Anatomy and physiology of the brain and spinal cord

The brain is a spongy organ made up of nerve and supportive tissues. It is located in the head and is protected by the boney covering called the skull. The base or lower part of the brain is connected to the spinal cord. Together, the brain and spinal cord are known as the central nervous system (CNS). The brain and spinal cord contain nerves that send information to and from the brain.

The CNS works with the peripheral nervous system (PNS). The PNS is made up of nerves that branch out from the spinal cord to relay messages from the brain to different parts of the body. Together, the CNS and PNS allow a person to walk, talk, throw a ball, and so on.

Structure and function of the brain

The brain is the body's control centre. It constantly receives and interprets nerve signals from the body and starts responses based on this information. Different parts of the brain control movement, speech, emotions, consciousness and internal body functions, such as heart rate, breathing and body temperature.

The brain has 3 main parts: cerebrum, cerebellum and brain stem.

Cerebrum

The cerebrum is the largest part of the brain. It is in the upper area of the brain. It is divided into 2 parts (halves) called cerebral hemispheres. The 2 hemispheres are connected by a bridge of nerve fibres called the corpus callosum.

The right half of the cerebrum controls the left side of the body. The left half of the cerebrum controls the right side of the body.

The outer surface of the cerebrum is called the cerebral cortex or grey matter. It contains nerve cells that control brain activity. The inner area of the cerebrum contains insulated (myelinated) nerve cells (white matter) that relay information between the brain and spinal cord.

The cerebrum is further divided into 4 sections called lobes. These include the frontal (front), temporal (side), parietal (top) and occipital (back) lobes.

Each lobe has different functions:

• The frontal lobe controls movement, speech, behaviour, memory, emotions and intellectual functioning, such as thought processes, reasoning, problem solving, decision making and planning.
• The parietal lobe controls sensations such as touch, pressure, pain and temperature. It also controls spatial orientation (understanding of size, shape and direction).
• The temporal lobe controls hearing, memory and emotions. The left temporal lobe also controls speech.
• The occipital lobe controls vision.

Cerebellum

The cerebellum is the next largest part of the brain. It is under the cerebrum at the back of the brain. It is divided into 2 parts or hemispheres and has grey and white matter, much like the cerebrum. The cerebellum controls balance, coordination and movement.

Brain stem

The brain stem is a bundle of nerve tissue at the base of the brain. It connects the brain to the spinal cord and sends messages between different parts of the body and the brain. The brain stem controls breathing, heart rate and rhythms, blood pressure, body temperature, hunger, thirst, sleep patterns. Ten pairs of cranial nerves emerge from the brainstem and control facial sensation and movement, eye movement, hearing, swallowing, taste and speech. The brain stem has 3 areas:

• midbrain
• pons
• medulla oblongata

Other important parts of the brain

Cerebrospinal fluid

The cerebrospinal fluid (CSF) is a clear, watery liquid that surrounds, cushions and protects the brain and spinal cord. The CSF also carries nutrients from the blood to the brain and spinal cord, and removes waste products. It circulates through chambers called ventricles and over the surface of the brain and spinal cord.

Meninges

The brain and spinal cord are covered and protected by 3 thin layers of tissue called the meninges:

• dura mater – thickest outer layer
• arachnoid layer – middle, thin membrane
• pia mater – inner, thin membrane that is in direct contact with the surface of the brain

CSF flows in the space between the arachnoid layer and the pia mater. This space is called the subarachnoid space.

The tentorium is a flap made of a fold in the meninges. It separates the cerebrum from the cerebellum.

• The supratentorial area of the brain is above the tentorium. It contains the cerebrum, the 2 lateral ventricles and third ventricle, and glands and structures in the centre of the brain.
• The infratentorial area is located at the back of the brain below the tentorium. It contains the cerebellum and brain stem. This area is also called the posterior fossa.

Corpus callosum

The corpus callosum is a bundle of nerve fibres between the 2 cerebral hemispheres. It bridges the 2 hemispheres together.

Thalamus

The thalamus is a two-lobed structure in the middle of the brain. It acts as a relay station for almost all information that comes and goes between the brain and the rest of the nervous system in the body.

Hypothalamus

The hypothalamus is a small structure in the middle of the brain below the thalamus. It plays a part in controlling body temperature, hormone secretion, blood pressure, emotions, appetite and sleeping habits.

Pituitary gland

The pituitary gland is a small, pea-sized organ in the centre of the brain. It is attached to the hypothalamus. It receives messages from the hypothalamus and releases hormones that control the thyroid gland, the adrenal gland, growth and physical development, and sexual development.

Pineal gland

The pineal gland is an outgrowth from the third ventricle in the middle of the brain. In some mammals, it controls the response to darkness and light. In humans, it may play a role in sexual maturation, but its exact function is not clear.

Ventricles

The ventricles are fluid-filled cavities within the brain. There are 4 ventricles:

• The first 2 ventricles are in the cerebral hemispheres. They are called lateral ventricles.
• The third ventricle is in the centre of the brain, surrounded by the thalamus and hypothalamus.
• The fourth ventricle is at the back of the brain between the brain stem and the cerebellum.

The ventricles are connected to each other by a series of tubes. The fluid in the ventricles is cerebrospinal fluid (CSF). CSF is made by a structure called choroid plexus, which is located in each ventricle. The CSF flows through the ventricles, around the brain in the space between the layers of the meninges (subarachnoid space) and down the spinal cord.

Blood-brain barrier

The blood-brain barrier is a specialized system of blood vessel cell structures and enzymes that protect the brain from chemicals or toxins produced by bacteria. It helps maintain a constant environment for the brain.

The blood-brain barrier is made up of very small blood vessels (capillaries) that are lined with thin, flat endothelial cells. In other parts of the body, endothelial cells have small spaces between them that allow substances to move in and out of the capillaries, and to other cells and tissues. In the brain, the endothelial cells are packed tightly together so substances cannot pass out of the bloodstream into the brain. The enzymes also restrict the types of substances that can be carried from the bloodstream to the brain.

Some substances can pass through the blood-brain barrier, such as very small molecules and molecules that can be dissolved in fat (are lipid soluble).

Cranial nerves

There are 12 pairs of cranial nerves that start in the brain and perform specific functions in the head and neck area. Cranial nerves are identified by name or Roman numeral.

Cranial nerves and their functions

Types of cells in the brain

The brain is made up of 2 types of cells:

• neurons
  • receive and send nerve impulses or signals
  • cannot be replaced or repaired if they are damaged
• glial cells (also called neuroglial cells or neuroglia)
  • support, feed and protect the neurons
  • types of glial cells include:
    • astrocytes
    • oligodendrocytes
    • ependymal cells
    • microglial cells

Neuroectodermal cells are immature cells that have not fully developed into mature brain cells. They are found throughout the brain but do not play a role in the function of the brain.

Structure and function of the spine

The spine is made up of:

• 31 vertebrae – boney sections that house and protect the spinal cord (commonly called the spine); 24 are articulating and 9 are fused
• spinal cord – a column of nerves inside the vertebrae that runs from the brain to the bottom of the spine
• disc – a layer of cartilage between each vertebrae that cushions and protects the vertebrae

The spine is divided into 5 sections:

• cervical – from the base of the skull to the lowest part of the neck; contains 7 articulating vertebrae
• thoracic – from the chest to the mid-back; contains 12 articulating vertebrae
• lumbar – from the mid-back to the hips ; contains 5 articulating vertebrae
• sacral – the base of the spine; the vertebrae in this section are fused and do not flex
• coccyx – the "tail bone" at the end of the spine; the vertebrae in this section are fused and do not flex

Spinal nerves

The spine relays nerve messages between the body and the brain. These nerve messages control body functions like movement, bowel and bladder control and breathing, as well as sensation or feeling. Each pair of spinal nerves receives messages from the body (sensory impulses) and sends messages from the brain to the body (motor impulses). The spinal nerves are numbered from the cervical spine to the sacral spine.

Spinal nerves and their functions