The nervous system is the network of specialised cells and structures that receives information, processes it and coordinates the body’s responses. Its central division comprises the brain and spinal cord, while its peripheral division connects them with the skin, muscles, joints and internal organs.
What are the main divisions of the nervous system?
The central nervous system processes information within the brain and spinal cord. The peripheral nervous system consists of nerves outside these structures and ganglia, which are collections of nerve cell bodies outside the central nervous system.
Neurons are nerve cells that transmit signals. Their axons are long extensions that carry signals away from the cell body; bundles of axons form nerves in the peripheral nervous system. Grey matter contains many nerve cell bodies, while white matter contains many axons covered by myelin, an insulating sheath.
Functionally, the somatic division carries sensation from the body and controls skeletal muscles, the muscles that move the skeleton. The autonomic division supplies smooth muscle in organ walls, cardiac muscle in the heart and glands. Its sympathetic and parasympathetic divisions have different distributions and often complementary effects on organs.
What are the main parts of the brain?
The brain comprises the cerebrum, brainstem and cerebellum. The cerebrum includes the large paired cerebral hemispheres and the diencephalon. Its cortex, the outer layer of grey matter, is folded into ridges separated by grooves.
The cerebral lobes have broad associations rather than completely separate functions:
- The frontal lobe is associated with voluntary movement, planning, behaviour and speech production.
- The parietal lobe processes bodily sensation and contributes to spatial awareness.
- The temporal lobe is associated with hearing, memory and language comprehension.
- The occipital lobe processes visual information.
- The insula, a lobe buried beneath neighbouring cortex, contributes to taste and awareness of internal bodily sensations.
The diencephalon lies deep between the cerebral hemispheres. It includes the thalamus, which relays much sensory information to the cortex, and the hypothalamus, which participates in autonomic control and regulation of hormone release. Other components include the epithalamus, containing the pineal gland, and the subthalamus, associated with movement control.
The brainstem connects the brain with the spinal cord. Its parts are the midbrain, pons and medulla oblongata. It contains pathways linking different regions and collections of nerve cell bodies involved in cranial nerve functions, breathing and cardiovascular control.
The cerebellum lies behind the brainstem. It contributes to coordination, balance and the adjustment of movements, rather than initiating voluntary movement itself.
How is the spinal cord organised into segments?
The spinal cord lies within the vertebral canal, the passage formed by the vertebrae. In adults, it usually ends around the level of the first or second lumbar vertebra. Its lower end tapers into the conus medullaris, meaning the cone-shaped end of the cord.
A spinal cord segment is the region associated with a pair of spinal nerves. There are 31 pairs of spinal nerves:
- Eight cervical pairs serving the neck and contributing to upper limb supply.
- Twelve thoracic pairs supplying much of the trunk.
- Five lumbar pairs contributing to lower trunk and lower limb supply.
- Five sacral pairs contributing to pelvic and lower limb supply.
- One coccygeal pair supplying a small region near the coccyx.
Each spinal nerve forms from a posterior root carrying sensory signals towards the cord and an anterior root carrying motor signals away from it. The posterior root contains a ganglion holding sensory neuron cell bodies.
The lower nerve roots descend below the end of the cord before leaving the vertebral canal. Together they form the cauda equina, named for its resemblance to a horse’s tail. Cord segments therefore do not all lie opposite the vertebrae with corresponding names.
What are the meninges and cerebrospinal fluid?
The meninges are connective tissue coverings surrounding the brain and spinal cord. From outside to inside, they are the dura mater, arachnoid mater and pia mater. The dura mater is tough, the arachnoid mater bridges over surface grooves, and the pia mater closely follows the nervous tissue.
The subarachnoid space lies between the arachnoid mater and pia mater. It contains cerebrospinal fluid, a clear fluid that surrounds and supports the brain and spinal cord.
Cerebrospinal fluid is produced mainly by choroid plexuses, specialised vascular tissue within the ventricles, the brain’s connected fluid-filled cavities. It passes through the ventricular system into the subarachnoid space. Drainage pathways include arachnoid granulations, projections associated with the arachnoid mater that return fluid to the venous circulation, and routes into lymphatic vessels.
In the adjacent 3D model, you can rotate and isolate the brain or spinal cord, then cut a sagittal section, a plane dividing left and right portions, to examine their spatial relationships.
What are the cranial nerves and their main roles?
The 12 pairs of cranial nerves are numbered with Roman numerals. They carry sensory signals, motor signals or both; some also carry parasympathetic fibres, the signal-carrying extensions of neurons in the parasympathetic division. Most serve the head and neck, but the vagus extends into the chest and abdomen.
| Cranial nerves | Main roles |
|---|---|
| I — Olfactory; II — Optic | I: smell. II: vision. |
| III — Oculomotor; IV — Trochlear | III: most eye movements, raising the upper eyelid and narrowing the pupil. IV: supplies the superior oblique muscle, which helps turn the eye downwards when it is directed towards the nose. |
| V — Trigeminal; VI — Abducens | V: most facial sensation and muscles of chewing. VI: supplies the lateral rectus muscle, which turns the eye away from the nose. |
| VII — Facial; VIII — Vestibulocochlear | VII: facial expression, taste from the anterior two-thirds of the tongue except the vallate papillae, and secretion from the lacrimal gland and several salivary glands. VIII: hearing and balance. |
| IX — Glossopharyngeal; X — Vagus | IX: sensation and taste from the posterior third of the tongue, taste from the vallate papillae, supply to stylopharyngeus, a swallowing muscle, and parotid gland secretion. X: swallowing, voice, internal organ sensation and parasympathetic supply to many chest and abdominal organs. |
| XI — Accessory; XII — Hypoglossal | XI: supplies sternocleidomastoid and trapezius, muscles involved in head turning and shoulder movement. XII: supplies most tongue muscles. |
Which plexus and nerves supply the upper limb?
A nerve plexus is a network in which nerve branches exchange fibres before forming named nerves. The brachial plexus is formed mainly by the anterior branches of spinal nerves C5–T1, meaning the fifth cervical through first thoracic nerves. It passes from the neck towards the armpit and supplies the upper limb.
Its major terminal nerves include:
- The musculocutaneous nerve supplies muscles that bend the elbow and carries sensation from the lateral forearm, the thumb-side surface.
- The axillary nerve supplies the deltoid and teres minor muscles and skin over the outer shoulder.
- The radial nerve supplies muscles that straighten the elbow, wrist and fingers, and carries sensation from parts of the posterior limb and hand.
- The median nerve supplies most forearm muscles that bend the wrist and fingers, several thumb muscles, and skin over much of the thumb-side palm and fingers.
- The ulnar nerve supplies many small hand muscles, some forearm muscles, and skin on the little-finger side of the hand.
Which plexuses and nerves supply the lower limb?
The lumbar and sacral plexuses supply much of the lower limb. The lumbar plexus forms within the posterior abdominal wall, while the sacral plexus lies against the posterior wall of the pelvis.
The femoral nerve arises from the lumbar plexus. It supplies muscles that straighten the knee and contributes to bending the hip. Its sensory branches supply the anterior thigh and the medial leg, the side facing the opposite limb.
The obturator nerve also arises from the lumbar plexus. It mainly supplies thigh muscles that draw the limb towards the body’s midline and may carry sensation from a variable area of the medial thigh.
The sciatic nerve arises from the sacral plexus and passes into the posterior thigh. Its tibial and common fibular components supply muscles in the posterior thigh and, through their branches, the muscles and much of the skin of the leg and foot. Skin over the posterior thigh is supplied principally by the posterior femoral cutaneous nerve. Other sacral plexus branches, including the superior and inferior gluteal nerves, supply muscles around the buttock and hip.