The nervous system is the body's network of nerve tissue, organised into the central nervous system—the brain and spinal cord—and the peripheral nervous system, which connects them with the rest of the body. Its pathways carry sensory information inward and motor signals outward, with somatic and autonomic divisions serving different structures.
What makes up the central nervous system?
The central nervous system, or CNS, consists of the brain and spinal cord. It receives information, integrates it and produces outgoing signals. The brain lies within the skull, while the spinal cord lies within the vertebral canal, the passage enclosed by the vertebrae of the spine.
The brain includes the cerebrum, the large region associated with conscious perception and voluntary movement; the cerebellum, which contributes to coordination and balance; and the brainstem, which connects higher brain regions with the spinal cord. The brainstem comprises the midbrain, pons and medulla oblongata. Other central structures include the thalamus, which relays much sensory information, and the hypothalamus, which contributes to autonomic control.
The spinal cord carries pathways between the brain and peripheral structures. It also contains circuits for reflexes: responses organised through neural connections without requiring a conscious decision. A spinal cord segment is a region associated with a particular pair of spinal nerves.
What makes up the peripheral nervous system?
The peripheral nervous system, or PNS, comprises nerves and ganglia outside the brain and spinal cord. A nerve is a bundle of nerve fibres enclosed by connective tissue; a ganglion is a collection of nerve cell bodies outside the CNS. Nerve fibres include axons, the long processes of nerve cells that conduct signals.
There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial nerves are associated with the brain and mainly supply the head and neck, although the vagus nerve extends into the chest and abdomen. The optic nerve is an important anatomical exception to the peripheral grouping: it is structurally a continuation of the CNS.
The spinal nerves are grouped as follows:
- Eight pairs of cervical nerves, associated with the neck region.
- Twelve pairs of thoracic nerves, associated with the chest region.
- Five pairs of lumbar nerves, associated with the lower back region.
- Five pairs of sacral nerves, associated with the sacrum at the back of the pelvis.
- One pair of coccygeal nerves, associated with the lowest spinal region.
Each spinal nerve forms from a posterior root carrying sensory fibres and an anterior root carrying motor fibres. The posterior root has a sensory ganglion containing nerve cell bodies. After the roots unite, the resulting spinal nerve contains both sensory and motor fibres.
What is the difference between afferent and efferent pathways?
For pathways connecting peripheral tissues with the CNS, afferent means carrying signals towards the CNS; efferent means carrying signals away from it. Sensory receptors are specialised cells or nerve endings that respond to changes such as touch, stretch or temperature. Afferent fibres carry information from these receptors towards the spinal cord or brain.
Peripheral efferent fibres carry motor signals to muscles and glands. The terms afferent and efferent therefore describe direction, not whether a nerve belongs to the central or peripheral system. A mixed nerve contains both kinds of fibre.
The somatic nervous system serves the body wall and limbs. Its sensory component carries information from skin, skeletal muscles and joints, including information about body position. Its motor component supplies skeletal muscle, the muscle type that produces most body movements, and participates in both voluntary movements and reflexes.
How is the autonomic nervous system divided?
The autonomic nervous system is the motor system supplying smooth muscle, cardiac muscle and glands. Smooth muscle forms the muscle layers of many hollow organs and blood vessels; cardiac muscle forms the muscular wall of the heart. Autonomic output does not ordinarily depend on conscious control.
Its principal divisions are sympathetic and parasympathetic. Sympathetic outflow arises from thoracic and upper lumbar spinal cord levels. Parasympathetic outflow arises from the brainstem and sacral spinal cord levels.
Most autonomic motor pathways use a chain of two nerve cells. The preganglionic cell has its body in the CNS and sends an axon to an autonomic ganglion. The postganglionic cell has its body in that ganglion and sends an axon to the supplied tissue.
Sympathetic ganglia lie beside the vertebral column or farther forward near major abdominal arteries. Parasympathetic ganglia generally lie close to, or within, their target organs. Sensory fibres from internal organs often travel alongside autonomic motor fibres, but they are afferent pathways rather than sympathetic or parasympathetic motor output.
The enteric nervous system is a network of nerve cells within the digestive tract wall. It organises local activity and receives sympathetic and parasympathetic input.
What is the difference between grey and white matter?
Grey matter contains many nerve cell bodies, their branching receiving processes and connections between nerve cells. White matter consists mainly of axons, many covered by myelin, an insulating sheath that supports rapid signal conduction. The distinction describes the predominant components, not an absolute separation of tissue types.
In the spinal cord, grey matter forms a central, roughly H-shaped region surrounded by white matter. The posterior projections of this region, called posterior horns, receive sensory input. The anterior horns contain the motor nerve cell bodies that supply skeletal muscles.
In the cerebrum and cerebellum, grey matter forms an outer cortex, meaning a surface layer, with white matter beneath it. Collections of grey matter also lie deeper within the brain. In the brainstem, grey matter groups lie among bundles of white matter rather than forming a simple outer layer.
What do nucleus, ganglion, tract and nerve mean?
These terms distinguish collections of nerve cell bodies from bundles of axons, and central locations from peripheral locations. In nervous system anatomy, a nucleus means a collection of nerve cell bodies within the CNS, not the smaller nucleus inside an individual cell. A tract is a bundle of axons within the CNS that follows a defined pathway.
| Term | Location | Main anatomical meaning |
|---|---|---|
| Nucleus | Central nervous system | Collection of nerve cell bodies |
| Ganglion | Peripheral nervous system | Collection of nerve cell bodies |
| Tract | Central nervous system | Bundle of axons following a defined pathway |
| Nerve | Peripheral nervous system | Bundle of nerve fibres enclosed by connective tissue |
A nerve can contain fibres serving several functions, whereas a named tract usually identifies a particular central pathway. These terms describe anatomical organisation; they do not by themselves specify whether signals are sensory or motor.
What are the main spinal nerve plexuses?
After forming, a spinal nerve divides into posterior and anterior rami, meaning branches. Both carry sensory and motor fibres. Posterior rami supply deep back muscles and overlying skin; anterior rami supply much of the remaining trunk and the limbs.
A plexus is a network in which nerve branches mix and redistribute fibres. The main anterior ramus plexuses are:
- Cervical plexus: supplies parts of the neck and contributes the phrenic nerve, which supplies the diaphragm, the main muscle of breathing.
- Brachial plexus: supplies the upper limb. Its branches include the median nerve, which carries fibres from several spinal levels.
- Lumbar plexus: supplies parts of the abdominal wall and the anterior and medial regions of the thigh.
- Sacral plexus: supplies the buttock and much of the lower limb. It gives rise to the sciatic nerve.
Most thoracic anterior rami retain a segmental arrangement rather than forming plexuses. In the adjacent 3D model, rotate and zoom, then select and isolate a structure to examine its position relative to neighbouring structures.
What are dermatomes and myotomes?
A dermatome is an area of skin supplied by sensory fibres associated with one spinal nerve. Neighbouring dermatomes overlap, so their boundaries are not sharp lines. Dermatome maps describe spinal nerve territories, not the territories of named peripheral nerves formed after fibres mix in a plexus.
A myotome, in this context, is the group of skeletal muscles supplied by motor fibres associated with one spinal nerve. Most limb muscles receive fibres from more than one spinal level. Dermatomes and myotomes therefore describe segmental sensory and motor supply, while named peripheral nerves describe the routes those fibres take to reach the tissues.