5.1.3

• Sensory receptors


  Groups of specialised cells that detect changes in the surroundings and convert them to neuronal signals by initiating or changing the frequency of action potentials
• Sensory receptors


  • Rods and cones in the eye (photoreceptors)
  • Chemoreceptors, such as the olfactory receptors in the nose
  • Thermoreceptors in the end-bulbs of Krause in the tongue
  • Vibration receptors in the cochlea of the ear
• Pacinian corpuscle


  A mechanoreceptor—detecting pressure and movement—found in the epidermis of the skin. It comprises a neuron with concentric layers of connective tissue. When pressure is placed on the skin, this deforms the layers of connective tissue, and because the membrane of the neuron contains stretch-mediated sodium ion channel, the neuron is depolarised when the connective tissue is deformed, generating a generator potential, which, by positive feedback, will generate an action potential which is transmitted along neurones to the central nervous system.
• Sensory neurons


  • Carry the action potential from a sensory receptor to the central nervous system. They have a long dendron carrying the action potential from a sensory receptor to the cell body, which is position just outside the CNS. They have a short axon carrying the action potential into the CNS.
• Relay neurons


  • Connect sensory and motor neurons, mainly in the central nervous system. They have many short dendrites to receive impulse transmissions from a number of sensory receptors and a short axon with variable numbers of synaptic endings to carry the action potential to the cell bodies of motor neurones in the CNS.
• Motor neurons


  • Carry an action potential from the central nervous system to an effector such as cells in a muscle or gland. They have their cell body in the CNS and have a long axon that carries the action potential to effector cells.
• Cell body


  Contains the nucleus, surrounded by cytoplasm containing large amounts of endoplasmic reticulum and mitochondria to produce neurotransmitters used at the synapses of all three neurones.
• Dendrons


  Short extensions which carry action potentials toward the cell body where dendrites of relay neurons synapse with sensory receptors, or relay neurons synapse with motor neurones.
• Axons


  Singular, elongated nerve fibres that carry action potential potentially long distances away from the cell body.
• Peripheral neurones in vertebrates

  • Are myelinated. Most sensory and motor neurones are associated with many Schwann cells which wrap around the neurone creating a multi-layered fatty sheath, which insulates the neurone from adjacent ones, and preventing the membrane from depolarising. Every 1-3 mm, there are gaps in the myelin sheath—the nodes of Ranvier, where the membrane can be depolarised.
• Neurones in the CNS and unmyelinated peripheral neurones

  • Are associated with Schwann cells, but several neurones may be enshrouded in one loosely-wrapped Schwann cell, insulting adjacent neurones, but not the axonal membrane.
• Resting potential


  At -70mV across the membrane, caused by the movement of sodium and potassium ions across the axonal membrane, with the sodium-potassium pump actively transporting three sodium ions out of the axon for every two potassium ions that are pumped in.
• Action potential generation


  1. Stimulus detected by sensory receptor

  2. Temporary depolarisation of axonal membrane

  3. Voltage-gated sodium ion channels open

  4. Sodium ions diffuse into axon

  5. Positive feedback loop further depolarises membrane

  6. Voltage-gated sodium ion channels close, voltage-gated potassium ion channels open

  7. Potassium ions diffuse out of axon, repolarising membrane

  8. Sodium-potassium pump restores resting potential
• Threshold


  +40mV, the potential difference at which an action potential is generated
• Action potential propagation


  1. Depolarisation acts as stimulus for depolarisation further down axon

  2. Sodium ions diffuse laterally, creating local current

  3. Voltage-gated sodium ion channels open, generating next action potential
• Refractory period


  Period after an action potential is generated where another cannot be generated, due to high internal sodium ion concentration and external potassium ion concentration
• Saltatory conduction


  In myelinated neurons, the action potential 'jumps' from node of Ranvier to node of Ranvier, increasing transmission speed
• Factors affecting transmission speed

  • Wider axon diameter
  • Higher temperature (before denaturation)
• All-or-nothing principle


  Action potentials are either generated at threshold or not at all, but their intensity is communicated by frequency
• Synapse


  Junction between two or more neurones, where one neurone can communicate an action potential across the 20nm-wide synaptic cleft using neurotransmitters
• Cholinergic synapse structure


  • Pre-synaptic bulb contains vesicles of acetylcholine and voltage-gated calcium channels
  • Post-synaptic membrane contains acetylcholine-responsive sodium ion channels
• Synaptic transmission


  1. Action potential depolarises pre-synaptic membrane

  2. Calcium ions diffuse into pre-synaptic bulb

  3. Acetylcholine-containing vesicles fuse with membrane, releasing acetylcholine

  4. Acetylcholine binds to post-synaptic receptors, opening sodium ion channels

  5. Sodium ions diffuse into post-synaptic neurone, depolarising membrane

  6. Acetylcholinesterase hydrolyses acetylcholine in synaptic cleft
• Summation


  Several excitatory post-synaptic potentials must combine to reach threshold and generate an action potential, either through temporal summation or spatial summation
• Inhibitory post-synaptic potentials


  Reduce the effect of summation and prevent an action potential in the post-synaptic neurone
• Sensory receptors


  Groups of specialised cells that detect changes in the surroundings and convert them to neuronal signals by initiating or changing the frequency of action potentials
• Transducers


  Convert one type of energy, or a change in the magnitude of frequency of a type of energy, and converting it to chemical, and then electrical, energy
• Sensory receptors


  • Rods and cones in the eye (photoreceptors)
  • Chemoreceptors, such as the olfactory receptors in the nose
  • Thermoreceptors in the end-bulbs of Krause in the tongue
  • Vibration receptors in the cochlea of the ear
• Pacinian corpuscle


  • A mechanoreceptor—detecting pressure and movement—found in the epidermis of the skin
  • Comprises a neuron with concentric layers of connective tissue
  • When pressure is placed on the skin, this deforms the layers of connective tissue, and because the membrane of the neuron contains stretch-mediated sodium ion channel, the neuron is depolarised when the connective tissue is deformed, generating a generator potential, which, by positive feedback, will generate an action potential which is transmitted along neurones to the central nervous system
• Sensory neurons


  Carry the action potential from a sensory receptor to the central nervous system
• Relay neurons


  Connect sensory and motor neurons, mainly in the central nervous system
• Motor neurons


  Carry an action potential from the central nervous system to an effector such as cells in a muscle or gland
• Neuron structures


  • Cell body
  • Dendrons
  • Axons
• Cell body


  Contains the nucleus, surrounded by cytoplasm containing large amounts of endoplasmic reticulum and mitochondria to produce neurotransmitters used at the synapses
• Dendrons


  Short extensions which carry action potentials toward the cell body where dendrites of relay neurons synapse with sensory receptors, or relay neurons synapse with motor neurones
• Axons


  Singular, elongated nerve fibres that carry action potential potentially long distances away from the cell body
• Peripheral neurones in vertebrates

  • Myelinated
  • Most sensory and motor neurones are associated with many Schwann cells which wrap around the neurone creating a multi-layered fatty sheath, which insulates the neurone from adjacent ones, and preventing the membrane from depolarising
  • Every 1-3 mm, there are gaps in the myelin sheath—the nodes of Ranvier, where the membrane can be depolarised
• Neurones in the CNS and unmyelinated peripheral neurones

  • Associated with Schwann cells, but several neurones may be enshrouded in one loosely-wrapped Schwann cell, insulting adjacent neurones, but not the axonal membrane
  • Speed is less imperative with unmyelinated neurones carrying action potentials over small distances, or coordinating bodily functions, such as breathing and the action of the digestive system
• Resting potential


  • 70mV across the membrane, caused by the movement of sodium and potassium ions across the axonal membrane, with the sodium-potassium pump actively transporting three sodium ions out of the axon for every two potassium ions that are pumped in
• Action potential generation


  1. Stimulus detected by sensory receptor

  2. Temporarily depolarises axonal membrane

  3. Voltage-gated sodium ion channels open, allowing sodium ions to diffuse into the axon

  4. Positive feedback loop further depolarises the membrane until +40mV

  5. Voltage-gated sodium ion channels close, and voltage-gated potassium ion channels open, allowing potassium ions to diffuse out of the axon

  6. Axon repolarises and hyperpolarises before sodium-potassium pump restores resting potential
• Threshold


  +40mV, when reached an initial action potential is generated