C2.2: neural signaling

Dendrites: receive nerve impulses and carry them towards the cell body.

Axon: conducts nerve impulses away from the cell body.

Cell body: contains the nucleus and organelles and integrates incoming signals.

Sensory neurons have receptor endings connected to dendrites and carry impulses from receptors to the CNS.
Motor neurons have cell bodies in the CNS and carry impulses from the CNS to effectors.

Membrane potential is the difference in electrical charge across a cell membrane.

Resting potential is the stable negative membrane potential of a neuron when it is not transmitting an impulse. (-70mV)

Resting potential is created by the sodium–potassium pump actively transporting 3 Na⁺ out and 2 K⁺ in, by greater permeability of the membrane to K⁺ causing K⁺ diffusion out, and by negatively charged proteins remaining inside the neuron.

Approximately –70 mV.

1. Three Na⁺ bind to the pump inside the neuron.
2. ATP hydrolysis provides energy to change pump shape.
3. Na⁺ is released outside the cell.
4. Two K⁺ bind from the outside.
5. The pump reverts to its original shape, powered by ATP.
6. K⁺ is released inside, maintaining the resting potentia

A nerve impulse is a self-propagating electrical signal travelling along a neuron.

An action potential is a rapid temporary reversal of membrane potential due to sodium influx followed by potassium efflux.

Larger axon diameter increases conduction speed because internal resistance to ion flow is lower.

Myelinated fibres conduct impulses faster because action potentials occur only at nodes of Ranvier, causing saltatory conduction.
Unmyelinated fibres conduct impulses more slowly because depolarisation must occur continuously along the axon.

Larger animals generally have faster nerve conduction speeds to allow rapid communication over longer distances.

Synapse: junction between neurons or between a neuron and an effector.

Synaptic gap (cleft): small space between pre- and postsynaptic membranes.

Effector: muscle or gland that produces a response.

Muscle cells and gland cells.

1 direction

Neurotransmitters are chemical messengers that transmit signals across synapses.

Depolarisation of the presynaptic membrane opens voltage-gated calcium channels.
Calcium ions enter the synaptic knob and trigger exocytosis of vesicles containing neurotransmitter.
Neurotransmitter diffuses across the synaptic gap.

Calcium ions act as a signal that triggers exocytosis of neurotransmitter vesicles.

Neurotransmitter diffuses across the synaptic gap and binds to specific receptors on the postsynaptic membrane.
This opens ligand-gated ion channels allowing ion movement.
If threshold potential is reached, an action potential is generated.

Acetylcholine is a common neurotransmitter in both invertebrates and vertebrates and transmits signals across synapses, including at neuromuscular junctions between neurons and muscle cells.

Acetylcholinesterase breaks down acetylcholine in the synaptic gap into acetate and choline, stopping continuous stimulation of the postsynaptic cell.