6.2. Nervous Coordination Flashcards
Describe the general structure of a motor neurone
- Cell body: contains organelles and high proportion of RER
- Dendrons: branch into dendrites which carry impulses towards cell body
- Axon: long, unbranched fibre carries nerve impulse away from cell body
Describe the additional features of a myelinated motor neurone
- Schwann cells: wrap around axon many times
- Myelin sheath: made from myelin-rich membranes of Schwann cells
- Nodes of Ranvier: 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 regulation
How does an action potential pass along an unmyelinated neurone?
- Stimulus leads to influx of Na+ ions. First section 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 axons
- Saltatory conduction: impulse ‘jumps’ from one node of Ranvier to another
- Depolarisation cannot occur where myelin sheath acts as electrical insulator
- So impulse does not travel along whole axon length
What is resting potential?
- Potential difference (voltage) across neurone membrane when not stimulated (-70mV)
How is resting potential established?
1) Membrane is more permeable to K+ than Na+
2) Sodium potassium pump actively transports 3Na- out of cell and 2K+ into cell
3) Establishes electrochemical gradient: cell contents more negative than extracellular environment
Name the stages in generating an action potential
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to resting potential
What happens during depolarisation?
1) Stimulus -> facilitated diffusion of Na+ ions into cell down electrochemical gradient
2) Potential difference across membrane becomes more positive
3) If membrane reaches threshold potential (-50mV), voltage gated Na+ channels open
4) Significant influx of Na+ ions reverses potential difference to +40mV
What happens during repolarisation?
1) Voltage gated Na+ channels close and voltage gated K+ channels open
2) Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
3) Potential difference across membrane becomes more negative
What happens during hyperpolarisation?
1) ‘Overshoot’ when K+ ions diffuse out = potential difference becomes more negative than resting potential
2) Refractory period: no stimulus is large enough to raise membrane potential threshold
3) Voltage gated K+ channels close and sodium potassium pump re-establishes resting potential
What is the refractory period?
No action potentials can be generated in the hyperpolarised sections of the membrane
Explain the importance of the refractory period
- Ensures unidirectional impulse
- Ensures discrete impulse
- Limits frequency of impulse transmission
What is the ‘all or nothing’ principle?
Any stimulus that causes the membrane to reach threshold potential will generate an action potential
Name the factors that affect the speed of conductance
- Myeline sheath
- Axon diameter
- Temperature
How does axon diameter affect the speed of conductance?
- Greater diameter = faster
- Less resistance to flow of ions (depolarisation and 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 and repolarisation
- Faster rate of respiration = more ATP for active transport to re-establish resting potential
What happens to the speed of conductance if the temperature is too high?
Membrane proteins denature so slower speed of conduction
Suggest an appropriate statistical test to determine whether a factor has a significant effect on the speed of conductance
Student’s t-test (comparing means of continuous data)
Suggest appropriate units for the maximum frequency of impulse conduction
Hz
How can an organism detect the strength of a stimulus
Larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses
Function of synapses
- Electrical impulse cannot travel over junction between neurones
- Neurotransmitters send impulses between/from neurones to effectors
- New impulses can be initiated in several different neurones for multiple simultaneous responses
Describe the structure of a synapse
- Pre-synaptic neurone ends in a synaptic knob: contains lots of mitochondria, endoplasmic reticulum and vesicles of a neurotransmitter
- Synaptic cleft: 20-30 nm gap between neurones
- Post synaptic neurone: has complementary receptors to neurotransmitter
Outline what happens in the presynaptic neurone when an action potential is transmitted from one neurone to another
1) Wave of depolarisation travels down presynaptic neurone, causing voltage gated Ca2+ channels to open
2) Vesicles move towards and fuse with presynaptic membrane
3) Exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross the synaptic cleft?
Simple diffusion
Outline what happens in the post synaptic neurone when an action potential is transmitted from one neurone to another
1) Neurotransmitter binds to specific receptor on post synaptic membrane
2) Ligand-gated Na+ channels open
3) If influx of Na+ ions raises membrane to threshold potential, action potential is generated
Explain why synaptic transmission is unidirectional
- Only presynaptic neurone contains vesicles of neurotransmitter
- Only post synaptic membrane has complementary receptors
Define summation
- Neurotransmitter from several sub-threshold impulses accumulates to generate action potential
- No summation at neuromuscular junctions
Name the two types
- Temporal summation
- Spatial summation
What is the difference between temporal and spatial summation?
- Temporal: one presynaptic neurone releases neurotransmitter several times in quick succession
- Spatial: multiple presynaptic neurones release neurotransmitter
What are cholinergic synapses?
- Use acetylcholine as primary neurotransmitter
- Excitatory or inhibitory
Where are cholinergic synapses located?
- Motor end plate (muscle contraction)
- Preganglionic neurones (excitation)
- Parasympathetic postgangionic neurones (inhibition e.g. of heart or breathing rate)
What happens to acetylcholine from the synaptic cleft?
1) Hydrolysis of acetyl and choline by acetylcholinesterase (AChE)
2) Acetyl and choline diffuse back into presynaptic membrane
3) ATP is used to reform acetylcholine for storage in vesicles
Explain the importance of AChE
- Prevents overstimulation of skeletal muscle cells
- Enables acetyl and choline to be recycled
What happens in an inhibitory synapse?
1) Neurotransmitter binds to and opens Cl- channels on post synaptic membrane and triggers K+ channels to open
2) Cl- move in and K+ move out via facilitated diffusion
3) Potential difference becomes more negative: hyperpolarisation
Describe the structure of a neuromuscular junction
Synaptic cleft between a presynaptic neurone and a skeletal muscle cell
Contrast a cholinergic synapse and a neuromuscular junction
- Cholinergic has AChE on synaptic cleft, neuromuscular has AChE on postsynaptic membrane
- Cholinergic response is excitatory or inhibitory, neuromuscular is always excitatory
- Cholinergic has motor, sensory, relay neurones involved, neuromuscular only has motor
How might drugs increase synaptic transmission?
- Inhibit AChE
- Mimic shape of neurotransmitter
How might drugs decrease synaptic transmission?
- Inhibit release of neurotransmitter
- Decrease permeability of post synaptic membrane to ions
- Hyperpolarise postsynaptic membrane
