5: Nerves Flashcards
Which organelle is found in large amounts in nerve cells and what is its function?
rough endoplasmic reticulum
involved in the production of proteins and neurotransmitters
Nerve cells are made of axons and dendrons. In relation to the cell body, which direction does each carry the impulse?
Axons carry impulses away from cell body
Dendrons carry impulses towards the cell body
Describe structure and function the 3 types of neurone
Motor - carries impulses to effectors
- long axon and short dendrons, cell body at one end
Sensory - carries impulses away from receptors
- one dendron and one axon
- cell body in the middle
Intermediate - transmit impulses between neurones
- short axons, short dendrons
How is a resting potential maintained in a neurone?
What is the resting potential in mV?
Large negative proteins
Na+K+ATPase pumps 3 Na+ out for every 2 K+ in
Potassium channels more leaky than sodium channels so more K+ leaks out than Na+ leaks in
-77mV
What is the threshold for an action potential in mV?
At what potential difference does depolarisation stop and depolarisation start?
-55mV
+30/+40 mV
Describe depolarisation
Stimulus causes Na+ channels to open
Na+ diffuses in down concentration gradient
This makes the cell more positive
Na+ ions cause more sodium channels to open
Describe repolarisation
At +30mV, sodium channels close so Na+ stops diffusing in
potassium channels open and K+ diffuses out
This makes the potential difference more negative and returns to resting potential
What is hyperpolarisation?
Outward diffusion of K+ ions causes a temporary overshoot of the electrical gradient
What is the refractory period and what 3 purposes does it serve?
The time when an action potential has been created in a region of an axon and it is impossible for a further action potential to be generated
Ensures that action potentials is propagated in one direction
Produces discrete impulses that don’t overlap
Limits the number of action potentials
What is the refractory period and what 3 purposes does it serve?
The time when an action potential has been created in a region of an axon and it is impossible for a further action potential to be generated
Ensures that action potentials is propagated in one direction
Produces discrete impulses that don’t overlap
Limits the number of action potentials
Why are impulses described as all or nothing?
There is a certain level of stimulus, called the threshold value, which triggers an action potential. Below this, no action potential and therefore no impulse. And it does not matter how far above the threshold it is, only one action potential is generated
How is the size of a stimulus perceived? x2
Frequency of action potentials
By having different neurones with different threshold values. The brain interprets the number and type of neurones and therefore determines its size
What 3 factors affect the speed at which an action potential passes along the axon and why?
temperature, particles more energy etc. and respiration to provide ATP is enzyme controlled
axon diameter, less leakage of ions from a large axon
myelin sheath, provides insulation, saltatory conduction
Why do impulses travel faster along myelinated neurones?
electrical insulator
localised circuits only occur at nodes of ranvier
action potentials ‘jump’ from node to node
saltatory conduction is faster than if depolarisation happens along entire length of axon
Why are mitochondria found near synaptic knobs?
Provide ATP to provide energy for movement of vesicles, Na+K+ATPase, neurotransmitter synthesis
What makes synapses unidirectional?
vesicles only in pre-synaptic knob
receptors only on post-synaptic knob
Describe the two types of summation
Temporal - frequent impulses lead to a release of a large amount of neurotransmitter, quantity is above threshold to trigger action potential
Spatial - action potentials from two or more neurones all synapsing with one other neurone, total neurotransmitter from all neuroes is enough to exceed threshold and trigger action potential
Describe synaptic transmission
Action potential causes depolariastion of membrane
Calcium channels open and Ca2+ ions diffuse in
This causes vesicles to fuse with presynaptic membrane and release neurotransmitter in the synaptic cleft
NT diffuses across gap, down concentration gradient
NT binds to recpetors on postsynaptic membrane
This opens sodium channels and new action potential is generated
Enzymes break down NT
Products diffuse back over gap
Taken back up into presynaptic knob
How wide are nodes of ranvier and how often to they appear along human neurones?
2-3 micrometers long
1-3mm
Which potential is passive and which is active?
resting potential is active as the Na+K+ATPase uses ATP
action potential is passive as ions diffuse in and out of channel proteins in the membrane
