6.5 Syllabus

U1. Neurons transmit electrical impulses.

  • One form of internal communication in the body occurs through nerve impulses in the nervous system
  • Neurons transmit electrical impulse nu allowing the passage of charged ions across their membranes in response to stimuli
  • Neurons consist of a cell body with a nucleus and cytoplasm, an elongated nerve fibre called an axon, and short-branched nerve fibres called dendrites. 

U2. ​The myelination of nerve fibres allows for salutatory conduction.

  • Nerve fibres conduct electrical impulses along the length of their axons. Some of these axons such as interneurons are unmyelinated, and therefore the impulse travels much slower
  • The greater the diameter, the greater the speed of the nerve impulse
  • Some axons are surrounded by a mixture of protein and phospholipids called myelin that collectively form a myelin sheath
  • Many layers of myelin are deposited around the axon by special cells called Schwann cells
  • The myelin sheath insulates the axon and greatly increases the speed of nerve impulse
  • In between the myelin are gaps called the nodes of Ranvier
  • In Myelinated neurons, the impulse can jump from one node to the next. This is called saltatory conduction
  • This allows myelinated neurons to conduct impulses up to 100 x faster than unmyelinated axons. 
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U3. ​Neurons pump sodium and potassium ions across their membranes to generate a resting potential.

  • The time period when a neuron that is not conducting a nerve impulse, but is ready to conduct one, is called the resting potential
  • This membrane potential is due to an imbalance of positive and negative charges across the membrane
  • Sodium-potassium pumps pump Na+ out of the axon and K+ into the axon
  • Sodium-potassium pumps pump Na+ out of the axon and K+ into the axon
  • Three Na+ are pumped out of the neuron and two K+ are pumped into the neuron
  • This creates a concentration gradient of Na+ (outside to in) and of K+ (inside to out)
  • The membrane is also much more permeable to K+ as Na+, so K+ leaks back out of the neuron through leak channels
  • This means the Na+ concentration is much greater outside the neuron
  • There are also negatively charged ions permanently located in the cytoplasm of the neuron
  • These conditions create a resting membrane potential of -70mv inside the neuron. 
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U4. ​An action potential consists of depolarization and repolarization of the neuron.

  • Action potentials are rapid changes in membrane potentials
  • This consists of rapid depolarization (change from negative to positive when sodium diffuses into the neuron) and a rapid repolarization (change from positive to negative when potassium diffuse out of the neuron
  • The arrival of an action potential caused by a stimulus causes a depolarization of the membrane as Na+ channels begin to open
  • If the membrane potential reaches a threshold level of -50mv. Many more voltage gated Na+ channels open and Na+ rapidly diffuses into the neuron
  • The inside of the neuron becomes more positively charged than the outside of the neuron (depolarization) 
  • K+ channels open and K+ ions diffuse out of the neuron making the inside negative again (repolarization) 
  • After the action potential, there is a refractory period where the impulse cannot go back in the same direction. This ensures a one-way nerve impulse 
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U5. ​Nerve impulses are action potentials propagated along the axons f neurons.

  • As a depolarization occurs in one part of the neuron, the positive charge triggers the Na+ channels to open in the nearby regions causing an action potential to occur
  • This action potential will cause a depolarization in the next region
  • The propagation of action potentials will continue along the axon of the neuron
  • Nerve impulses move in one direction along the neuron from one end of the neuron to the other end
  • A refractory period occurs after depolarization which prevent the electrical impulses from traveling backwards along the axon
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U6. ​Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

  • Propagation of nerve impulses along the axon results from the diffusion of Na+ ions from the area that was just depolarized to the neighbouring area that is still polarized inside the axon
  • When a part of the axon depolarizes, the localized are inside the axon become more positive as Na+ diffuses into the axon through voltage gated channels
  • Outside the axon the concentration of Na+ is less in the depolarized region, so sodium diffuses from the polarized region towards the depolarized region. 
  • The adjacent area inside the axon that is still polarized (more negative) 
  • The higher the concentration of Na+ inside the depolarized region region diffuses towards the polarized (more negative) region inside the axon
  • When this happens, the membrane potential of the adjacent region becomes more positive from -70mv to -50mv (threshold potential) 
  • This results in a depolarization in the neighbouring region, as Na+ voltage-gated channels open and Na+ diffuses into the axon. 
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U7. ​Synapses are junctions between neurons and between neurons and receptors or effector cells.

  • Synapses are junctions or structure between the pre-synaptic and post-synaptic membrane of two cells in the nervous system
  • The junction can be between a neuron and an effector such as a muscle or a gland
  • It can be between two different neurons. Many of these connections occur in the CNS (brain and spinal cord) 
  • A junction also exists between the sense receptor cells and the sensory neurons 
  • Neurotransmitters are chemicals diffuse across a synapse from pre-synaptic membrane to post-synaptic membrane to send a signal to the next cell 
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u8. When presynaptic neurons are depolarized they release a neurotransmitter into the synapse.

  • As the nerve impulse reaches the axon terminal of the presynaptic neuron, the positive charge from the depolarization causes voltage-gated channels permeable to Ca2+ to open
  • Ca2+ flows into the presynaptic neuron increasing the amount of Ca2+ in the presynaptic neuron
  • This Ca2+ causes vesicles containing neurotransmitters to bind to the membrane and release their neurotransmitters into the synaptic cleft (space between pre and post synaptic neuron) 
  • These neurotransmitters diffuse across the synaptic cleft and bind to receptor sites on the membrane of the post synaptic neuron
  • The binding of these neurotransmitters open ion channels allowing ions such as Na+ to diffuse into the post synaptic neuron
  • This influx of positive charge possibly leads to an action potential and a depolarization in the post synaptic neuron
  • The neurotransmitter is reabsorbed by the presynaptic neuron or broken down in the synapse by enzymes 
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U9. ​A nerve impulse is only initiated if the threshold potential is reached.

  • The threshold potential is the critical level to which a membrane potential must be reach in order to initiate an action potential
  • Neurons fire or a nerve impulse is generated by an 'all or nothing'
  • When a stimulus occurs, some Na+ channels open causing the membrane potential to become more positive 
  • If enough Na+ diffuses into the neuron(-50mv to -70mv) and action potential is generated
  • At a synapse, binding of a neurotransmitter at the post-synaptic membrane causes Na+ to diffuse into the neuron
  • This can cause a depolarization of the neuron if enough neurotransmitters are released 
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