Structure and Function: Brain, Spinal Cord, and Nerves

The brain contains more than 100 billion neurons and weighs about 1.4 kg (3 lb). The brain and spinal cord contain two main types of tissue: grey matter, which originates and processes nerve impulses; and white matter, which transmits them. The largest structure in the brain is the cerebrum, which is divided into two halves or hemispheres. Other structures include the cerebellum, the brain stem, and a central region that includes the thalamus and hypothalamus. The spinal cord is an extension of the brain stem and continues downwards from the base of the skull.

MRI scan of the brain

Protective cerebrospinal fluid is made in four ventricles, or cavities, in the brain. This section of the brain shows the lateral ventricles.

Major lobes of the brain

The brain is divided into several lobes that take their names from the skull bones that cover them.

Brain tissue

The outer layer of the brain, the cerebral cortex, is made of grey matter. Beneath are white matter and islands of grey matter.

Cranial nerves

Twelve pairs of cranial nerves emerge directly from the underside of the brain. Most of these nerves supply the head, face, neck, and shoulders. Certain organs in the chest and abdomen, including the heart, lungs, and much of the digestive system, are supplied by the vagus nerve.

Cranial nerve functions

The cranial nerves contain sensory and/or motor fibres, which control various conscious and unconscious functions.

MRI scan of optic nerves

The optic nerves from the left and the right eyes merge and then diverge in the centre of the brain so that each cerebral hemisphere receives data from both eyes.

Spinal nerves

Thirty-one pairs of spinal nerves emerge from the spinal cord and extend through the protective, bony spinal column. These nerves divide to supply all parts of the trunk and the limbs. Before reaching the limbs, bundles of nerves converge to form braid-like plexuses, called the brachial and lumbar plexuses, which then branch further along.

Role of the spinal nerves

The spinal nerves (shown here from the front) carry sensory nerve impulses towards the spinal cord and brain. They also carry motor nerve impulses from the brain to the rest of the body.

Structure of the spinal cord

The spinal cord has a core of grey matter containing nerve cell bodies, dendrites, and supporting cells. Surrounding the grey matter is white matter containing columns of nerve fibres that carry signals to and from the brain along the length of the spinal cord.

Nerve root pathways

Each spinal nerve has motor and sensory nerve roots. The sensory nerve roots enter the back of the spinal cord to join fibres that lead to the brain. Nerve fibres carrying signals from the brain join motor nerve roots leaving the front of the cord.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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