Nerves (yellow) in the arm
The human nervous system. Sky blue is PNS; yellow is CNS.
Cross-section of a nerve
3D Medical Animation still shot of Lumbosacral Plexus
Micrograph demonstrating perineural invasion of prostate cancer. H&E stain.

A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.

- Nerve

The PNS consists of the nerves and ganglia outside the brain and spinal cord.

- Peripheral nervous system
Nerves (yellow) in the arm

6 related topics

Alpha

An axon of a multipolar neuron

Axon

Long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body.

Long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body.

An axon of a multipolar neuron
A typical myelinated axon
A dissected human brain, showing grey matter and white matter
Detail showing microtubules at axon hillock and initial segment.
TEM of a myelinated axon in cross-section.
Cross section of an axon: (1) Axon (2) Nucleus 
(3) Schwann cell (4) Myelin sheath (5) Neurilemma
(A) pyramidal cell, interneuron, and short durationwaveform (Axon), overlay of the three average waveforms;
(B) Average and standard error of peak-trough time for pyramidal cells interneurons, and putative axons;
(C) Scatter plot of signal to noise ratios for individual units againstpeak-trough time for axons, pyramidal cells (PYR) and interneurons (INT).
Axon of nine-day-old mouse with growth cone visible

In certain sensory neurons (pseudounipolar neurons), such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon.

A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals.

Left View of the human brain from below, showing origins of cranial nerves. Right Juxtaposed skull base with foramina in which many nerves exit the skull.

Cranial nerves

Left View of the human brain from below, showing origins of cranial nerves. Right Juxtaposed skull base with foramina in which many nerves exit the skull.
The oculomotor (III), troclear (IV) and abducens (VI) nerves supply the muscle of the eye. Damage will affect the movement of the eye in various ways, shown here.
The facial nerve (VII) supplies the muscles of facial expression. Damage to the nerve causes a lack of muscle tone on the affected side, as can be seen on the right side of the face here.
A damaged glossopharyngeal nerve (IX) may cause the uvula to deviate to the affected side.
The cranial nerves in the horse.
Ventral view of a sheep's brain. The exits of the various cranial nerves are marked with red.

Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs.

The cranial nerves are considered components of the peripheral nervous system (PNS), although on a structural level the olfactory (I), optic (II), and trigeminal (V) nerves are more accurately considered part of the central nervous system (CNS).

The human nervous system

Nervous system

Highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body.

Highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body.

The human nervous system
Diagram showing the major divisions of the vertebrate nervous system.
Horizontal section of the head of an adult female human, showing skin, skull, and brain with gray matter (brown in this image) and underlying white matter
Nervous system of a bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front
Area of the human body surface innervated by each spinal nerve
Earthworm nervous system. Top: side view of the front of the worm. Bottom: nervous system in isolation, viewed from above
Internal anatomy of a spider, showing the nervous system in blue
Major elements in synaptic transmission. An electrochemical wave called an action potential travels along the axon of a neuron. When the wave reaches a synapse, it provokes release of a small amount of neurotransmitter molecules, which bind to chemical receptor molecules in the membrane of the target cell.
Illustration of pain pathway, from René Descartes's Treatise of Man
Simplified schema of basic nervous system function: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs that results in signals sent back to the spinal cord and then out to motor neurons
Layers protecting the brain and spinal cord.

In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS).

The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.

Schematic diagram showing the central nervous system in yellow, peripheral in orange

Central nervous system

Part of the nervous system consisting primarily of the brain and spinal cord.

Part of the nervous system consisting primarily of the brain and spinal cord.

Schematic diagram showing the central nervous system in yellow, peripheral in orange
Dissection of a human brain with labels showing the clear division between white and gray matter.
Diagram of the columns and of the course of the fibers in the spinal cord. Sensory synapses occur in the dorsal spinal cord (above in this image), and motor nerves leave through the ventral (as well as lateral) horns of the spinal cord as seen below in the image.
Different ways in which the CNS can be activated without engaging the cortex, and making us aware of the actions. The above example shows the process in which the pupil dilates during dim light, activating neurons in the spinal cord. The second example shows the constriction of the pupil as a result of the activation of the Eddinger-Westphal nucleus (a cerebral ganglion).
A map over the different structures of the nervous systems in the body, showing the CNS, PNS, autonomic nervous system, and enteric nervous system.
Schematic image showing the locations of a few tracts of the spinal cord.
Reflexes may also occur without engaging more than one neuron of the CNS as in the below example of a short reflex.
Diagram depicting the main subdivisions of the embryonic vertebrate brain, later forming forebrain, midbrain and hindbrain.
Development of the neural tube

Microscopically, there are differences between the neurons and tissue of the CNS and the peripheral nervous system (PNS).

From and to the spinal cord are projections of the peripheral nervous system in the form of spinal nerves (sometimes segmental nerves ).

Autonomic nervous system innervation, showing the parasympathetic (craniosacral) systems in blue.

Parasympathetic nervous system

One of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system.

One of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system.

Autonomic nervous system innervation, showing the parasympathetic (craniosacral) systems in blue.

Nerve fibres of the parasympathetic nervous system arise from the central nervous system.

The parasympathetic nerves are autonomic or visceral branches of the peripheral nervous system (PNS).

A top-down view of skeletal muscle

Skeletal muscle

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton.

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton.

A top-down view of skeletal muscle
3D rendering of a skeletal muscle fiber
Muscle types by fiber arrangement
Types of pennate muscle. A – unipennate; B – bipennate; 
C – multipennate
ATPase staining of a muscle cross section. Type II fibers are dark, due to the alkaline pH of the preparation. In this example, the size of the type II fibers is considerably less than the type I fibers due to denervation atrophy.
Structure of muscle fibre showing a sarcomere under electron microscope with schematic explanation.
Diagram of sarcoplasmic reticulum with terminal cisternae and T-tubules.
Human embryo showing somites labelled as primitive segments.
When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap.
Contraction in more detail
(a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
Exercise-induced signaling pathways in skeletal muscle that determine specialized characteristics of slow- and fast-twitch muscle fibers
Jogging is one form of aerobic exercise.
In muscular dystrophy, the affected tissues become disorganized and the concentration of dystrophin (green) is greatly reduced.
Prisoner of war exhibiting muscle loss as a result of malnutrition.

The efferent leg of the peripheral nervous system is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement.

Nerves move muscles in response to voluntary and autonomic (involuntary) signals from the brain.