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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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 left optic nerve and the optic tracts.

Optic nerve

Paired cranial nerve that transmits visual information from the retina to the brain.

Paired cranial nerve that transmits visual information from the retina to the brain.

The left optic nerve and the optic tracts.
A fundus photograph showing the back of the retina. The white circle is the beginning of the optical nerve.
MRI scan of human eye showing optic nerve.
The ophthalmic artery derived from internal carotid artery and its branches. (optic nerve is yellow)
Superficial dissection of brain-stem. Lateral view.
Dissection of brain-stem. Lateral view.
Scheme showing central connections of the optic nerves and optic tracts.
Nerves of the orbit. Seen from above.
Nerves of the orbit, and the ciliary ganglion. Side view.
The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section.
Structures of the eye labeled
This image shows another labeled view of the structures of the eye
Optic nerve.Deep dissection.Inferior view.
Optic nerve.Deep dissection.Inferior view.
Optic nerve
Optic nerve
Human brain dura mater (reflections)
Optic nerve
Optic nerve
Optic nerve
Cerebrum.Inferior view.Deep dissection
Cerebral peduncle, optic chasm, cerebral aqueduct. Inferior view. Deep dissection.

The optic nerve has been classified as the second of twelve paired cranial nerves, but it is technically part of the central nervous system, rather than the peripheral nervous system because it is derived from an out-pouching of the diencephalon (optic stalks) during embryonic development.

1. (Brain) Precentral gyrus: the origin of nerve signals initiating movement.
2. (Cross section of Spinal cord) Corticospinal tract: Mediator of message from brain to skeletal muscles.
3. Axon: the efferent nerve fiber that carries the command to contract muscles.
4. Neuromuscular junction: muscle cells are stimulated to contract at this intersection

Somatic nervous system

1. (Brain) Precentral gyrus: the origin of nerve signals initiating movement.
2. (Cross section of Spinal cord) Corticospinal tract: Mediator of message from brain to skeletal muscles.
3. Axon: the efferent nerve fiber that carries the command to contract muscles.
4. Neuromuscular junction: muscle cells are stimulated to contract at this intersection

The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.

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).

The spinal cord (in yellow) connects the brain to nerves throughout the body.

Spinal cord

Long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column.

Long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column.

The spinal cord (in yellow) connects the brain to nerves throughout the body.
Part of human spinal cord. 1 – central canal; 2 – posterior median sulcus; 3 – gray matter; 4 – white matter; 5 – dorsal root + dorsal root ganglion; 6 – ventral root; 7 – fascicles; 8 – anterior spinal artery; 9 – arachnoid mater; 10 – dura mater
Diagram of the spinal cord showing segments
A model of segments of the human spine and spinal cord, nerve roots can be seen extending laterally from the (not visible) spinal cord.
Spinal cord seen in a midsection of a five-week-old embryo
Spinal cord seen in a midsection of a 3 month old fetus
Spinal cord tracts.
Spinal Cord Sectional Anatomy. Animation in the reference.
Diagrams of the spinal cord.
Cross-section through the spinal cord at the mid-thoracic level.
Cross-sections of the spinal cord at varying levels.
Cervical vertebra
A portion of the spinal cord, showing its right lateral surface. The dura is opened and arranged to show the nerve roots.
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.
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).
Cross-section of rabbit spinal cord.
Cross section of adult rat spinal cord stained using Cajal method.
An overview of the spinal cord.
Sagittal section of pig vertebrae showing a section of the spinal cord.
The base of the brain and the top of the spinal cord
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Cerebrum.Inferior view.Deep dissection
Cerebrum.Inferior view.Deep dissection
Spinal cord. Brachial plexus. Cerebrum.Inferior view.Deep dissection.
Spinal cord. Brachial plexus. Cerebrum.Inferior view.Deep dissection.
Spinal cord
Medulla spinalis of 8-week-old human embryo

The spinal cord is the main pathway for information connecting the brain and peripheral nervous system.

Micrograph of a ganglion. H&E stain.

Ganglion

Micrograph of a ganglion. H&E stain.
A dorsal root ganglion (DRG) from a chicken embryo (around stage of day 7) after incubation overnight in NGF growth medium stained with anti-neurofilament antibody. Note the axons growing out of the ganglion.

A ganglion is a group of neuron cell bodies in the peripheral nervous system.

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.

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).

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).

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.