The Nervous System

Neurones (nerve cells) are the basic structural and functional units of the nervous system, specialised for transmitting electrical and chemical signals. Each neurone typically consists of three main parts: 1. Dendrites: Short, branched extensions that receive nerve impulses from other neurones or receptors and transmit them towards the cell body. 2. Cell Body (Soma): Contains the nucleus and most of the cytoplasm. It controls the metabolic activities of the neurone and integrates incoming signals. 3. Axon: A long, slender projection that transmits nerve impulses away from the cell body towards other neurones, muscles, or glands. Axons can be very long. Many axons are covered by a Myelin Sheath, a fatty insulating layer formed by Schwann cells (in the PNS) or oligodendrocytes (in the CNS). The myelin sheath speeds up the transmission of nerve impulses through saltatory conduction (impulses 'jump' between Nodes of Ranvier, which are gaps in the myelin sheath). At the end of the axon, there are Axon Terminals (or synaptic knobs) which contain neurotransmitters for chemical transmission at synapses. There are three main types of neurones: * Sensory Neurones: Transmit impulses from receptors to the Central Nervous System (CNS). * Motor Neurones: Transmit impulses from the CNS to effectors (muscles or glands). * Interneurones (Relay Neurones): Connect sensory and motor neurones within the CNS.

A Synapse is the junction between two neurones, or between a neurone and an effector cell (e.g., muscle or gland). It is where a nerve impulse is transmitted from one cell to another, typically chemically. Key components of a synapse: * Presynaptic Neurone: The neurone transmitting the impulse, ending in the presynaptic terminal. * Synaptic Cleft: A tiny gap (about 20 nm wide) between the presynaptic and postsynaptic membranes. * Postsynaptic Neurone: The neurone or effector cell receiving the impulse, with receptors on its postsynaptic membrane. Process of Synaptic Transmission: 1. An action potential (nerve impulse) arrives at the presynaptic terminal. 2. This depolarisation opens voltage-gated calcium ion (Ca²⁺) channels in the presynaptic membrane, causing Ca²⁺ to rush into the terminal. 3. The influx of Ca²⁺ triggers synaptic vesicles (sacs containing neurotransmitters) to fuse with the presynaptic membrane. 4. Neurotransmitters are released into the synaptic cleft by exocytosis. 5. Neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane. 6. This binding causes ion channels on the postsynaptic membrane to open, leading to a change in its membrane potential. If the change reaches the threshold, a new action potential is generated in the postsynaptic neurone (excitation). Some neurotransmitters can also inhibit the postsynaptic neurone. 7. Neurotransmitters are quickly removed from the synaptic cleft, either by enzymatic breakdown (e.g., acetylcholine by acetylcholinesterase) or by reuptake into the presynaptic terminal, ensuring precise and brief signalling.

A Reflex Action is a rapid, involuntary, and automatic response to a stimulus, designed to protect the body. The neural pathway involved in a reflex action is called a Reflex Arc. Most reflex arcs bypass the brain, with the integration centre located in the spinal cord, allowing for very fast responses. Components of a Reflex Arc: 1. Receptor: Detects the stimulus (e.g., pain receptors in the skin). 2. Sensory Neurone (Afferent Neurone): Transmits the nerve impulse from the receptor to the CNS (spinal cord). 3. Interneurone (Relay Neurone): Located within the CNS (spinal cord), it connects the sensory neurone to the motor neurone. (Some simple reflexes, like the knee-jerk, are monosynaptic and do not involve an interneurone). 4. Motor Neurone (Efferent Neurone): Transmits the nerve impulse from the CNS to an effector. 5. Effector: A muscle or gland that carries out the response (e.g., a muscle contracting to withdraw a limb).

The nervous system is broadly divided into two main parts: 1. Central Nervous System (CNS): * Components: Consists of the Brain and Spinal Cord. * Function: The main processing centre of the body. It receives and integrates sensory information, processes thoughts, emotions, and memories, and initiates motor responses. * Brain: The command centre, responsible for complex functions. Major parts include the Cerebrum (thought, voluntary movement), Cerebellum (coordination, balance), and Medulla Oblongata (vital involuntary functions like breathing and heart rate). * Spinal Cord: A long, slender bundle of nerves extending from the brainstem. It serves as a major communication link between the brain and the rest of the body, and also acts as the integration centre for many reflex arcs. 2. Peripheral Nervous System (PNS): * Components: Consists of all the nerves and ganglia (clusters of neurone cell bodies) located outside the brain and spinal cord. * Function: Connects the CNS to the limbs and organs, essentially serving as a communication relay going back and forth between the brain and spinal cord and the rest of the body. * Subdivisions of the PNS: * Somatic Nervous System: Controls voluntary movements by carrying signals from the CNS to skeletal muscles. It also carries sensory information from receptors (e.g., skin, muscles) to the CNS. * Autonomic Nervous System (ANS): Controls involuntary functions of internal organs, such as heart rate, digestion, respiration, and glandular secretions. It operates without conscious thought. * Sympathetic Nervous System: Prepares the body for 'fight or flight' responses during stress or danger. It increases heart rate, dilates pupils, inhibits digestion, and redirects blood flow to muscles. * Parasympathetic Nervous System: Promotes 'rest and digest' activities, conserving energy. It decreases heart rate, constricts pupils, stimulates digestion, and promotes relaxation.