Nervous System

 

Divided into three main parts

 

•The central nervous system (The brain and spinal cord)

•The peripheral nervous system (sensory, motor and majority of the cranial nerves)

•The autonomic nervous system (sympathetic and parasympathetic nervous systems)

 

These systems receive information from the environment about what is happening to the body, which they process and respond by sending messages to the nerve cells

 

Nerves are made up of nerve cells or neurons. Neurons are highly specialised cells that conduct impulses throughout the central and peripheral nervous system.

 

 

 

 

 

 

 

                                                 

 

 

Structure of a neuron

 

Cell body (soma)

The cell body contains a nucleus, cytoplasm and other typical organelles e.g. mitochondria, ribosomes, Golgi apparatus etc. It has a number of processes, which vary depending on the type of nerve cell, which is called dendrites.

 

Dendrites

The dendrite is a receptor site and its function is to conduct impulses towards the cell body.

 

Axon

The axon is usually a single long, thin process, that conducts impulses to the effector site e.g. muscle, or a connecting neuron where it can release neurotransmitter chemicals at its terminal points.

 

Myelin sheath

Myelin is produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. They produce myelin which forms a sheath around the axon. Myelin insulates the nerve fibre and speeds up the rate of impulse conduction. 

 

Nodes of Ranvier

These are the spaces between the myelin coating on the neuron's axon. These breaks in the myelin allow the conduction of the impulses down the neuron.

 

 

 

 

The central nervous system

The central nervous system is made up of the brain and spinal cord

 

The spinal cord

The spinal cord is a long cylindrical organ that runs inside the vertebrae. There are thirty-one pairs of nerves branch off the cord, which provide the nerve supply to the whole of the body.

 

The spinal cord is protected by the bony vertebral column, the meninges, and the CSF (cerebrospinal fluid).

 

The spinal cord extends from the medulla oblongata of the brain to the first lumbar vertebra, where beyond this it extends into a bundle of nerve roots called the cauda equina (Latin for horses tail).

 

The spinal cord consists of white matter and grey matter.

 

The white matter contains the different nerve tracts that consist of either sensory or motor nerves that travel vertically up to the brain and back down. There are several different tracts, and each will transmit particular types of impulse e.g. pain, temperature, pressure.

 

The tracts that carry sensory nerves, transmit information from the skin or organs via the spinal cord to the brain and are called ascending tracts.

 

Example:

The spinothalamic tract carries information to the brain about pain and temperature

 

The dorsal white column carries information to the brain about touch, pressure and proprioception (the sensation of movement)

 

The tracts that carry motor nerves transmit commands from the brain to the muscles or organs via the spinal cord and are called descending tracts.

 

Example:

The corticospinal tracts conduct impulses concerned with voluntary movement from the motor cortex of the brain down the spinal cord, which are then conducted to the muscles via the spinal nerves.

 

The extrapyramidal tracts conduct impulses concerned with involuntary movement from the midbrain down the spinal cord, which are then conducted to the muscles via the spinal nerves. These help to modify and coordinate movement and maintain posture.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The brain

The brain is the control centre for the body and is made up of approx. 100 billion nerve cells, called neurons. The brain coordinates the body’s movements and allows it to function efficiently. It controls all the functions of the body e.g. sleeping, feeding, water balance, temperature regulation.

 

The brain receives and interprets impulses from the sense receptors in the body and sends impulses to make your muscles and glands work. It stores information in the memory and deals with emotional and intellectual processes.

 

Structures of the brain and their functions

 

• The brain stem (midbrain, pons and medulla oblongata) - This controls involuntary reflex actions e.g. breathing rate, heart rate and swallowing.

 

• The cerebellum - This controls our sense of balance and lets us make precise and accurate movements. It also maintains our posture.

 

• The diencephalon (includes the thalamus and hypothalamus) - This is the centre part of the brain, that controls homeostatic mechanisms and the autonomic nervous system.

 

• The cerebrum - The left and right cerebral hemispheres form the largest part of the brain. They are divided into four main areas that control movement, interpret sensory impulses and control thought, memory, emotions and personality traits.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                                                                                                                                                  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The peripheral nervous system

 

The peripheral nervous system consists of the sensory and motor nerves that supply the body. 

 

The motor nerves control our muscles by sending messages to either contract or relax.

 

The sensory nerves involve the five senses; sight, hearing, touch, taste and smell. They pick up sensations such as light, sound, heat and pain in receptors, and send the information to the brain via the spinal cord for interpretation.

 

The autonomic nervous system

 

The autonomic nervous system works without us being aware of it. It regulates the functions of our internal organs. It is responsible for many processes that happen all the time, such as maintaining homeostasis and blood pressure.

 

There are two parts:

• The sympathetic nervous system

• The parasympathetic nervous system