An axon of a multipolar neuron
The human nervous system. Sky blue is PNS; yellow is CNS.
A typical myelinated axon
3D Medical Animation still shot of Lumbosacral Plexus
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.

- Axon

However, the remaining ten cranial nerve axons extend beyond the brain and are therefore considered part of the PNS.

- Peripheral nervous system
An axon of a multipolar neuron

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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).

The white matter consists of axons and oligodendrocytes, while the gray matter consists of neurons and unmyelinated fibers.

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.

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.

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.

In cross-section, the peripheral region of the cord contains neuronal white matter tracts containing sensory and motor axons.