Peripheral nervous system

The human nervous system. Sky blue is PNS; yellow is CNS.
3D Medical Animation still shot of Lumbosacral Plexus

One of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS).

- Peripheral nervous system
The human nervous system. Sky blue is PNS; yellow is CNS.

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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).

Nerves (yellow) in the arm

Nerve

Nerves (yellow) in the arm
Cross-section of a nerve
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.

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.

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).

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

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

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

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.

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).

Autonomic nervous system innervation.

Autonomic nervous system

Autonomic nervous system innervation.
Autonomic nervous system, showing splanchnic nerves in middle, and the vagus nerve as "X" in blue. The heart and organs below in list to right are regarded as viscera.
Function of the autonomic nervous system
A flow diagram showing the process of stimulation of adrenal medulla that makes it release adrenaline, that further acts on adrenoreceptors, indirectly mediating or mimicking sympathetic activity.

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

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.

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.

Acetylcholine

Organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter.

Organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter.

Acetylcholine pathway.
Acetylcholine processing in a synapse. After release acetylcholine is broken down by the enzyme acetylcholinesterase.
Muscles contract when they receive signals from motor neurons. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows: (1) The action potential reaches the axon terminal. (2) Calcium ions flow into the axon terminal. (3) Acetylcholine is released into the synaptic cleft. (4) Acetylcholine binds to postsynaptic receptors. (5) This binding causes ion channels to open and allows sodium ions to flow into the muscle cell. (6) The flow of sodium ions across the membrane into the muscle cell generates an action potential which induces muscle contraction. Labels: A: Motor neuron axon B: Axon terminal C: Synaptic cleft D: Muscle cell E: Part of a Myofibril
Components and connections of the parasympathetic nervous system.
Micrograph of the nucleus basalis (of Meynert), which produces acetylcholine in the CNS. LFB-HE stain.

Acetylcholine functions in both the central nervous system (CNS) and the peripheral nervous system (PNS).

The formation of the spinal nerve from the posterior and anterior roots

Spinal nerve

Mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.

Mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.

The formation of the spinal nerve from the posterior and anterior roots
Spinal nerve
Typical spinal nerve location
Scheme showing structure of a typical spinal nerve
1. Somatic efferent.
2. Somatic afferent.
3,4,5. Sympathetic efferent.
6,7. Autonomic afferent.
Cervical nerves
Lumbar plexus and branches
Plan of sacral and pudendal plexuses
Areas of distribution of the cutaneous branches of the posterior divisions of the spinal nerves. The areas of the medial branches are in black, those of the lateral in red
A portion of the spinal cord, showing its right lateral surface. The dura is opened and arranged to show the nerve roots.
Distribution of the cutaneous nerves. Ventral aspect.
Distribution of the cutaneous nerves. Dorsal aspect.
The spinal cord with dura cut open, showing the exits of the spinal nerves.
The spinal cord showing how the anterior and posterior roots join in the spinal nerves.
A longer view of the spinal cord.
Projections of the spinal cord into the nerves (red motor, blue sensory).
Projections of the spinal cord into the nerves (red motor, blue sensory).
Schematic diagram of cervical plexus.
Cerebrum. Inferior view. Deep dissection.
Cerebrum. Inferior view. Deep dissection.
Spinal nerves. Spinal cord and vertebral canal. Deep dissection.

The spinal nerves are part of the peripheral nervous system.