Nerve Impulse Conduction

• Neurone repolarisation

  1. Sodium ion channels close and potassium ion channels open, potassium ions diffuse out of the neurone down the concentration gradient, restoring the resting potential

  2. Closing of potassium ion channels is slightly delayed, leading to hyperpolarisation where potential difference becomes greater than the resting potential

  3. Resting potential is reestablished with the help of the sodium-potassium pump, returning the potential difference to -70mV
• Synaptic transmission

  1. Upon arrival of an action potential, presynaptic membrane depolarises, opening calcium ion channels, allowing calcium ions to enter the neurone

  2. Entry of calcium ions fuses the synaptic vesicles containing neurotransmitters to the presynaptic membrane, releasing neurotransmitters into the synaptic cleft 3. The neurotransmitter diffuses across the synaptic cleft, where they bind to the receptors on the postsynaptic membrane therefore opening cation channels which enable sodium ions to enter the neurone
• Refractory period
• Synapses are junctions between two neurones
• Action potential propagation

  Action potential travels along the neurone as a wave of depolarisation, sodium ions move through the cytoplasm to adjacent resting regions, triggering a change in potential difference and stimulating another action potential
• Neurone depolarisation

  1. Stimulation of neurone cell triggers sodium ion channels to open, making it more permeable to sodium ions which diffuse into the neurone, reducing the negativity inside

  2. Upon reaching -55mV threshold, more sodium channels open, leading to a potential difference of +30mV, marking the end of depolarisation and start of repolarisation
• Cardiac muscle is found only in the heart and allows involuntary contraction.
• Skeletal muscles are attached to bones by tendons and allow voluntary movement.
• The three main types of muscle tissue are skeletal, cardiac, and smooth.
• Neuronal cell signalling

  • Faster and short term
• Structure of neurones


  • All neurones have a cell body containing the nucleus and organelles like mitochondria
  • Neurones have extensions called dendrites conducting impulses towards the cell body and axons conducting them away
  • Three types of neurones: sensory, motor, relay with different functions based on the position of the cell body
• Nerve impulse conduction


  1. Nerve cells are polarised in resting state due to imbalance between sodium and potassium ions

  2. Resting potential maintained by sodium-potassium pump creating electrochemical gradient

  3. Upon stimulation, neurone cell membrane becomes depolarised, sodium channels open, potassium channels open for repolarisation, restoring resting potential
• Neuronal communication


  Nerve cells called neurones coordinate communication within the nervous system
• Sensory neurones

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

  • Located within the central nervous system, transmit electrical impulses from sensory neurones to motor neurones
• Motor neurones


  • Transmit electrical impulses from the central nervous system to muscles and glands
• Neuronal structure for impulse conduction

  • Length of axons, polarised nature of neurone membrane enables carrying of electrical impulses called action potentials
  • Myelin sheath increases speed of electrical potential conduction via saltatory conduction between nodes of Ranvier
• Refractory period


  Ensures that action potentials can only pass in one direction as discrete impulses
• Pacinian corpuscles are pressure receptors found in the skin converting mechanical energy into electrical energy
• Short period during which the neurone membrane cannot be excited is known as the refractory period
• Process only occurs if the neurotransmitter originates from an excitatory neurone
• Receptors are described as transducers as they convert one form of energy into another form
• Digestive enzymes in the synaptic cleft break down neurotransmitter to prevent overstimulation of the post-synaptic membrane
• Action potential travels along the neurone

  As a wave of depolarisation where sodium ions move through the cytoplasm to the adjacent resting region triggering another action potential
• Cells specialised for detection of stimuli are known as receptors
• Repolarisation

  1. Sodium ion channels closing and potassium ion channels opening

  2. Potassium ions diffuse out of the neurone down the concentration gradient and eventually restore the resting potential
• Synaptic transmission

  1. Presynaptic membrane depolarises causing calcium ion channels to open

  2. Calcium ions enter the neurone causing fusion of synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane

  3. Neurotransmitter is released into the synaptic cleft

  4. Neurotransmitters bind to receptors on the postsynaptic membrane stimulating the opening of cation channels enabling sodium ions to enter the neurone

  5. Post-synaptic membrane depolarises triggering another action potential
• Closing of potassium ion channels is slightly delayed, leading to hyperpolarisation
• Inhibitory neurones cause hyperpolarisation of the post synaptic membrane making triggering a new action potential more difficult
• Synapses


  Junctions between two neurones
• All action potentials have the same magnitude, and the brain interprets the strength of stimulus based on the frequency of action potentials
• Receptors ensure action potential can only travel in one direction at the synapse
• Resting potential is reestablished with the help of the sodium-potassium pump which returns the potential difference to the value of -70mV