A-Level Biology OCR Notes

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Receptors

Sensory receptors are specialised cells in the nervous system that detect physical stimuli and convert them into electrical signals (the generator potential)
Sensory receptors tend to be specific to one type of stimulus because they have specialised structures that are specific to one type of physical property
Pacinian corpuscles detect changes in pressure in the skin

Increases in pressure cause a deformation of the concentric rings of the Pacinian corpuscle, opening stretch-mediated sodium channels in the membrane. Sodium ions enter the sensory neuron, causing a generator potential which can trigger an action potential

Neurones & The Resting Potential

The resting potential is the difference in electrical charge across the membrane while the neurone is at rest
The sodium-potassium pump uses ATP to pump 3 sodium (Na^+) ions out of the cell and 2 potassium (K^+) ions into the cell. The membrane is permeable to K^+ but impermeable to Na^+ ions. These factors allow an electrochemical gradient to be set up, with the cell negatively charged at -70mV.

Action Potentials

When the neurone receives an impulse from sensory receptors, sodium channels on the dendrites open, leading to the movement of Na^+ ions into the cell causing depolarisation. If this depolarisation reaches the threshold potential it activates voltage-gated sodium channels causing an action potential. After Voltage-gated sodium ion channels close, and voltage-gated potassium channels open, causing Repolarisation as K^+ ions leave the cell. Outward diffusion of K^+ ions causes hyperpolarisation and the voltage-gated potassium channels close. Finally, the Sodium-potassium pump returns the cell to the resting membrane potential.
Action potentials are an all or nothing response because once the threshold is reached each action potential always depolarises the axon to the same voltage by voltage-gated sodium channels.
The refractory period is the period in an action potential where the axon can't be depolarised to initiate a new action potential. It limits the frequency of action potentials and ensures action potential are discrete & only travel in one direction.

Transmission of Action Potentials

Action potential are transmitted in non-myelinated axons because when a depolarisation happens, it causes voltage-gated sodium channels to open further down the axon. By the time the depolarisation has spread, part of the axon is repolarising
In myelinated axons, action potentials only occur at the nodes of Ranvier, with charge diffusing along the cell where myelin is present (saltatory conduction).
Factors affecting transmission speed:

Cholinergic Synapse

Structure of a synapse:
At a cholinergic synapse (acetylcholine is the neurotransmitter), an action potential arrives at the pre-synaptic knob, depolarising the membrane and causes voltage-gated calcium ion channels to open. The influx of Ca2+ ions causes the synaptic vesicles to fuse with the membrane, releasing the neurotransmitter into the synaptic cleft. The neurotransmitter diffuses and binds receptors on the post synaptic membrane, causing an action potential.

Acetylcholinesterase breaks down acetyl choline in the cleft.
The synapses can be excitatory if the neurotransmitter opens Na+ channels or inhibitory if the neurotransmitter opens chloride or potassium channels causing hyperpolarisation.
Spatial summation is when action potentials from multiple presynaptic neurones are added together in a post-synaptic neurone
Temporal summation is when multiple action potentials from a single presynaptic neurone are added together in a post-synaptic neurone over time.

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