Neurones Flashcards
Give descriptions of: Cell body Dendrons Axon Schwann cell Myelin shealth Nodes of ranvier
Cell body, the nucleus and nucleolus containing many rough endoplasmic reticulum
Dendrons, extensions of cell body, subdivide into dendrites
Axon,long fibre taking impulses from cell body
Schwann cells- surrond axon, insulating layer
Myelin sheath - covers axon made of schwann cells
Nodes of ranvier - gaps between schwann cells
Define a nerve impulse
A self propagating wave of electrical excitation, where the resting potential is reversed to an action potential
Describe how the resting potential is formed, give a value in volts for the resting potential
Sodium potassium pump, 3Na+ pumped out for 2K+ pumped in
Voltage gated Na+ channels closed
Voltage gated K+ channels open hence k+ diffuse back out
Forms a potential difference between inside and outside the membrane.
Around -70mv
Describe how an action potential is formed
Receptors create a generator potential
If the generator potential meets the threshold value of -55mV the voltage gated Na+ channels will open
Na+ floods in via diffusion and hence the potential is reversed
What happens after an impulse has been generated
Where the potential reaches +40mV inside the axon
The voltage gated sodium channels close and depolarisation occurs to resume the resting potential
Describe the refractory period
Na+ channels remain closed,
And due to hyper polarisation, there is a time gap between when the organism can fire impulses.
Sodium potassium pump takes time to re induce the resting potential, another impulse cannot be sent in this period
Describe how an action potential is carried in a myelinated neurone opposed to an unmyelinated neurone
Unmyelinated:
- action potential opens next V-G gated Na+ channel
Myelinated
- action potential occurs at the nodes of ranvier
- travels through the myelinated section
- meets the next node and opens Na+ volated gated channels there
What is saltatory conduction?
Saltatory conduction is much faster, describes the movement within a myelinated neurone
Give three factors that effect speed of a neurone s
Myelination
Axon diameter
Temperature - ATP production needs enzymes, atp needed for Na+/K+ pump
- Diffusion occurs quicker at higher temperatures
What three purposes does the refactory period serve?
Unidirectional - because an action potential cant be triggered in a refractory period area
Discrete impulse - ensures a constant amount of time between impulses
Limits the number of action potentials
Describe nervous transmission as an all or nothing response
Stimulus must meet threshold value to meet action potential
Any stimulus that has greater than threshold produces one single impulse
Hence an action potential, if transmitted is the same irrespective of stimulus intensity
Nervous transmission is an all or nothing response, how then does the body perceive intensity of a stimulus
Impulse frequency
Some neurones may have different thresholds, brain interprets which neurone has sent impulse and hence intensity
The sodium channels on the post synaptic membrane are said to be ligand gated, what does this mean?
They open when a ligand attaches to them e.g. Acetylcholine
How many molecules of acetylcholine need bind to a ligand gated Na+ channel to open it?
2
Describe how an impulse is transmitted across a synapse
Action potential arrives at pre synaptic knob
Ca2+ v.g gated channels open
Ca2+ diffuses in
Ca2+ causes vesicles to fuse to presynaptic membrane
Exocytosis allows acetylcholine to move into the synaptic cleft
Acetylcholine binds to ligand gated Na+ channels on post synaptic membrane, allows depolarisation
