The nervous system and neurons
The divisions of the nervous system
The nervous system divides into the central nervous system (CNS) and the peripheral nervous system (PNS). It provides the biological basis to an individual’s psychological experiences.
The CNS comprises the brain and spinal cord and is concerned with maintaining life functions and psychological processes.
• There are many different brain areas with differing functions. Some brain areas are seen as primitive and involved in basic functioning to maintain life, while other areas are more complex and involved with higher level functioning such as thinking and decision making.
• The function of the spinal cord is to facilitate the transfer of information to and from the brain to the PNS.
The PNS conveys information to and from the CNS and in essence is the messaging service for the limbs and torso. It sub-divides into 2 divisions: the somatic nervous system (SNS) and the autonomic nervous system (ANS).
• The SNS mainly transmits and receives information from the senses, such as auditory information from the ears. It also directs muscles to react and move.
• The ANS helps to transmit and receive information from bodily organs, such as the stomach, and divides into 2 further sub-systems: the sympathetic nervous system, which generally helps to increase bodily activities, and the parasympathetic nervous system, which generally conserves bodily activities by maintaining or decreasing activity.
Fig 6.1 Divisions of the nervous system, with an indication of the function of each division
The structure and function of sensory, relay and motor neurons
Neurons are cells that transmit nerve impulses around the nervous system, acting as a kind of bodily communication system. There are about 100 billion neurons in the brain and 1 billion in the spinal cord. There are 3 main types of neurons: sensory, relay and motor, each having a different specialised role to play. The structure of all neurons is generally the same, though there are structural differences in size relating to their function — for instance, motor neurons tend to be longer than other neurons. In all neurons the dendrite/receptor cell receives the signal, which then travels through the neuron to the pre-synaptic terminal.
• Sensory neurons inform the brain about a person’s external and internal environment by processing sensory information received by the sensory organs. As sensory neurons only transmit information, they are known as unipolar neurons, while relay and motor neurons are bipolar, as they send and receive information.
• Relay neurons transmit information from one area of the CNS to another. Relay neurons also connect motor and sensory neurons together.
• Motor neurons transmit information from the CNS to help the functioning of bodily organs (including glands — important for the endocrine system) and muscles.
The process of synaptic transmission
Synaptic transmission is the process by which nerve impulses are carried across synapses (small gaps between neurons). The nerve impulses transmitted through neurons are electrical in nature and are transmitted across synapses by chemicals called neurotransmitters. Initially a nerve impulse travels down a neuron and initiates release of neurotransmitters (brain chemicals) at the pre-synaptic terminal. The neurotransmitters are then released into the synaptic fluid within the synapse. The adjoining neuron takes up the neurotransmitters from the synaptic fluid and converts them to an electrical impulse, which travels down the neuron to the next pre-synaptic terminal, and so on.
Not all signals prompt activation in the same way. How this occurs is dependent on the action potential of the post-synaptic neuron and the type of information received.
• Excitatory potentials act like the accelerator pedal in a car, as they make it more likely to cause the post-synaptic neuron to fire. When this occurs it is known as an excitatory synapse.
• Inhibitory potentials act like the brake on a car, as they make it less likely for the neuron to fire, and when this occurs and the signal is stopped at the post-synaptic neuron it is known as an inhibitory synapse.
Fig 6.2 The anatomical differences between neurons
Fig 6.3 A typical synapse between 2 neurons. The nerve impulse travels from the pre-synaptic neuron, across the synaptic cleft, to the post-synaptic neuron