Nervous coordination Flashcards
Describe the general structure of a motor neuron.
Cell body: Contains
Dendrons: branch
Axon: long
Dendron - towards
Axon: away
Cell body: contains organelles & high proportion of RER.
Dendrons: branch into denrites which carry impulses towards cell body.
Axon: a single long unbranched fibre that carriers nerve impulses away from cell body.
Describe the additional features of a myelinated motor neuron.
Wrap
made from
short between
- Schwann cell: wrap around the axon many times.
- Myelin sheath: made from myelin-rich membranes of schwann cells.
- Nodes of Ranveir: very short gaps between neighbouring Schwann cells where there is no myelin sheath,
Name 3 processes Schwann cells are involved in.
- Electrical insulation
- Phagocytosis
- Nerve regeneration.
How does an action potential pass along an unmyelinated neuron?
- points
- Stimulus leads to influx of Na ions. First setion of membrane depolarises.
- Local electrical currents cause sodium voltage-gated channels
further along membrane to open. Meanwhile, the section behind begins to repolarise. - Sequential wave of depolarisation
Explain why myelinated axons conduct impulses faster than unmyelinated axon
what happens to impulse
what cannot occur
what does this mean for the impulse
Saltatory conduction: Impulse “jumps’ from one node of Ranivier to another.
why?: Depolaristion cannot occur where myelin sheath acts as electrical insulator.
meaning: So impulse does not travel along whole axon length.
What is resting potential?
Potential difference (voltage) across neuron membrane when not stimulated (-50 to -90mV< usually about -70 mV in humans).
How is resting potential established?
adaptation of membrane
what comes in and out
cell contents are more..
- Membrane is more permeable to K⁺ than Na⁺.
- Sodium-potassium pump activetly transports 2Na⁺ out of cell & 2K⁺ into cell.
- Establishes electrochemical graident: cell contents more negative than extracelluary enviroment
Name the stages in generating an action potential.
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to resting potential.
What happens during depolarisation?
- Stimulus → facillitated diffusion of Na⁺ ions into cell down electrochemical gradient.
- p.d. across membrane becomes more postive.
- If membrane reaches theshold potential (-50mV), voltage-gated Na⁺ Channels open.
- Significantly influx of Na⁺ ions reverses p.d. to +40mV.
What happens during repolarisation?
- Voltage-gated Na⁺ channels close and voltage-gated K+ channels open.
- Facillitated diffusion of K⁺ ions our of ccell down their electrochemical graident.
- p.d. across membrane become more negative.
What happens during hyperpolarisation?
- ‘Overshoot” when K⁺ ions diffuse out = p.d becomes more negative than resting potential.
- Refractory period: no stimulus is large enough to raise membrane potential to threshold.
- Voltage-gated K⁺ channels close & sodium-potassium pump re-establishes resting potential.
Explain the important of the refractory period.
No action potential can be generated in hyperpolarised sections of membrane:
- Ensures unidirectional impulse
- Ensures discrete impulses
- Limits frequency of impulse transmission
What is the ‘all or nothing” principle?
Any stimulus that causes the membrane to reach threshold potenital will generate an action potential
All action potential have same magnitude.
Name the factors that affect the speed of conductance.
- Myelin sheath
- Axon diameter
- Temperature
How does axon diameter affect the speed of conductance?
Greater diameter = faster
- Les resistance to flow of ions (depoloraisation & repolarisation).
- Less ‘leakage’ of ions (easier to maintain membrane potential).
How does temperature affect speed of conductance?
Higher temperature = faster
- Faster rate of diffusion (depolarisation & repolarisation)>
- Faster rate of respiration (enzyme-controlled_ = more ATP for active transport to re-establish resting potential.
Temperature too high = membrane potential denature.
Suggest an approraite statistical test to determine whether a factor has a significant effect on the speed of conductance.
Student’s t-test (comparing means of continous data.)
Suggest appropriate units for the maximum frequency of imoulse conduction.
Hz.
How can an organism detect the strength of a stimulus?
Larger stimulus raises membrane to theshold potential more quickly after hyperpolarisation = greater frequency of impulses.
What is the function of synapses>
Electrical impulse cannot travel over junction between neurons.
- Neurotransmitters send impulses between neurons/ from neurons to effectors.
- New impulses can be intitated in several different neurons for multiple simultaneous responses.
Describe the structure of a synapse.
Presynaptic neuron ends in a synaptic knob: contain lots of mitochondria, endoplasmic reticulum & vesciles of neurotransmitter.
Synaptic clef: 20-30 nm gap between neurons.
Postsynaptic neuron: has complemntary receptors to neurotransmitter (ligand-gated Na⁺ channels).
Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another.
- Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca²⁺ channels to open.
- Vesciles move towards & ruse with presynaptic membrane.
- Exocytosis of neurotransmitter into synaptic cleft.
How do neurotransmitters cross the synaptic cleft?
Via simple diffusion
Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another.
- Neurotransmitter binds to specific receptor on postynaptic membrane.
- Ligand-gated Na⁺ channels open.
- If influx of Na⁺ ions raises membrane to threshold potential, action is generated.
Explain why synaptic transmission is unidirectional.
Only presynaptic neuron contain vesicles of neurotransmitter & only postynaptic membrane has complementary receptors.
so impulse always travels presynaptic → postsynaptic.
Define summation and name the 2 types.
Neurotransmitter from several sub-theshold impulses accumulates to generate action potential:
- Temportal summation
- Spatial summation
NB no summation at neuromuscular junctions.
What is the difference between temportant and spatial summation>
Temportal: one presynaptic neuron releases neurotransmitter several times in quick succession.
Spatial: multiplse presynaptic neurons release neurotransmitter.
What are cholinergic synapses?
Use acetylcholine as primary neurotransmittery,
Exicitaroy or inhibitiory. Located at:
- Motor end plate (muscle contraction).
- Preganglionic neurons (exicitation).
- Parasympathetic postganglionic neurons (inhibitation e.g. of heart of breathing rate).
What happens to acetylcholune from the synaptic cleft?
- Hydrolysis into acetyl choline by acetylcholinesterase (AChE).
- Acetyle & choline diffuse back into presynaptic membrane.
- ATP is used to reform acetylecholine for storage in vesicles.
Explain the importance of AChE.
- Prevents overstimulation of skeletal musles cells.
- Enables acetyl and choline to be recyled.
What happens in an inhibitory synapse?
- Neurotransmitter binds to and open Cl⁻ channels on postynaptic membrane & triggers K+ channels to open.
- Cl⁻ moves in & K+ out via facilitated diffusion.
- p.d. becomes more negative: hyperpolarisation.
Describe the structure of a neuromuscular junction.
Synaptic cleft between a presynaptic neuron and skeletal muscle cell.
Contrast a cholinergic synapse and a neuromuscular junctio.
Refer to PMT table
Difference (1) Postsynaptic cell AChE location Action potential Response Neurons involved
Cholinergic (2)
Refer to PMT table
Neuromuscular (3)
Refer to PMT table
How might drugs increase synaptic transmission?
- Inhibit AChE
- Mimic shape of neurotransmitter.
How might drugs decrease synaptic transmission?
- Inhibit release of neurotransmitter.
- Decrease peremabiity of postsynaptic membrane to ions
- Hyperpolarise postsynaptic membrane.
