensitivities to various drugs. Neurons in the central nervous system contain α1- and α2-adrenergic receptors and β1- and β2-adrenergic receptors. All four kinds of receptors are also found in the various organs of the body aside from the brain. They are responsible for the effects of epinephrine and norepinephrine when they act as hormones outside the central nervous system. In the brain, all autoreceptors appear to be of the α2 type. (The drug idazoxan blocks α2 autoreceptors and hence acts as an antagonist.) All adrenergic receptors are metabotropic, coupled to

G proteins G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their act ...

that control the production of second messengers. Adrenergic receptors can produce both excitatory and inhibitory effects. In general, the behavioral effects of the release of norepinephrine are excitatory. In the brain the α1 receptors produce a slow depolarizing (excitatory) effect on the postsynaptic membrane, while α2 receptors produce a slow hyperpolarization (inhibitory) effect. Both types of β receptors increase the responsiveness of the postsynaptic neuron to its excitatory inputs, which presumably related to the role this neurotransmitter plays in vigilance.

In the Body

Adrenergic fibres innervate

smooth muscle Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...

cardiac muscle Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...

, visceral glands, and various

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...

structures and sense organs. Their function is enhancing, compared to the inhibiting action of the cholinergic fibres of the

parasympathetic system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...

. Peripheral adrenergic neurons integrate signals from other nerves of the central nervous system and peripheral sense organs. An adrenergic nerve impulse is triggered when one nerve fires repeatedly or when several nerves fire simultaneously which can cause an additive effect leading to a greater stimulus. Adrenergic neurons, in particular the α2 autoreceptors found in the brain, are also involved in sexual behavior and in the control of appetite.

References

{{DEFAULTSORT:Adrenergic Nerve Fibre Sympathetic nervous system