The nervous system

• Receptors


  Cells specialised for detection of stimuli
• Photoreceptors


  Light receptors located in the eye
• Light entering the eye

  1. Passes through pupil

  2. Amount controlled by iris muscles
• Light focusing


  1. Lens focuses light on retina

  2. Photoreceptors located in fovea
• Nerve impulses from photoreceptors


  Carried via optic nerve to brain
• Blind spot


  Point where optic nerve leaves eye, no photoreceptors located there
• Photoreceptors in retina


  • Cones
  • Rods
• Cones


  • Involved in colour vision
• Rods


  • Produce monochromatic vision
  • More sensitive, work in dim light
• Rhodopsin in rods


  1. Absorbs light energy

  2. Splits into retinal and opsin
• Dark adaptation in rods

  1. Sodium ions diffuse in, actively pumped out

  2. Membrane slightly depolarised

  3. No neurotransmitter released
• Light adaptation in rods

  1. Rhodopsin splits

  2. Opsin binds to membrane

  3. Sodium ions close

  4. Membrane hyperpolarised

  5. Action potential formed and transmitted to brain
• Spinal cord


  Column of nervous tissue running along back, protected by vertebral column
• Components of spinal cord

  • Grey matter
  • White matter
  • Central canal with cerebrospinal fluid
• Sensory neurones


  Enter spinal cord via dorsal route
• Motor neurones


  Leave spinal cord via ventral route
• Function of spinal cord


  Relay information in and out, up and down body and to brain
• Reflex arc


  Basis for rapid, protective involuntary actions
• Reflex response to stepping on sharp object

  1. Stimulus detected by mechanoreceptors

  2. Sensory neurone transmits to spinal cord

  3. Relay neurone passes to motor neurone

  4. Motor neurone transmits to muscle

  5. Muscle contracts to move away
• Cnidarian nervous system


  Simple nerve net of interconnected neurones
• Neurones


  Nerve cells that coordinate communication in nervous system
• Types of neurones


  • Sensory
  • Motor
  • Relay
• Motor neurones


  Transmit signals from central nervous system to muscles and glands
• Sensory neurones


  Transmit impulses from receptors to central nervous system
• Relay neurones


  Transmit impulses from sensory to motor neurones
• Neurone structure


  • Cell body with nucleus and organelles
  • Dendrites conduct impulses towards cell body
  • Axons conduct impulses away from cell body
• Resting potential


  • 70mV difference across neurone membrane, outside positive, inside negative
• Maintaining resting potential


  1. Sodium-potassium pump moves Na+ out, K+ in

  2. K+ ions diffuse back out through channels
• Depolarisation


  1. Stimulus opens Na+ channels

  2. Na+ diffuses in, making inside less negative

  3. Threshold of -55mV reached, more Na+ channels open

  4. Potential reaches +30mV
• Refractory period


  Period after action potential when neurone cannot be excited
• Synapses


  Junctions between two neurones
• Synaptic transmission


  1. Action potential depolarises presynaptic membrane

  2. Ca2+ enters, vesicles fuse and release neurotransmitter

  3. Neurotransmitter binds to receptors on postsynaptic membrane

  4. Cation channels open, depolarising postsynaptic membrane
• Excitatory vs inhibitory synapses

  Excitatory - Na+ channels open, depolarisation

  Inhibitory - Cl- channels open, hyperpolarisation
• Neurotransmitter removal


  1. Broken down by enzymes in synaptic cleft

  2. Reuptake by presynaptic membrane
• Psychoactive drugs


  Chemicals that affect brain function, mood and perception
• Drugs affecting impulse transmission

  • Organophosphates inhibiting neurotransmitter breakdown

  Amphetamines stimulating neurotransmitter release