Receptors and neurones

What is the structure of neurone

• Cell body- contains nucleus and surrounded by cytoplasm

• Dendrons- extensions of cell body, divide into dendrites and responsible for transmitting electrical impulses towards cell body

• Axons- singular elongated nerve fibres that transmit impulse away from cell body

What are myelinated neurones

• Some neurons covered in myelin sheath made of Schwann cells which produce layers by growing around axon many times

• Myelin sheath acts like an insulator so impulses are carried much faster

• Nodes of Ranvier between sheath in which impulses jump from one node to the next

What are types of receptor are there

• Mechanical- pressure or bending opens ion channel

• Thermoreceptors- temperature affects enzyme controlling ion channel

• Photoreceptors- light alters membrane protein which controls ion channels

• Chemoreceptors- chemical binds to receptor to open ion channels

How is resting potential reached

-70mV

1. Created due to protein pump Na⁺K⁺ATPase which uses ATP to pump 3 sodium ions out of cell and 2 potassium ions in

2. Sets up electrochemical gradient where charge outside higher than inside

3. Creates a higher Na⁺ concentration outside and a higher K⁺ concentration inside the axon.

4. K⁺ channels open, allowing K⁺ to diffuse out of the axon down its concentration gradient.

What is depolarisation

• Stimulus results in sodium channels opening and sodium ions diffusing out of neurone

• This occurs until threshold potential achieved at -55mV so voltage gated sodium ions open and more diffuse into cell until +40mV reached

What is repolarisation

• Once potential difference reaches 40mV voltage gated sodium ions close and voltage gated potassium channels open

• Potassium ions rush out of axon so it is more negative inside than outside

What is hyperpolarisation

• Rush of K⁺ ions out of axon results in a resting potential more negative than -70mV so voltage gated potassium channels close

• Action of sodium-potassium pump returns axon to resting potential

What is an action potential

A nerve impulse which is carried along neurone

What is a generator potential

Occurs when neurone becomes depolarised

Action potential graph

What are waves of depolarisation

• When action potential occurs sodium ions enter neurone and diffuse sideways stimulating sodium channels in next region of neurone to open and sodium ions to diffuse into the area

What is the refactory period

• Period of time in which ion channels cannot be stimulated again

• This prevents propagation of action potentials backwards along axon

What is salutatory conduction

• Only occurs in myelinated neurones as depolarisation can only occur at nodes of Ranvier

• Electrical impulses jump from node to node as waiting for Na⁺ channels to open takes time

What other ways can action potential transmission be sped up

• Axon diameter

• Temperature

How can bigger stimulus be detected

As size of action potential is always the same the more frequent an action potential the bigger the stimulus

What is the action of neurotransmitter GABA

GABA reduces the number of action potentials in the postsynaptic neurone by opening chloride ion channels in the post-synaptic membrane which cause hyperpolarisation

What are transducers

1. Detect a stimulus

2. Stimulus energy can be converted into electrical energy

3. This can trigger generator potential

4. If threshold is reached action potential is generated transmitting impulses

How can the immune system damage the nervous system

1. Antigens on a neurone activate immune system

2. Antibodies against antigens produced

3. Phagocytes break down neurones