A-Level Biology NERVOUS SYSTEM

Structure of a neurone

Structure of a neurone

• cell body

• Dendrons - short extensions combing from the cell body

• Axons- long nerve fibres that transmit impulses away from the cell body

Different types of neurones

• sensory neurones - transmit impulses from a sensory receptor cell to a relay/motor neurone or the brain

• Relay neurones - transmit impulses between neurones

• Motor neurones - transmit impulses from a relay/sensory neurone to an effector

Electrical impulse pathway

• receptor

• Sensory neurone

• Relay neurone

• Motor neurone

• Relay neurone

Which neurone is this

Motor neurone

Which neurone is this

Relay neurone

Which neurone is this

Sensory neurone

What is the myelin sheath made of

Schwann cells

Purpose of myelinated sheath

Conducts electrical impulses at faster speeds compared to non myelinated neurones as they allow for saltatory conduction between nodes of ranvier

Pacinian corpuscle

Specific sensory receptors which detect mechanical pressure

How does the pacinian corpuscle convert mechanical pressure into a nervous impulse

• in normal state, stretch mediated Na+ channels are too narrow for Na+ to move pass them, the neurone of the pacinian corpuscle has a resting potential

• When pressure is applied, the corpuscle changes shape, causing membrane surround the neurone to stretch

• Stretch mediated Na+ channels widen, allowing Na+ to diffuse into the neurone

• Influx of Na+ channels causes the membrane to become depolarised

• This creates an action potential which passes along the sensory neurone

What is the resting potential across the membrane

-70mV

How is a resting potential formed

• Na+ ions are actively pumped out of the axon whilst K+ ions are actively transported in at a ratio of 3:2

• This means there are more Na+ ions outside the membrane compared to inside the axon cytoplasm

• As the axon cytoplasm is more negative, Na+ ions move down an electrochemical gradient whereas K+ ions diffuse out the axon

• However, most of the VG Na+ ion channels are closed whilst K+ channels are open, this means the amount of positive ions outside of the axon is greater than inside, creating the -70mV resting potential

What is the depolarisation charge

+40mV

How does an action potential occur

Protein channels in axon membrane change shape as a result of a voltage across a membrane

Outline process of action potential

1. Neurone has a resting potential of -70mV, it is not transmitting an impulse, some K+ channels are open but VG Na+ channels are closed

2. Energy of the stimulus triggers some VG Na+ channels to open, making the membrane more permeable to Na+ ions, Na+ ions diffuse into the axon down an electrochemical gradient making inside the neurone less negative

3. This change in charge causes more Na+ ions to open, resulting in a positive feedback loop

4. When potential difference reaches +40mV, VG Na+ channels close and VG K+ channels open

5. K+ ions diffuse out of the axon down an electrochemical gradient resulting the inside of the axon becoming more negative

6. Hyperpolarisation occurs which causes Na+-K+ pump to open and move K+ in the axon, causing it to return to resting potential- repolarised