What is the central nervous system?

The central nervous system (CNS) is made up of the brain and spinal cord.

It is one of 2 parts of the Nervous system.

The other part is the peripheral nervous system, which consists of nerves that connect the brain and spinal cord to the rest of the body.

The central nervous system is the body’s processing centre. The brain controls most of the functions of the body, including awareness, movement, thinking, speech, and the 5 senses of seeing, hearing, feeling, tasting and smelling.

The spinal cord is an extension of the brain. It carries messages to and from the brain via the network of peripheral nerves connected to it.

Nerves also connect the spinal cord to a part of the brain called the brainstem.

What are the parts of the central nervous system?

The nervous system is made up of basic units called neurons. The neurons are arranged in networks that carry electrical or chemical messages to and from the brain.

The tissue of the central nervous system is made up of grey matter and white matter. Grey matter is made up of neurons, cells and blood vessels. White matter is made up of axons, which are long cords that extend from the neurons. They are coated in myelin, which is a fatty insulation.

The brain and spinal cord are protected from damage by a clear liquid called cerebrospinal fluid, 3 layers of membranes called the meninges, and the hard bones of the skull and backbone.

The brain

The brain is made up of different parts. These include the cerebrum, the cerebellum, the thalamus, the hypothalamus and the brainstem.

The cerebrum is the largest part of the brain. It controls intelligence, memory, personality, emotion, speech, and ability to feel and move. It is divided into left and right hemispheres, linked by a band of nerve fibres in the centre of the brain called the corpus callosum.

Each hemisphere is divided into 4 lobes, or sections, which are all connected.

• The frontal lobes control movement, speech and some of the functions of the mind like behavioral, mood, memory and organization.
• The temporal lobes play an important part in memory, hearing, speech and language.
• The parietal lobes play an important part in taste, touch, temperature and pain, and also in the understanding of numbers, awareness of the body and feeling of space.
• The occipital lobes are vital for being able to see clearly.

Deep inside the brain are the thalamus and the hypothalamus. The thalamus moves information to and from the lobes, and controls movements and memory. The hypothalamus controls appetite, thirst and body temperature, and produces hormones that control the release of other hormones in the pituitary gland.

At the base of the brain is the brainstem. It is important for breathing, blood pressure and how the body reacts to danger.

Spinal Cord

The spinal cord connects to the brain via the brain stem and then runs down through the spinal canal, located inside the vertebrae. The spinal cord carries information from various parts of the body to and from the brain.

While it varies from one individual to the next, the spinal cord is about 18 inches in length. At the brainstem, 31 spinal nerves enter into the spinal cord. The nerves of the spinal cord consist of:

• 8 cervical nerves
• 12 thoracic nerves
• 5 lumbar nerves
• 5 sacral nerves
• 1 coccygeal nerve

In the case of some reflex movements, responses are controlled by spinal pathways without involvement from the brain. Examples include the Golgi tendon reflex, the crossed extensor reflex, and the stretch reflex.

Recap

The spinal cord carries information from the brain to the rest of the body and transmits signals from the body to the brain. 

Neurons

Neurons are the building blocks of the central nervous system. Billions of these nerve cells can be found throughout the body and communicate with one another to produce physical responses and actions.

Neurons are the body's information superhighway. up to 100 billions of neurons are connected together.

Most neurons are divided into three basic sections: dendrites, cell body, and axon. These cells also differ in terms of function. The three types of neurons are afferent neurons, efferent neurons, and interneurons. 

Efferent neurons are motor neurons that carry signals from the brain to the peripheral nervous system. Afferent neurons are sensory neurons that bring information from the senses to the brain. Interneurons are association neurons that connect efferent and afferent neurons to the central nervous system.

Recap

Neurons are the cells that make up the central nervous system. They are responsible for communicating information throughout the body.

Protective Structures

Since the CNS is so important, it is protected by a number of structures. First, the entire CNS is enclosed in bone. The brain is protected by the skull. The spinal cord is encased by the vertebrae that make up the spinal column.³ 

The brain and spinal cord are both covered with a protective tissue known as meninges. There are three layers of meninges protecting the brain and spinal cord:

• Dura mater: From the Latin words meaning "hard mother," this is the top layer of the meninges found directly under the bones of the skull and vertebrae. It is composed of dense connective tissue.
• Arachnoid mater: The second layer of the meninges is a spider-like, transparent membrane made up of collagen and elastic fibers.
• Pia mater: From the Latin for "soft mother," this protective layer is the innermost layer of the meninges. It is made of delicate connective tissue that is filled with tiny blood vessels that provide nourishment for the brain.

The entire CNS is also immersed in a substance known as cerebrospinal fluid, which forms a chemical environment that allows nerve fibers to transmit information effectively as well as offering yet another layer of protection from potential damage.

Recap

The CNS is protected by structures including the skull, spinal vertebrae, meninges, and cerebrospinal fluid.

Diseases of the Central Nervous System

Damage or disease to the central nervous system can produce a range of effects.  Some of the conditions that can impact the CNS include:

• Degenerative : Diseases such as Parkinson's disease and other dementias can cause the degeneration of cells in pivotal areas of the brain, affecting functions such as movement and memory.
• Infections: Viruses, bacteria, and fungi can invade the central nervous system, causing symptoms and/or damage.
• Stroke: A blockage of blood flow to the brain prevents oxygen from reaching the tissues of the brain. This results in damage to the affected area and can lead to impairment or death.
• Trauma: Injury to the CNS can cause a number of problems ranging from paralysis to death.
• Tumors: Cancerous and benign tumors can grow in different areas of the CNS. The impact of these tumors depends on their location and size. 

Recap

A variety of diseases and other problems can affect the CNS, including infections, trauma, tumors, and degenerative conditions. Such diseases and damage can lead to impairment and sometimes death.

The brain oversees bodily functions and allows consciousness. This delicate organ can be affected by a wide range of disorders and events such as trauma, stroke and tumours. Symptoms and effects depend on which brain areas are affected.

The brain is connected to the sensory and motor nerves of the body by the spinal cord. The brain and spinal cord are wrapped in membranes (meninges) and nourished by a special liquid called cerebrospinal fluid. The average adult brain weighs around one to two kilograms.

Neurones

The brain consists of specialised cells called neurones, and electrical impulses are sent from one neurone to another. A neurone has three basic parts:

• The cell body (soma)
• A long ‘tail’ (axon)
• A branching of tiny projections (dendrites) that connect to other neurones.

There may be around 100 billion neurones, with each neurone connected to around 10,000 to 100,000 others. This complicated circuitry makes the human brain sophisticated enough to oversee all bodily systems and allow higher functions such as awareness, judgement and thought.

Main components

The brain is a soft, gelatinous organ made up of multiple folds. The main components of the brain include:

• Brain stem
• Cerebellum
• Thalamus
• Cerebrum (including cerebral cortex and two hemispheres)
• Corpus callosum.

Brain stem

The brain stem is the bridge between the brain and the spinal cord. Messages relay from the brain to the motor and sensory nerves of the body and vice versa in a constant ‘conversation’. The three portions of the brain stem include:

• Medulla oblongata - responsible for functions of the autonomic nervous system such as breathing, heart beat and digestion.
• Pons - regulates sleep and wakefulness and breathing.
• Midbrain - oversees hearing and seeing reflexes (startle response) and produces the brain chemical, dopamine, which is important to regulate movement.

Cerebellum

The cerebellum is located behind the brain stem. It is responsible for motor skills such as movement, coordination and balance.

Thalamus

The thalamus sits on top of the brain stem. Sensory information from the body is sorted by the thalamus, which then routes the information to the most appropriate ‘receiving stations’ in the cerebral cortex.

Cerebrum, cerebral cortex and brain hemispheres

The cerebrum is involved in functions of higher consciousness. Most of the brain, around 85 per cent, is taken up by the cerebrum. It is this structure that gives the brain its distinctive shape of two halves - the right and left hemispheres. The cortex is essentially a sheet of neurons about 3mm thick, which forms the ‘grey matter’.

The cerebral cortex is folded in on itself; this provides the characteristic ridges and valleys of the brain. Connections between neurons and with remote nuclei are carried by a myriad of axons, forming the ‘white matter’. The right and left hemispheres are further divided into four lobes. These include:

• Frontal - located behind the forehead. Functions include consciousness, thinking, emotions, language and memory.
• Parietal - located on the top and rear of the head. These lobes process information from the sensory nerves and contribute to voluntary movement.
• Temporal - located over each ear. Functions include memory and processing sounds, face recognition, recognition of complex objects and multi-sensory integration.
• Occipital - located at the back of the head. The main function is interpretation of sensory information from the eyes.

Corpus callosum

The two hemispheres are connected by a thick band of nerve fibres called the corpus callosum. The brain halves are able to communicate with each other via this ‘bridge’.

Other important structures

Other structures of the brain include:

• Hypothalamus - located underneath the thalamus. This structure plays an important role in regulating many bodily processes including temperature, thirst, hunger, sleep and libido. It works hand in hand with another brain structure, the pituitary gland. The pituitary gland is considered to be the ‘master gland’ of the endocrine (hormonal) system.
• Limbic system - made up of various structures including the hippocampus and amygdala. This primitive area of the brain is responsible for basic emotions like anger, happiness and sexual desire.

A range of disorders

The brain can be affected by a wide range of disorders and events. The specific symptoms or losses of functioning depend on which brain areas are affected. Some of the causes of brain damage include:

• Diseases affecting the nervous system - such as Parkinson’s disease, multiple sclerosis or Alzheimer’s disease.
• Tumours - growths usually start in the cells of the meninges, the glial cells of the cerebellum or cerebral hemispheres, or in the pituitary gland.
• Stroke - the rupturing of a blood vessel in the brain or a blockage of blood vessels.
• Congenital defects - including genetic disorders such as Down syndrome, or developmental problems caused by maternal consumption of alcohol or drugs during pregnancy.
• Trauma - such as a blow to the head or a penetrating injury.
• Drugs - including alcohol. Some drugs are neurotoxins (poisonous to nerve cells).
• Lack of oxygen - brain cells can be damaged or killed by a lack of oxygen, such as in the case of near-drowning.

Symptoms of disease

The symptoms depend on which area of the brain is affected but, generally, may include:

• Speech difficulties
• Swallowing difficulties
• Paralysis or weakness
• Numbness
• Tremors
• Loss of balance or coordination
• Loss of certain senses, such as sight or feeling
• Visual problems
• Vertigo
• Personality changes
• Severe mood changes
• Confusion
• Sleeping problems
• Memory loss.

Diagnosis and treatment

Diagnosis methods depend on the underlying cause, but may include general tests, x-rays, computed tomography (CT) scans and magnetic resonance imaging (MRI).

Treatment depends on the cause. In some cases, the degree of damage and long term function loss depends on the speed of first aid. For instance, if a person suspects they are having a stroke and seeks immediate medical help, the bleeding - and its associated destructive effects - may be able to be stopped quickly. Generally, children recover better from brain injury or damage because their developing brains are more flexible than those of adults.