Mammalian nervous system

The mammalian nervous system is a complex communication network that coordinates actions and transmits signals between different parts of the body. It is divided into two main parts. The Central Nervous System (CNS) consists of the brain and spinal cord. It acts as the main processing centre, receiving information, making decisions, and sending out commands. The Peripheral Nervous System (PNS) is made up of all the nerves that branch out from the CNS to the rest of the body. These nerves connect the CNS to sense organs, muscles, and glands, forming a complete circuit for information flow.

Neurones (or nerve cells) are the basic functional units of the nervous system. They are specialised cells that transmit electrical signals called nerve impulses. There are three main types: 1. Sensory Neurones: Transmit impulses from receptors (e.g., in skin, eyes) to the CNS. 2. Motor Neurones: Transmit impulses from the CNS to effectors (muscles or glands). 3. Relay Neurones: Found within the CNS, they transmit impulses between sensory and motor neurones. A typical neurone has a cell body containing the nucleus, dendrites to receive signals, and a long axon to transmit signals. Many axons are covered in a fatty myelin sheath, which insulates the axon and speeds up impulse transmission. A 'nerve' is not a single neurone, but a bundle of many axons from different neurones.

A reflex is a rapid, automatic, and involuntary response to a stimulus. Reflexes are protective, helping to prevent injury. For example, you automatically pull your hand away from a hot object. This action occurs via a pathway called the reflex arc. The brain is not involved in the initial reflex decision, which is why it is so fast. The sequence is: 1. Stimulus: A change is detected (e.g., heat). 2. Receptor: Cells in a sense organ detect the stimulus (e.g., pain receptors in the skin). 3. Sensory Neurone: Transmits an impulse from the receptor to the spinal cord (part of the CNS). 4. Relay Neurone: In the spinal cord, it passes the impulse from the sensory neurone to a motor neurone. 5. Motor Neurone: Transmits the impulse from the spinal cord to an effector. 6. Effector: A muscle or gland that carries out the response (e.g., biceps muscle contracts). 7. Response: The action taken (e.g., hand is pulled away).

Neurones do not connect directly to each other. There is a microscopic gap between them called a synapse or synaptic cleft. For a nerve impulse to continue its journey, it must cross this gap. This is achieved through a chemical process. When an electrical impulse arrives at the end of the first neurone (the presynaptic neurone), it triggers the release of chemical messengers called neurotransmitters from tiny sacs called vesicles. These neurotransmitter molecules diffuse across the synaptic gap and bind to specific receptor proteins on the membrane of the next neurone (the postsynaptic neurone). This binding triggers a new electrical impulse in the second neurone. This chemical mechanism ensures that impulses only travel in one direction, as neurotransmitters are only released from one side and receptors are only present on the other.

Our nervous system controls two main types of actions: voluntary and involuntary. A voluntary action is one that we consciously decide to perform. It involves thought and is controlled by the cerebrum, the main part of the brain. Examples include picking up a pen, walking, or speaking. The pathway involves the brain making a decision and sending motor impulses to the relevant muscles. An involuntary action is one that occurs automatically, without conscious thought or control. There are two types. Reflex actions (like pulling away from heat) are one type. Other involuntary actions are controlled by a part of the brain called the medulla and regulate vital body functions like heartbeat, breathing rate, and digestion. These actions happen continuously without us having to think about them.