SL NEURAL SIGNALLING Flashcards
Neurons as cells within the nervous system that carry electrical impulses
Students should understand that cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres of varying length projecting from it.
An axon is a long single fibre.
Dendrites are multiple shorter fibres. Electrical impulses are conducted along these fibres.
Neurons transmit information along nerve fibers in the form of electrical impulses. The electrical impulse is not like an electrical current that flows along wires. An impulse is a momentary reversal in electrical potential difference in the membrane – a change in the position of charged ions between the inside and outside of the membrane of the nerve fibres.
Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions
- They should understand the concept of a membrane polarization and a membrane potential
A nerve impulse is a result of a change in concentration of sodium (Na+) and
potassium (K+) ions along the cell membrane.
Resting potential: this is the potential difference across a nerve cell membrane when it is not stimulated. It is at approximately -70mV.
Action potential: This is the reversal (depolarisation) and restoration (repolarisation) of the electrical potential across a plasma membrane as a nerve impulse passes along a neuron.
- Reasons that the resting potential is negative.
- Students should understand how energy from ATP drives the pumping of sodium and potassium ions in opposite directions across the plasma membrane of neurons.
Sodium-potassium pumps
Leakage of ions back across the membrane by simple diffusion (K+ ions more so)
Negatively charged proteins inside the nerve fibre.
Nerve impulses as action potentials that are propagated along nerve fibres
- Students should appreciate that a nerve impulse is electrical because it involves movement of positively charged ions.
Depolarization: The cell membrane’s charge becomes positive. This is caused by positive sodium ions going into the cell.
Repolarization: The cell membrane’s charge returns to negative. This is caused by positive potassium ions moving out of the cell.
A stimulus causes sodium ions (Na+) to flow into the cytoplasm of the axon, reversing the polarity of the axon. This makes the membrane potential more positive (from -70mV to +40mV).
Towards the end of the action potential the flow of sodium ions stopps and potassium channels open up. This causes the flow of potassium ions (K+) out of the axon, bringen the membrane potential back down to -70mV.
Variation in the speed of nerve impulses
- Compare the speed of transmission in giant axons of squid and smaller non-myelinated nerve fibres.
- Also compare the speed in myelinated and non-myelinated fibres.
**note: gaps = nodes of Ranvier
- a larger axon diameter allows for faster transmission speeds (helps w/ faster movements + reaction to danger)
- myelin = lipid-rich substance surrounding the nerve fibre.
- The presence of a myelin sheath surrounding the axon fibre increases the speed of transmission of the action potential. Only at the junctions in the sheath (nodes of Ranvier) the axon membrane exposed. Elsewhere along the fibre, the electrical resistance of the myelin sheath prevents depolarization of the nodes.(saltatory conduction)
Synapses as junctions between neurons and between neurons and effector cells
- Limit to chemical synapses, not electrical, and these can simply be referred to as synapses.
- Students should understand that a signal can only pass in one direction across a typical synapse.
A synapse is the link point between neurons.
- ex: synapses btw. neurons and effectors
An action potential cannot cross the synaptic cleft between neurons – the nerve impulse is carried by chemical transmitter substances called neurotransmitters.
These chemicals are made in the Golgi apparatus of the cell that is sending the impulse (the pre-synaptic neuron) and stored in vesicles at the end of the axon.
Release of neurotransmitters from a presynaptic membrane
- Include uptake of calcium in response to depolarization of a presynaptic membrane and its action as a signalling chemical inside a neuron.
- A nerve impulse (action potential) reaches the terminal end of the pre-synaptic neuron.
- Depolarisation causes voltage gated calcium channels to open. Ca2+ rushes into the neuron.
- Ca2+ causes synaptic vesicles containing a neurotransmitter to move to the membrane and fuse.
Generation of an excitatory postsynaptic potential
- Include diffusion of neurotransmitters across the synaptic cleft and binding to transmembrane receptors.
- Use acetylcholine as an example. Students should appreciate that this neurotransmitter exists in many
types of synapse including neuromuscular junctions.
- Neurotransmitters (e.g. acetylcholine), that were stored in the synaptic vesicle now diffuse (from high to low concentration) rapidly across the synaptic gap.
- The neurotransmitter binds with the neuroreceptor (protein channels) in the post-synaptic membrane.
- The proteins channels of the receptors open upon binding, allowing the influx of Na+ ions. An excitatory action potential is initiated, and the nerve impulse is propagated along the post-synaptic neuron.
preventing overstimulation
The transmitter substance on the receptors is immediately inactivated by enzyme action to avoid overstimulation.
As a consequence, the ion channel of the receptor protein closes, and the resting potential in the post-synaptic neuron is re-established.
