The Nervous System

The nervous system is the means by which the body co-ordinates bodily systems and informs the body about any changes in the environment.

The nerves carry brief electro-chemical messages that trigger appropriate responses in the various parts of the body. The messages (impulses) then react and will do certain tasks such as make the muscles contract, the glands secrete and the blood vessels widen or narrow.

The nervous system is a very complex system in the body but is divided up into two main parts. The Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

The CNS

The Central Nervous System consists of the brain and spinal cord. The main function of this part of the system is to get information from the body and send out instructions, and to maintain equilibrium in the body. The CNS receives sensory information from all parts of the body. On receipt of this information, the CNS analyses the information, and thoughts, emotions and memories are then generated and stored. The CNS usually responds to nerve impulses by stimulating muscles or glands, which creates an appropriate response to the original stimulus such as a change in temperature.

The Brain

The brain is the most highly developed part of the nervous system and is protected by the skull. A vast network of arteries supply the brain with blood and twelve pairs of cranial nerves originate from the brain. Most of these nerves supply the sense organs and muscles in the head, but some do extend to other parts of the body. The cranial nerves are not part of the CNS but are part of the peripheral nervous system. The surface layer of the brain is called the cerebral cortex, and is often referred to as the gray matter because of the lack of insulation which gives it the white appearance.

The largest part of the brain is called the cerebrum, which in Latin means “brain” and is divided into two sections called hemispheres, which are joined by a band of nerve fibres. These hemispheres are both responsible for different behaviours such as hearing smell and touch.

Each hemisphere has four lobes, as you can see from the diagram.

  • The front part of the frontal lobe deals with problem solving and personality and the rear part are concerned with movement. There is a strip here called the motor area which moves different muscles.
  • The parietal lobe deals with sensation and perception, with an area called the somatosensory. It is also concerned with speech and processing information.
  • The temporal lobes at the side of the head contain auditory areas which deal with hearing as well as emotional responses. The Wernicke’s area found in the left temporal lobe deals with understanding speech.
  • The occipital lobe is the prominent bone which can be felt at the back of the head. This part deals with receiving visual information.

The rear part of the brain is called the cerebellum and is responsible for monitoring the position of the limbs. Fine movements are controlled by this part of the brain.

The base of the brain extends into the spinal cord.

Meninges

Around the brain and spinal cord is a covering called the meninges. This is made up of three layers, with the outer layer called the dura mater, the middle layer called the arachnoid and the inner layer called the pia mater. The dura mater is thick and tough and can restrict the movement of the brain within the skull. The arachnoid contains projections which transfers cerebrospinal fluid from the ventricles back into the bloodstream. The pia mater contains tissues which adhere to the brain, making it difficult to dissect.

Brainstem

The brainstem is the lower extension of the brain where it connects to the spinal cord. Neurological functions located in the brainstem include those necessary for survival (breathing, digestion, heart rate, blood pressure) and for arousal (being awake and alert).

Most of the cranial nerves come from the brainstem. The brainstem is the pathway for all fibre tracts passing up and down from peripheral nerves and spinal cord to the highest parts of the brain

The Spinal Cord

The spinal cord is a long nerve tract that runs from the base of the brain, down through the vertebral column. It consists of millions of nerve fibres which will allow messages to be transmitted.

The spinal cord allows the brain to communicate to all areas of the body. It does this using 31 pairs of spinal nerves which branch off from the spinal cord and are part of the PNS.

The main function of the spinal cord is to transmit information from sense organs located in the skin and in muscles, to the brain, and to transmit information back from the brain to the muscles and glands. The nervous system, therefore works closely with other systems, such as the Integumentary, the muscular and the endocrine systems.

Nerves exit the spinal cord at the vertebra and are numbered accordingly. For example, a nerve that exits the foramen of the 3rd Thoracic vertebra will be numbered T3.

Neurons

The spinal cord is a thick bundle containing millions of nerve cells called neurons. A neuron is a cell which is very long and is specialised to be able to transmit nerve impulses. Most of this length is made up of the part of the cell called an axon – this is a nerve fibre thinner than a hair.

A neuron is triggered to fire a nerve impulse, which travels along the axon, which then passes from one neuron to another by means of transmitter chemicals. Long axons enable nerve impulses to be transmitted very quickly. Most nerve fibres are encased in a fatty layer called the myelin sheath. This acts like insulation and gives the nerve its white appearance. Unlike an electrical wire, the axon of a neuron can only transmit impulses in one direction. This means that there has to be two types of neurons:

  • Sensory neurons are needed to send information from the sense organ to the brain.
  • Motor neurons are needed to transmit the information from the brain to muscles and glands.

The dendrites receive the nerve impulses, which are then carried away by the axon. The terminal button touches a dendrite of another neuron at a junction called a synapse. Dopamine; a chemical neurotransmitter then allows the nerve impulse to be transmitted across the synapse, which allows the message to be conveyed.

The Peripheral Nervous System

This part of the system is made up of all of the nerves and the wiring. This system sends the messages from the brain to the rest of the body. The 31 pairs of spinal nerves are part of the peripheral nervous system.

There are two types of cells in the peripheral nervous system which carries information to the sensory neuron cells and from the motor neuron cell. Cells of the sensory nervous system send information to the CNS from internal organs or from external stimuli. Much of the peripheral nervous system is concerned with voluntary response, but there are still involuntary responses that are dealt with. This part of the PNS is called the autonomic nervous system as it deals with automatic responses such as smooth and cardiac muscle. The autonomic nervous system comprises of the sympathetic and parasympathetic system. The differences between both of these are the responses that are generated as they work in opposition to each other. For example, the medulla of the adrenal glands is supplied with sympathetic fibres which trigger the release of adrenaline into the blood. The parasympathetic nervous system releases acetylcholine that decreases the heart.

Types of Nerves

Sensory nerves send messages from the muscles to the spinal cord and the brain. Special sensors in the skin and deep inside the body help people identify if an object, for example if it is hot. Sensory nerve damage often results in tingling, numbness, pain, and extreme sensitivity to touch.

Motor nerves enable the brain to stimulate muscle contraction, by sending impulses from the brain and spinal cord to all of the muscles in the body. Damage to the motor nerve can lead to muscle weakness, difficulty walking or moving the arms, cramps and spasms.

Autonomic nerves control involuntary or semi-voluntary functions, such as heart rate. If the autonomic nerves are damaged, then a person’s heart may beat faster or slower, and dizziness may occur. In addition, autonomic nerve damage may result in difficulty swallowing, nausea, vomiting, diarrhoea or constipation, problems with urination, abnormal pupil size, and sexual dysfunction.

Cerebrospinal Fluid

This fluid circulates throughout the CNS and is located between the ventricles of the brain and within the spinal canal. The choroid plexus is the area on the ventricles of the brain where cerebrospinal fluid is produced at the rate of 500 ml/day. It has two important functions. Firstly, it is needed to deliver nutrients to structures of the nervous system and to remove any waste. It also acts as a shock absorber in the case of trauma to the head through an injury or accident.

Pathologies of the Nervous System

DiseaseSigns & SymptomsCause
Bell’s PalsyTemporary paralysis of the muscles on one side of the face.Due to an injury of the facial muscle where it becomes inflamed or compressed
EpilepsyCondition affecting the brain, causing repeated seizures.Not always known
MeningitisAn infection of the meninges, the membrane that surrounds the spinal cord and brain.Bacteria or a virus.
Multiple SclerosisScarring of the myelin sheath that protects and coats the nerves, creating problems with vision, sensation etc.Many theories, such as genetics, environment, autoimmune disease.
NeuritisAn inflammation of a nerve.Trauma or injury
Cerebral PalsyBrain damage causing problems with movement and function.Usually from birth or from trauma.
DepressionA mental or psychological condition which affects mood.No single cause but can be linked to life events.
Alzheimer’s DiseaseA form of dementia with memory loss and action slips.No single factor.
SciaticaPain, tingling and numbness in the leg.When there is damage or pressure on the sciatic nerve.
VertigoSensation that you are moving when you are notUsually caused by problems to do with the balance mechanism in the ear.

The Olfactory System

The Olfactory System is basically the body’s system of smell and it begins at the roof of the mouth and nasal cavity.

Tiny molecules of aroma are inhaled by the nose and are trapped in the nose by hair like nerve endings that pass the aroma on to receptors. These are then carried to the Olfactory Bulb within the limbic region of the brain. As a neural circuit, the olfactory bulb has one source of sensory input (axons from olfactory receptor neurons of the olfactory epithelium), and one output (mitral cell axons).

The olfactory region of each of the two nasal passages in humans is a small area of about 2.5 square centimetres containing in total approximately 50 million primary sensory receptor cells.