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.
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.
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
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
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.
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
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
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