Topic 8

Whats a sensory neuron?

• conduct electrical impulses from receptors to the central nervous system (CNS)

• cell body is found along the axon in middle of neurone.

• region of the axon before the cell body is called the dendron.

• cell body long dendrites and short axon

What’s a relay neurone?

• cell body found in the CNS and us short dendrites and axons.

• integrate input from sensory neurones and output via motor neurones

• have short axons and dendrites but many dendrites

what’s a motor neuron?

• conduct electrical impulses from the CNS to effectors

• cell body at one end

• Dendrites receive impulses from other neurones by forming synapses with other neurones → action potential initiates at dendrites and is propagated along the axon until the terminal branches are reached

• Cell body → contains the nucleus and other organelles e.g. mitochondria

• short dendrites long axon

whats a reflex?

• Rapid automatic responses which protect organisms from harmful stimuli which Involve the nervous system

whats the reflex response?

1) The stimulus is detected by a receptor.

2) The receptor sends electrical impulses to the central nervous system (CNS) via sensory neurones.

3) In the spinal cord, relay neurones conduct the impulses between the sensory neurones and the motor neurones (there are synapses between neurones).

4) The motor neurones conduct impulses to the effector.

5) The effector brings about the response.

what happens when light is the stimuli?

• light enters eye and hits photoreceptors

→ circular muscles contract

→ radial muscles relax

→ pupil constricts so less light enters the eye

when theres no light?

In dim light

→ circular muscles relax

→ radial muscles contract

→ pupil dilates so more light enters the eye

The sodium-potassium pump uses ATP to

actively transport sodium ions (Na+

) out of

the neurone and potassium ions (K+

) into

the neurone → maintains electrochemical

gradients of Na+ and K+

• Voltage-gated Na+ channels are closed

→ diffusion of Na+ into the neurone

down the electrochemical gradient is

very limited

• K+ ion channels are open

→ K+ diffuse out of the neurone down the

electrochemical gradient

Action potential

• The sodium-potassium pump is still working

as in resting potential

• Depolarisation (from -70mV to +40mV)

→ membrane potential rises to threshold

(-50mV) due to an increase in Na+ in the

cytoplasm which triggers some

voltage-gated Na+ channels open

→ Na+ diffuse into neurone down the

electrochemical gradient

→ even more voltage-gated Na+ channels

open, more Na+ diffuse in, and the

membrane depolarises

Voltage-gated channel

proteins only open at a

• The neurone cell membrane is more permeable to K certain potential difference. + than Na+

• The membrane is polarised → has a potential difference of -70mV

whats a synapse?

a small gap where two neurones meet wheres theres a small gap called synpatic cleft

How does the action potential cause the release of a neurotransmitter?

• action potential arrives at axon terminal

• voltage gated Ca2+ channels open

• Ca2+ enters presynaptic neurone

• Ca2+ signals neurotransmitter vesicles.

• Vesicles move to membrane and dock

• neurotransmitters released via exocytosis

• neurotransmitters bind to receptors

• signals iniated in postsynaptic cell