Nervous Coordination Flashcards
Describe the general structure of a motor neuron
Cell Body:Contains organelles and a 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 mylein sheath
Name 3 processes Schwann Cells are involved in
•Electrical insulation
•Phagocytosis
•Nerve regeneration
How does an action potential pass along an unmyelinated neurone?
1.Stimulus leads to influx of Na+.1st Section of membrane depolarises
2.Local electrical currents causes Na+ VG channels further along membrane to open.Meanwhile,section behind repolarises.
3.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 cant occur when myelin sheath acts as an electrical insulator. So impulse doesn’t travel along whole axon length
What is resting potential?
Potential difference across neurone membrane when not stimulated. Usually about -70mv in humans
How is resting potential established?
1.Membrane is more permeable to K+ to Na+
2.Na/K pump actively transports 3Na+ out of cell and 2K+ into cell
Establishes electrochemical gradient:Cell contents more negative than extracellular environment
Name the stages in generating an action potential
1.Depolarisation
2.Repolarisation
3.Hyperpolarisation
4.Return to resting potential
What happens during depolarisation?
1.Facilitated diffusion of Na+ ions into cell down electrochemical gradient
2.P.d across membrane becomes more positive
3.If membrane reaches threshold potential VG Na+ channels open
4.Significant influx of Na+ reverses p.d to +40mv
What happens during repolarisation
1.VG Na+ channels close and VG K+ channels open
2.Facilitated diffusion of K+ out of cell down their electrochemical gradient
3.P.d across membrane becomes more negative
What happens during hyperpolarisation?
1.’Overshoot’ when K+ ions diffuse out =p.d becomes more - than RP
2.Refractory period:no stimulus is large enough to raise membrane potential to threshold
3.VG K+ channels close & Na/K pump reestablishes resting potential (RP)
Explain the importance of the refractory period
No AP can be generated in hyper-polarised 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 potential will generate an action potential
All APs 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
•Less resistance to flow of ions (depolarisation & 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 reestablish resting potential
How can an organism detect the strength of a stimulus?
Larger stimulus raises to threshold potential more quickly after hyperpolarisation = greater frequency of impulses
What is the function of synapses?
Electrical impulse cannot travel over junction between neurones
Neurotransmitters send impulses between neurons/from neurons to effectors.
New impulses can be initiated in several different neurons from multiple simultaneous responses
Describe the structure of a synapse
Presynaptic neuron ends in synaptic knob:contains lots mitochondria, ER & vesicles of neurotransmitter
Synaptic Cleft: 20-30nm gap between neurons
Postsynaptic neuron:Has complementary receptors to neurotransmitters
Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another
1.Wave of depolarisation travels down presynaptic neuron,causing VG Ca2+ channels to open
2.Vesicles move towards & fuse with presynaptic membrane
3.Exocytosis of neurotransmitter into synaptic cleft
