The Nervous System

Basic Organisation of the Nervous System

  • The nervous system is made up of two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
  • The CNS is composed of the brain and spinal cord. It is the central processing and control hub for all nervous activities within the body.
  • The PNS comprises all nerves originating from the brain (cranial nerves) and spinal cord (spinal nerves). It connects the CNS to all other parts of the body.

Neurons

  • Neurons are the basic functional units of the nervous system.
  • Each consists of a cell body containing the nucleus, dendrites and an axon.
  • Dendrites receive signals from other neurons or sensory cells.
  • The axon transmits the nervous impulse away from the cell body towards other neurons or effectors.
  • The transmission of impulses is facilitated by myelin sheaths, which insulate the axons, and nodes of Ranvier, which are gaps between adjacent myelin sheaths.

Synapses and Neurotransmitters

  • A synapse is a junction between two neurons or between a neuron and an effector.
  • Electrical impulses are transmitted across synapses via neurotransmitters.
  • Neurotransmitters are chemicals released from the synaptic vesicles within a neuron when a nerve impulse arrives.
  • They diffuse across the synaptic cleft and bind to specific receptors on the post-synaptic neuron, triggering or preventing a new impulse.

Sensory, Relay and Motor Neurons

  • Sensory neurons transmit nerve impulses from receptors (e.g., in the eyes, ears, skin) to the CNS.
  • Within the CNS, the impulses are passed onto relay neurons.
  • Motor neurons receive these impulses from the CNS and transmit them to effectors (e.g., muscles, glands) resulting in a corresponding response.

Reflex Actions and Reflex Arc

  • Reflex actions are automatic, immediate responses to specific stimuli that occur without conscious control.
  • The pathway of transmission of nerve impulses during a reflex action is called a reflex arc, typically involving sensory, relay, and motor neurons.

Effectors

  • Effectors are muscles or glands that bring about a response when stimulated by nerve impulses from the nervous system.
  • Muscles contract in response to nerve impulses, causing movement.
  • Glands secrete specific substances (hormones, enzymes) in response to nerve impulses.