The Nervous System

• Receptors


  Cells specialised for detection of stimuli
• Sense organs


  Groups of receptors
• Photoreceptors


  Light receptors located in the eye
• Light entering the eye

  1. Enters through the pupil

  2. Amount controlled by iris muscles
• Light focusing


  1. Lens focuses light on the retina

  2. Photoreceptors located in the fovea
• Nerve impulses from photoreceptors


  Carried via optic nerve to the brain
• The point where the optic nerve leaves the eye is the blind spot as there are no photoreceptor cells located there
• Types of photoreceptors in the retina

  • Cones (involved in colour vision)
  • Rods (produce monochromatic vision)
• Cones and rods


  • Cones can only work in bright conditions
  • Rods are much more sensitive and dim light is sufficient for them to work
• Rhodopsin


  Light-sensitive pigment in rods that absorbs light energy and splits into retinal and opsin
• Rods in the dark

  1. Sodium ions diffuse into the cell through open sodium ion channels

  2. Sodium ions actively pumped out of the cell

  3. Inside of cell only slightly more negative than outside

  4. Membrane slightly depolarised

  5. Glutamate neurotransmitter released to inhibit neurons connecting to optic nerve
• Rods in the presence of light

  1. Rhodopsin splits into retinal and opsin

  2. Opsin binds to membrane, causing sodium ion channels to close without affecting sodium ion transport out of cell

  3. Membrane becomes hyperpolarised

  4. No neurotransmitter released

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


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

  • Grey matter (contains cell bodies of relay and motor neurons)
  • White matter (surrounds grey matter, contains myelinated axons)
  • Central canal (contains cerebrospinal fluid)
• Sensory and motor neuron pathways in the spinal cord

  1. Sensory neurons enter via dorsal route

  2. Motor neurons leave via ventral route
• Function of the spinal cord

  Relay information in and out of any point along it, as well as up and down the body and to the brain
• Reflex arc


  Basis for rapid, protective involuntary actions where information does not reach the brain before a response is carried out
• Example of a reflex (stepping on something sharp)


  1. Stimulus detected by mechanoreceptors in skin

  2. Sensory neuron passes impulse to spinal cord

  3. Relay neuron passes impulse to motor neuron

  4. Motor neuron carries impulse to muscle

  5. Muscle (effector) moves away from sharp object
• Cnidarians


  Have a very simple nervous system in the form of a nerve net of interconnected neurons, allowing them to respond to a limited number of stimuli
• Neuron


  Nerve cell that plays an important role in coordinating communication within the nervous system
• Components of a neuron

  • Cell body (contains nucleus and organelles)
  • Dendrites (conduct impulses towards cell body)
  • Axon (conducts impulses away from cell body)
• Types of neurons


  • Sensory
  • Motor
  • Relay
• Motor neurons


  Transmit electrical signals from the central nervous system to muscles and glands
• Sensory neurons


  Transmit impulses from receptors to the central nervous system
• Relay neurons


  Located within the central nervous system, transmit electrical impulses from sensory neurons to motor neurons
• Neuron structure


  • Length of axons
  • Polarised nature of neuron membrane in resting state (outside positively charged, inside negatively charged)
• Myelin sheath


  Insulator of axons and dendrons produced by Schwann cells, increases speed of electrical potential transmission via saltatory conduction
• Resting potential


  • 70mV voltage difference across neuron membrane, maintained by sodium-potassium pump
• Neuron depolarisation


  1. Stimulus excites neuron

  2. Sodium ion channels open, sodium ions diffuse in down electrochemical gradient

  3. Membrane potential reaches threshold of -55mV

  4. More sodium channels open, potential reaches +30mV

  5. Sodium channels close, potassium channels open, potassium ions diffuse out

  6. Hyperpolarisation occurs as potassium channel closing is delayed

  7. Sodium-potassium pump restores resting potential of -70mV
• Action potential


  Wave of depolarisation that travels along the neuron
• Refractory period


  Short period after an action potential when the neuron membrane cannot be excited, ensures action potentials can only pass in one direction
• Synapse


  Junction between two neurons
• Events at a synapse

  1. Action potential arrives at presynaptic membrane

  2. Calcium channels open, calcium ions enter

  3. Synaptic vesicles fuse with presynaptic membrane, release neurotransmitter into synaptic cleft

  4. Neurotransmitter binds to receptors on postsynaptic membrane

  5. Cation channels open, allowing sodium ions to enter and depolarise postsynaptic membrane
• Inhibitory synapses


  Chloride ions open, causing hyperpolarisation of postsynaptic membrane and making it more difficult to trigger a new action potential
• Neurotransmitter removal


  1. Digestive enzymes in synaptic cleft break down neurotransmitter

  2. Neurotransmitter taken up by presynaptic membrane and reused
• Receptors only present on postsynaptic side, ensures action potentials can only travel in one direction
• Psychoactive drugs


  Chemical substances that affect brain function, leading to changes in mood and perception
• Drugs affecting impulse transmission

  Can amplify or inhibit synaptic transmission
• Substances affecting impulse transmission

  • Organophosphates (inhibit breakdown of neurotransmitters, causing continual stimulation)
  • Amphetamines (stimulate release of neurotransmitters like noradrenaline)