Nervous system Flashcards
List the parts of a neurone
- Cell body
- Dendrites
- Axon
- Myelin sheath
- Nodes of Ranvier
- Axon terminal
Describe the cell body
Contains the organelles. Proteins + neurotransmitter chemicals are made here.
Describe the dendrites
Branch out from the cell body. Carry action potential to surrounding cells
Describe the axon
A long conductive fibre that runs all the way through the cell. Carries nervous impulses along neuron.
Describe Schwann cells
Wrap around the the axon to form the myelin sheath which is a lipid = acts as an insulator therefore charge can only pass through the gaps = Nodes of Ranvier
What is the resting potential and it’s charge?
- When a neuron isn’t conducting impulses = a difference between the charge inside/outside.
- There are more positive K+/Na+ outside than inside therefore inside more negative
- The charge is -70 mV
How is resting potential established and maintained?
- Through the Na+/K+ pump the resting potential is maintained
- 2 K+ in and 3 Na+ out are actively transported through the pump
- This creates a electrochemical gradient which allows facilitated diffusion of K+ out from higher to lower and Na+ to diffuse in from higher to lower
- But membrane is more permeable to K+ = more move out = -70mV charge
- There are more K+ protein channels than Na+ and some of the Na+ channels close while all of K+ are open
What is the action potential and it’s charge?
- When the neurons voltage increases beyond a set point from the resting potential = nervous impulse
- This is called depolarization
- Due to the membrane being more permeable to Na+ = positive charge produced
- The charge is +40mV
How is the action potential generated?
- A stimulus is introduced to the
-70mV = enough energy to open the voltage gated Na+ channels = Na+ going in at the same time as K+ out = voltage increase - If voltage increases more than -55 mV = more energy to open voltage gated Na+ channels = sharp increase in voltage to +40 mV = DEPOLARIZATION
- Voltage reaches +40 mV = action potential achieved but cannot go over +40 mV as this causes voltage gated Na+ channels close / 2 K+ channels to open so double K+ diffused out = voltage drop = REPOLARIZATION
- Voltage drop overshoots beyond charge for resting potential = HYPERPOLARIZATION
Explain the all or nothing principle
NOTHING:
If depolarization does not go over -55 mV = action potential not reached = no impulse produced
ALL:
- If depolarization goes over - 55 mV = action potential produced and will peak at the same maximum voltage of +40 mV
- A bigger stimuli will increase frequency of action potential peaks
Why is the refractory period important?
In this period the axon cannot be stimulated and the Na+ channels are recovering
1) Discrete impulses produces: action potentials are separate from each other so each impulse can be processed separately
2) Action potential travels in 1 direction: prevents spread of action potential in 2 directions = prevent response to stimulus
3) Limits number of impulses that can be produced: limit number of action potentials produced = prevent over reaction to stimulus and overwhelming senses that would hinder your survival instincts
What factors affect the speed of conducting an impulse?
1) Myelination
2) Axon diameter
3) Temperature
How does myelination affect the speed of conductance?
- Schwann cells which are lipids form myelin sheath = insulations so don’t let ions/impulses pass through them so they pass through the gaps = Nodes of Ranvier
- Action potential jump from node to node = SALTATORY CONDUCTION
- Action potential travels faster as it doesn’t need to be produced along the whole length just nodes
How does axon diameter affect the speed of conductance?
- Wider diameter = speed increases
- Wider = less ion leakage = action potential travels faster
How does temperature affect the speed of conductance?
- Higher temperature = faster
- Increases rate of diffusion of ions
- Respiration enzymes work faster = more ATP for active transport of Na+/K+ pump
What is a synapse and it’s functions?
- The gap between the end of the axon of 1 neuron and dendrite of another
- Action potential is transmitted between the synapse by diffusion of neurotransmitters
Explain how the synapse works
- Action potential arrives to the end of pre-synaptic membrane = voltage gated Ca2+ channels in the membrane open = Ca2+ ions diffuse to end of pre-synaptic
- The Ca2+ ions cause the vesicles with neurotransmitters to move to the end of pre-synaptic membrane = fuse with membrane = neurotransmitters released into the synaptic cleft
- Neurotransmitters only in pre-synaptic so when released = high concentration = neurotransmitters diffuse down concentration gradient across cleft to post-synaptic membrane
- Receptors on post-synaptic membrane are complementary to neurotransmitters = binding
- Na+ channels on the receptors open once neurotransmitters bind and Na+ from the cleft diffuse in to post-synaptic neuron
- If enough neurotransmitters/Na+ diffuse = voltage above threshold = action potential produced in post-synaptic neuron
- If neurotransmitters stay permanently attached = responses without stimulus
- Neurotransmitter breaks down and transported back to pre-synaptic neuron = recycled
- Na+ channels close and resting potential re-established
What is cholinergic synapse?
- Acetylcholine is the neurotransmitter
- Enzyme to break it down is AChE = acetate and choline which can be recycled
What is an inhibitory synapse?
- When the neurotransmitter binds to the receptor on the post-synaptic neuron Cl- channels open
- Cl- ions diffuse into post synaptic membrane from cleft
- Negative ions = drop the potential to -80 mV = HYPERPOLARIZATION
- Action potential not produced = prevents a response to the stimulus
What is summation?
Collective build up of neurotransmitters by pre-synaptic neuron to help generate an action potential when there is not a sufficient amount of neurotransmitters to produce an action potential
What are the 2 types of summation?
1) Spatial
2) Temporal
Describe spatial summation
Many pre-synaptic neurons collectively produce neurotransmitters that exceed threshold = action potential produced
Describe temporal summation
1 pre-synaptic neuron releases many sub-threshold impulses = many neurotransmitters repeatedly over short period = adds up to exceed threshold = action potential produced
Explain the neuromuscular junction
- Between the motor neuron and skeletal muscle cell
- End point for an action potential = instead of triggering an action potential in cells it triggers contraction
Compare a synapse and neuromuscular junction
SYNAPSE:
- Unidirectional
- Excitatory or inhibitory
- Connects 2 neurons
- New action potential is generated in post-synaptic neuron
- Acetylcholine binds to receptor on post-synaptic membrane
NEUROMUSCULAR JUNCTION:
- Unidirectional
- Only excitatory
- Connects motor neuron to muscle
- End point for action potential
- Acetylcholine binds to receptor on muscle fiber membrane
Describe the gross structure of the skeletal muscle
Muscle cells have fused together nuclei and cytoplasm that form bundles of myofibrils with a lot of mitochondria
Describe the ultrastructure of a myofibril
- 1 of the fibres that make a muscle cell bundle
- Made of actin (thin filament) and myosin (thick filament) = sarcomere
I BANDS: Only actin on the outsides
H ZONE: Only myosin
A BAND: Overlap of myosin + actin on the middle
Z LINES: Boundaries between sarcomeres
Explain the sliding filament theory
- Action potential travels from motor neuron to muscle cell to stimulate response
- Causes Ca2+ channels to open and Ca2+ ions diffuse from sarcoplasmic reticulum and binds to tropomyosin
- Binding = shifting of tropomyosin to uncover binding sites on actin
- Myosin head with ADP can now bind to exposed binding sites = crossbridge with actin
- Attachment creates tension = pulls actin filament across myosin = ADP released
- New ATP binds to myosin head and the energy causes the shape to change slightly = detachment from actin
- ATPase activated by Ca2+ ions will hydrolyze the ATP to ADP = enough energy to return myosin head to original position
- Continues as long as supply of Ca2+ is provided
What evidence proves the sliding filament theory?
1) H zone narrows
2) I band narrows
3) Z lines get closer: Sarcomere shortens
4) A zone remains the same: Shows that myosin does not shorten
Why is ATP so important for muscle contraction
- Have a lot of mitochondria because we need a very high concentration of ATP for contraction
- During anaerobic respiration phosphocreatine can phosphorylate ADP to regenerate ATP
Describe the slow twitch muscle
STRUCTURE:
- Has many myoglobin which has higher affinity for oxygen than haemoglobin
- Rich blood supply for constant supply of O2 and glucose
- Lots of mitochondria so continuous ATP production due to aerobic respiration
LOCATION:
- Calf muscles
PROPERTIES:
- Contract slowly with long term respiration due to blood supply and myoglobin
- Good for endurance work
Describe fast twitch muscles
STRUCTURE:
- Thicker with more myosin
- Lots of glycogen that can be hydrolyzed into glucose
- Large store of phosphocreatine to make ATP from ADP
- High concentration of enzymes involved in anaerobic respiration
LOCATION:
- Biceps
PROPERTIES:
- Contract fast for short bursts of energy
- Good for intense exercise
