Nervous system

• Nervous system

  Consists of the brain, spinal cord and complex network of nerves
• Function of the nervous system

  Sends messages to and from the brain and the body
• Brain

  • Controls all body functions
• Spinal cord

  • Has three basic functions: motor commands from the brain to the body, send sensory information from the body to the brain and coordinate reflexes
• Components of the nervous system

  • Brain
  • Spinal cord
  • Sensory and motor neurones: dendrites, cell body, nucleus, axon, myelin sheath of Schwann cells, nodes of Ranvier, axon endings/terminals and synaptic ends
  • Relay neurons
  • Synapses
• Central Nervous System (CNS)

  Consists of the brain and the spinal cord
• Brain

  • Where sensory inputs are processed and responses are initiated
• Spinal cord

  • Important in reflex actions
• Peripheral Nervous System (PNS)

  Consists of all the sensory and motor neurons that connect receptors with the CNS and to muscles or glands respectively, to bring about responses such as movement
• Sensory neurons

  • Collect information from the sensory receptors, which are located in specialised tissues of the ears, eyes, mouth, nose, skin and internal organs
• Afferent neurons


  Carry information to the CNS
• Efferent neurons

  Carry information away from the CNS
• Types of neurons

  • Sensory neurone
  • Motor neurone
• Sensory neurone

  • Carry sensory impulses from sensory organs to CNS, located in dorsal root ganglion of the spinal nerve, unipolar, short axon
• Motor neurone

  • Carry motor impulses from CNS to specific effectors, located in the ventral root ganglion of the spinal cord, multipolar, long axon
• An adult has about 10 million sensory nerves in the body
• About half a million of motor neurons are found in the body
• Location of sensory and motor neurons

  • Eyes
  • Skin
  • Ears
  • Tongue
  • Nose
  • Muscles
  • Glands
• Function of sensory neurons

  Carry impulses from receptors to the central nervous system (CNS)
• Function of motor neurons

  Carry impulses away from the CNS to effectors
• Motor neurone

  • Transmit nerve impulses only in one direction (away from the cell body), have dendrites that make connections with other neurons, cell body containing the nucleus and other organelles, axons which carries the nerve impulses from the cell body, myelin sheath (which consists of Schwann cells which produce them), nodes of Ranvier, axon endings or terminals that make connections, usually with muscle cells
• Synapses

  Connections between neurones
• Synaptic transmission

  Electrical transmission of nerve impulse across the synapse is not possible because of physical gaps between the cells, so the nerve impulse is carried across the synapse by chemicals known as neurotransmitters
• Neurotransmitters

  Chemicals released from the synaptic knob that diffuse across the synaptic cleft and bind to receptors on the membrane of the muscle cell or the next neurone
• Acetylcholine (ACH)


  The most common neurotransmitter, which if sufficient amount binds, then the post-synaptic membrane becomes depolarised and a new nerve impulse will be generated in the next neurone
• Mechanism of a reflex action

  1. Receptor detects a stimulus

  2. Sensory neurone sends a signal to the relay neuron

  3. Relay neurone sends the signal to the motor neurone

  4. Motor neurone sends the signal to the effector

  5. Effector produces an instantaneous response
• Reflex action

  Signal does not reach the brain, instead it is directed into the synapse in the spinal cord, giving an almost immediate reaction