Nervous system

Introduction

Communication is vital to the survival of living organisms. To interact with their environment, multicellular organisms have developed a communication system at the cellular level. Specialised cells (neurons) allow messages to be carried from one cell to another so that communication among all body parts is smooth and efficient.

Neurons

Neurons are cells that make up our nervous tissue. Due to their unique structure, these cells can transmit and receive impulses. Neurons have specialised projections called dendrites and axons. Dendrites bring information to the neuron cell body and axons take information away from the neuron cell body. Incoming information is received by the dendrites and passes through the neuron cell body and through the axon of the neuron. Neurons may have hundreds of dendrites but usually only one axon.

Information is transmitted from one neuron to another through a synapse. A synapse is the functional connection between a nerve cell's axon and target cells, which may be other nerve cells, muscle cells or gland cells. All messages are passed between connected neurons in the form of chemicals called neurotransmitters.

Neurotransmitters attach to the target cells by binding to the cell receptors. A cell receptor is a surface molecular structure that serves as a recognition mechanism for different substances. Nervous impulses between cells are passed in the form of chemicals called neurotransmitters. Messages take the form of electrical signals. A neuron can carry impulses only in only one direction.

Main components of the nervous system

The nervous system has two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central nervous system

The CNS consists of the brain and the spinal cord.

The spinal cord

The spinal cord carries impulses from other parts of the body to the brain where they are interpreted and analysed. The spinal cord consists of grey and white matter. The butterfly-shaped centre of the cord consists of grey matter. The front horns are made of motor nerves that transmit information from the brain or spinal cord to our muscles. These types of impulses stimulate our movements. The back horns are made of sensory nerves that transmit sensory information from other parts of the body through the spinal cord to the brain. The spinal cord is the link between the brain and the other parts of the body.

The central nervous system does not come into contact with the environment. The brain is the main 'switching unit' of the central nervous system. It is the place to which impulses are transmitted and from which impulses originate. See image 2.

The brain

The human brain can divided into the following structures:

The cerebrum

The cerebrum, or cortex, is the largest part of the human brain. It is divided into two hemispheres that in their turn are divided into four lobes: the frontal lobe, parietal lobe, occipital lobe and temporal lobe.

• Frontal lobe - associated with reasoning, planning, parts of speech, movement, emotions, and problem solving
• Parietal lobe - associated with movement, orientation, recognition, perception of stimuli
• Occipital lobe - associated with visual processing
• Temporal lobe - associated with perception and recognition of auditory stimuli, memory, and speech

The cerebellum

The cerebellum, or 'little brain', is similar to the cerebrum in that it has two hemispheres and has a highly folded surface or cortex. This structure is associated with regulation and coordination of movement, posture and balance.

The limbic system

The limbic system is often referred to as the 'emotional brain' because it is related to the control and expression of our emotions. The limbic system is buried within the cerebrum. This part of the brain is responsible for our primitive impulses and maintaining homeostasis. Homeostasis is the balance of mental and physical well- being or, in other words, health. From an evolutionary point of view, this part of the brain is very old. See image 3.

The peripheral nervous system (PNS)

The PNS consists of all the nerves in the body that lie outside the brain and spinal cord. It carries messages from all over the body to the central nervous system and vice versa. Some peripheral neurons collect information from the body and transmit it towards the CNS. These neurons are called afferent neurons. Other peripheral neurons transmit information away from the CNS. These neurons are called efferent neurons.

The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system conducts signals from sensory receptors to the CNS and signals from the CNS to skeletal muscles. The autonomic nervous system regulates involuntary body functions. It controls blood flow, digestion, temperature regulation and heart function.