Biological psychology

The nervous system has two structural divisions. The central nervous system (CNS) is the brain and the spinal cord. It receives information, integrates it and initiates responses. The peripheral nervous system (PNS) is every part of the nervous system outside the brain and spinal cord; its nerves carry information between the CNS and the rest of the body. The PNS has two divisions.

The somatic nervous system has a sensory role, carrying information from receptors in the skin, muscles and joints to the CNS, and a motor role, carrying messages from the CNS to skeletal muscles so that voluntary movement can occur. The autonomic nervous system regulates internal organs and glands, largely without conscious control.

The autonomic nervous system has two branches that usually work in opposition. The sympathetic branch prepares the body for action when a threat or stressor is perceived: heart rate and breathing rate rise, pupils dilate, stored glucose is released for energy and digestion slows. The parasympathetic branch maintains the body in its resting state and returns it to that state once the threat has passed: heart rate falls, digestion resumes and energy is conserved.

Many organs, such as the heart, receive input from both branches, and the balance between them determines the body's overall state.

The basic unit of the nervous system is the neuron. Dendrites are branched fibres that receive messages from other neurons. The soma (cell body) contains the nucleus and combines the incoming signals. The axon is a long fibre that carries the impulse away from the soma. Many axons are wrapped in a fatty myelin sheath, which insulates the axon and speeds up transmission. At the end of the axon, the axon terminals store and release neurotransmitter.

Sensory neurons carry information from receptors towards the CNS; motor neurons carry instructions from the CNS to muscles and glands; interneurons, found within the CNS (for example in the spinal cord), connect sensory and motor neurons and process information between them.

Neural transmission travels in one direction: dendrites → soma → axon → axon terminals. Within a neuron the message is electrical (an impulse called an action potential moves along the axon). Between neurons it is chemical: when the impulse reaches the axon terminals, neurotransmitter molecules are released into the synapse, the tiny gap between the terminal of one neuron and the dendrite of the next. The neurotransmitter binds to matching receptor sites on the receiving neuron.

An excitatory neurotransmitter makes the receiving neuron more likely to fire; an inhibitory neurotransmitter makes it less likely. This combination is why transmission is described as electro-chemical.