Nervous System

• Components of the nervous system

  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)

  
• Parts of the CNS

  • Brain
  • Spinal cord
• Parts of the PNS

  • Cranial nerves
  • Spinal nerves
• Roles of the nervous system

  • Allows organisms to detect changes in their surrounding and respond to it (sensory input)
  • Allows organisms to process sensory information
  • Send nervous impulses to effector cells (motor output)

  
• How the nervous system processes information

  1. Information is first gathered by receptors

  2. Information is then converted into electrical signals (nerve impulses)

  3. Nerve impulses are transmitted by cranial and spinal nerves to the central nervous system

  4. Brain processes information based on the pattern of nerve impulses

  5. Brain will send nerve impulses to the effectors

  6. Intended action is then carried out

  
• Nervous tissue


  Consists of nerve cells known as neurones

  
• Types of neurones


  • Sensory neurones
  • Motor neurones
  • Intermediate or relay neurones

  
• Structure of a neurone

  • Cell body, which is irregular in shape, contains a nucleus and cytoplasm
  • Axon are nerve fibers that carry impulses away from the cell body
  • Myelin sheath is a layer of fatty substance that shields and insulate the nerve fibre
  • Dendrites receive nerve impulses from other neurones
  • Dendrons are nerve fibres that transmit nerve impulses towards the cell body

  
• Synapse


  The junction between two neurones or between a neurone and an effector (muscle or gland)
• Chemical nature of synaptic transmission

  1. Ending (presynaptic) cell secretes a chemical signal, a neurotransmitter

  2. Neurotransmitter crosses the synaptic cleft

  3. Neurotransmitter binds to a specific receptor on the surface of the receiving (postsynaptic) cell
• Motor end plate


  The junction between the dendrite of a motor neurone and a muscle fibre

  
• Nerve


  A bundle of nerve fibres enclosed in a sheath of connective tissue
• Types of nerve fibres

  • Sensory nerve fibres
  • Motor nerve fibres
  • Spinal nerves (mixed fibres)
• How the brain, spinal cord and spinal nerves work together

  1. Sensory nerve fibres conduct nerve impulses from sense organs

  2. Motor nerve fibres conduct nerve impulses to the effectors

  3. Spinal nerves contain mixed fibres, which are made up of sensory and motor nerve fibres

  
• Grey matter


  Consists of cell bodies of neurones, located in the outer layers of the brain and central parts of the spinal cord
• White matter


  Consists of nerve fibres, located in the central parts of the brain and outer layers of the spinal cord
• Spinal cord


  • Passes through the vertebrate column (backbone)
  • 31 pairs of spinal nerves emerge at intervals along the length of the spinal cord
  • Grey matter of the spinal cord has the shape of the letter 'H'
  • A narrow central canal containing cerebrospinal fluid runs through the middle of the spinal cord
• Dorsal root


  • Joins the back part of the spinal cord, contains only sensory neurones
  • Cell bodies of sensory neurones are clustered together in a swelling called the dorsal root ganglion
• Ventral root


  • Joins the front part of the spinal cord, contains only motor neurones
  • Cell bodies and dendrons of the motor neurones lie in the grey matter of the spinal cord
• Spinal nerve


  • Contains nerve fibres from the dorsal root and ventral root
  • Spinal nerve subdivides into branches, supplying nerve fibres to various parts of the body
• Transmission of nerve impulses

  1. Relay neurones, lying parallel to the grey matter of the spinal cord, form synapses with the sensory and motor neurones

  2. They enable nerve impulses to be transmitted from the sensory to the motor neurones

  3. Relay neurones conduct nerve impulses from the sensory neurones to the brain

  4. They also conduct nerve impulses from the brain to the motor neurones

  
• Stimulus


  Change in environment that causes a reaction in an organism
• Response


  Reaction
• Types of actions


  • Voluntary actions
  • Involuntary actions
• Reflex action


  An immediate response to a specific stimulus without conscious control
• Types of reflexes


  • Spinal reflexes
  • Cranial reflexes
• Reflex arc


  1. Receptor or sense organ

  2. Sensory neurone

  3. Relay neuron in a reflex centre

  4. Motor neurone

  5. Effector

  
• Both the nervous system and endocrine system involve a stimulus and a target organ (effector), whereby a message is transmitted to the target organ that carries out the response
• Reflex actions


  Involuntary, rapid responses to stimuli
• Sense organ - Eye


  Organ that enables vision

  
• How do we see?

  1. Light rays refracted by cornea and lens

  2. Image formed on retina

  3. Rods and cones stimulated

  4. Nerve impulses transmitted to brain

  5. Brain processes image

  
• The principal functions of component parts of the eye are to produce a focused image of near and distant objects on the retina

  
• Pupil reflex


  1. Receptors in retina detect change in light intensity

  2. Sensory neuron in optic nerve transmits signal to brain

  3. Brain sends motor signal to iris muscles

  4. Iris muscles contract/relax to change pupil size

  
• Rods and cones


  • Rods for use in dim light and monochromatic vision
  • Cones for bright light and colour vision
• In bright light


  Circular muscles of iris contract, radial muscles relax, pupil constricts

  
• In dim light


  Circular muscles of iris relax, radial muscles contract, pupil dilates

  
• The image on the retina is inverted, laterally inverted, and smaller than the real object

  
• The brain corrects the inverted image on the retina to make it appear upright

  
• For a person with normal vision

  Light rays from near objects converge on retina, light rays from far objects converge on retina

  
• For a person with nearsightedness (myopia)


  Light rays converge in front of retina, causing blurred distant vision