C2.2.1 - C2.2.7 : SL (Neural Signalling)

Central Nervous System (CNS)

The body’s processing center

Made up of the brain and spinal cord

Neuron

Individual cell that carries electrical impulses from one point in the body to another very quickly

Nerve

Many individual neurons grouped in a single structure

Peripheral Nerves

Made up of nerves and branches that enter and leave the spinal cord and brain stem

• Sensory neurons = carry response to CNS

• Motor neurons = carry response information to muscles

3 Basic Neuron Components

• Dendrites

• Axon

• Soma

Dendrites

Short-branched fibers that convert chemical info from other neurons or receptor cells into electrical impulses

Axon

An elongated fiber that transmits the electrical signals to terminal regions for communication with other neurons or effectors

Soma

Cell body containing nucleus and organelles where essential metabolic processes occur to maintain cell survival

Synaptic terminal of axon

The end part of a neuron’s axon that releases neurotransmitters to communicate with another neuron, muscle, or gland across a synapse

Action potential

Sequence of events that allow an impulse to travel through a neuron

Polarized (neuron)

When a neuron is ready to send an impulse

→ Has a resting potential across membrane

Resting potential

Difference in charge across membrane when not firing

Created by Na+ (out) and K+ (in) through active transport (sodium-potassium pump = 3 Na+ out and 2 K+ in)

→ Outside is more positive relative to inside

Membrane potential

Potential difference in charge across cell membrane

Myelin sheath

Collection of surrounding membrane on axon

→ One of the factors which affect speed of action potential

It speeds up nerve impulses by allowing action potentials to “jump” from one node of Ranvier to another, skipping the long process of ion movement under insulated myelin

Schwann cells

Cells that wrap themselves around nerve fiber (axon) and acts as insulators

There is no ion movement when axon is covered

Nodes of Ranvier

Small, even gaps between Schwann cell

Synapses

Physical gaps that separate neurons from other cells

→ neurons align, so that synaptic terminals are next to dendrites of another neuron

Neurotransmitter

Chemicals that allow neurons to communicate

• Always released from synaptic terminal buttons of first neuron → results in the continuation of the impulse when neurotransmitter is received by dendrites of the second neuron

Presynaptic neuron

Neuron that releases neurotransmitter

Postsynaptic neuron

Neuron receiving neurotransmitter

Synaptic cleft

Very small gap (20nm) between two neurons

Neurotransmission process steps

1. Action potential reaches axon terminal → opens voltage-gated calcium channels (depolarization of presynaptic membrane and uptake of Ca²⁺ into terminal)

2. Ca²⁺ acts as a signaling chemical → makes vesicles with neurotransmitters, move to the presynaptic membrane and fuse

3. Neurotransmitter is released from vesicles into the synaptic cleft

4. Neurotransmitter binds to protein receptor on postsynaptic membrane

5. Binding opens ion channels → Na⁺ diffuse through

6. This starts action potential → causes electrical signal (depolarization)

7. Neurotransmitter on protein receptor is released back → the grades and synaptic cleft by enzymes (into two or more fragments)

8. Ion channel in postsynaptic membrane closes to Na⁺

9. Neurotransmitter fragments diffuse back across synaptic cleft → reassembled in terminal buttons of presynaptic neuron

Acetylcholine

Common neurotransmitter found at synapses between two neurons or between a neuron and a muscle cell (neuromuscular junction)

→ causes muscle to contract

Acetylcholinesterase

Enzyme in synaptic cleft and breaks down acetylcholine in fragments

• Allows for the transmission of the action potential to only happen once