Grey Matter

3 basic types of neurones 

SENSORY neurone 

• Transmit impulses from receptor --> CNS

RELAY neurone

• major component of the integration centre
• connects sensory to motor

MOTOR neurone

• Transmit impulses from CNS --> effectors

Schwann cells - Protect the nerves

      - Speed up transmission of nerve impulses

Saltatory conduction = Jumping of action potential from one  node of Ranvier to the next.

• Myelin sheath act as an electrical insulator
• Depolarisation only occers at the nodes of Ranvier
• Action potential jumps from node to node
• Increasing the conduction velocity

Speed of transmission influenced by

• Diameter of axon (increase diameter,increase speed)
• Myelin sheath
• Temperature(increase temp,increase speed)

Resting potential = Difference in charge which exists between the inside and outside of the cell membrane when neurone is NOT transmitting an impulse

Maintained by 

• Sodium-potassium pumps - 3 sodium out,2 potassium in
• Axon membrane more permeable to pottasium ions (because more potassium channels)
• More potassium can leak out back again faster tha sodium can diffuse in
• In more -ve ----> polarised

All or nothing law :

• Action potential is fixed in size
• action potential produced in full as result of stimulus greater than threshold or not at all.

• Sodium gates open
• Sodium ions diffuse into cytplasm down concentration gradient
• Membrane depolarises
• Threshold level reached --> action potential generated
• once reached +40mC,sodium gates close ---> potassium gates open
• Potassium diffuse out ---> repolarisation
• Potassium gates slow to close so excess potassium leave the axon
• Making inside more -ve ----> hyperpolarisation
• Sodium gates close --> sodium-potassium pump restarts

How resting potential restored?

• Potassium gates close
• Potassium ions diffuse back into axon
• Sodium potassium pump restarts

• Action potential arrives
• Causing calcium channels to open ---> Ca ions enter the presynaptic neurone
• Influx of calcium ions causes the synaptic vessicle containing neurotransmitter  to move to presynaptic membrane and fuse with the membrane.
• Neurotransmitters released into synaptic cleft
• By exocytosis
• Neurotransmitter diffuse across the synaptic cleft.
• Bind to receptors on postsynaptic membrane
• Na channels open --->Na ions diffuse into postsynaptic neurone
• Depolarisation of postsynaptic membrane
• Threshold level reached --->action potential generated

P.s. Enzymes break down the neurotransmitters

Summation - Simultaneous release of neurotransmitter from synapses of several neurones to cause enough depolarisation to generate action potential

• Temporal - Different neurones arrive at one time
• Spatial - One neurones in quick succession

Accomodation 

• Too many action potentials in rapid succession
• A short rest to restore more vessicles
• IGNORE REPEATED HARMLESS STIMULI

Adaptation

• Constant stimulation of the receptor cell -->gradual decline in response
• Concentrate on more important,new stimuli
• DO NOT RESPOND TO SAME STIMULI

In the dark

• Sodium pumo establish ion gradient
• High conc of Na+ outside rod cells
• Na+ channels in the membrane of outer segment opens
• Na+ constantly diffuse in
• But move out again by sodium pump
• Resting potential produced
• Inhibitory neurotransmitter released
• No depolarisation --> no action potential generated

In the light

• Light energy absorbed by rhodopsin
• Cis-retinal change to trans-retinal (bleaching)
• Can no longer bind to opsin
• Bleaching cause Na+ channel to close -->less Na+ ions diffuse in
• Causing hyperpolarisation
• No inhibitory neurotransmitter released
• Bipolar cells depolarised
• Action potential formed

• Light falling on the sensory cells of the retina causes impulse to travel along neurones in the optic nerve to the brain
• Impulses detected in control centre in the midbrain
• Synapse with branches of parasympathetic cranial nerve (oculomotor)
• Which transmit impulses to the iris
• Which then stimulate the muscles in the iris
• Circular muscles contract,radial muscles relax
• So pupul constricts

If the converse way..

-sympathetic nerve

-circular relax,radial contract

-pupil widen