Nerves and Synapses Flashcards
Neurone Structure
Cell body (contains cell organelles)
Dendrons branch into dendrites (carry nerve impulses to cell body)
Axon (carries nerve impulses away from cell body)
Schwann Cell (around axon for electrical insulation)
Myelin sheath (covers axons made up of Schwann cell membranes)
nodes of Ranvier (gaps in myelin sheath)
Differences in Types of Neurones
Motor: cell body on one end
Sensory: cell body in dorsal root ganglion (attached on the middle of axon)
Intermediate: central cell body with several dendrons and one axon branching off it.
Resting Potential
Established by sodium-potassium pump.
3 Na+ ions actively transported out, 2K+ actively transported in.
Sodium ion channels closed so Na+ ions can not diffuse back in.
K+ ion channels open so K+ ions diffuse out
Therefore negative resting potential of -70mV
Action Potential Generated
Stimulus causes some Na+ ion channels in axon to open, allowing Na+ ions to diffuse in.
Axon becomes depolarised (more positive) and this opens voltage gated Na+ ion channels. So more Na+ ions diffuse in.
This triggers an action potential of +40mV.
All or Nothing Action Potential
The stimulus has to be large enough to reach the threshold for an action potential to be triggered. If it isn’t no action potential generated.
All action potentials triggered are roughly the same size.
Repolarisation and Hyperpolarisation
Once +40mV is reached, voltage gated Na+ ion channels close, but the K+ ion channels remain open. And more voltage gated K+ ion channels open. K+ ions diffuse out repolarising the axon.
Hyperpolarisation occurs where too many K+ ions diffuse out so the axon is more negative than usual.
The voltage gated K+ ion channels close and sodium potassium pump establishes resting membrane potential
Action Potential Along Unmyelinated Axon
An action potential in one region of an axon, causes the voltage gated Na+ ion channels to open in the next region down the axon, causing it to be depolarised and triggering an action potential in that region. This continues and the action potential travels down the axon.
Action Potential Along Myelinated Axon
Myelin sheath provides electrical insulation so action potentials can occur, they can only occur at the nodes of Ranvier
So they jump from one node to the next.
This is faster as the action potential doesn’t have to travel the full length of the axon.
Refractory Period and Importance
After an action potential is created in one region of the axon, the voltage gated Na+ ion channels are closed in that region.
Prevents an action potential being triggered again for a period of time (refractory period)
1) Ensures the nerve impulse travels in one direction
2) Separates two consecutive impulses
Factors Affecting Speed at which Action Potential Travels
1) Myelin Sheath
2) Diameter of axon: larger diameter, faster speed, as less leakage of ions
3) Temperature, higher temp, faster speed, faster rate of respiration and diffusion. Temp to high proteins denature
Synapse Features and Function
Unidirectionality: as neurotransmitter only made in presynaptic neurone and their specific receptors only on postsynaptic neurone
Spatial Summation: there can be multiple presynaptic neurones to one post so that the threshold is reached in the postsynaptic neurone
Temporal Summation: single presynaptic releases transmitter very frequently over short period to reach threshold in post
Transmission across Synapse
Action potential in end of pre synaptic neurone.
Causes Ca2+ ion channels to open, Ca2+ ions diffuse into the presynaptic neurone.
Causes vesicles containing acetylcholine (ACh) to fuse with the neurone membrane and release ACh into the synaptic cleft.
ACh diffuses across synaptic cleft and binds to specific ACh receptors on post synaptic membrane which opens ligand gated Na+ ion channels.
Na+ ions diffuse into postsynaptic neurone
Depolarises neurone and causes voltage gated Na+ ion channels to open creating action potential.
Resetting Synapse
Acetylcholinesterase hydrolyses the ACh into choline and ethanoic acid which diffuse across synapse into presynaptic neurone where ATP is used to recombine into ACh. Recycles and prevents a second action potential in post.
Ca2+ ions actively transported out of presynaptic neurone
Inhibitory Synapse
Neurotransmitter binds to specific receptors on postsynaptic membrane and causes Cl- channels and K+ channels to open. Cl- ions diffuse in, K+ diffuse out, causing hyperpolarisation preventing an action potential being triggered.
Effect of Drugs on Synapses
Increase Action Potential: inhibit acetylcholinesterase. Mimic neurotransmitter (e.g similar shape to ACh)
Decrease action potential: inhibit release of ACh or action of ACh by binding to receptors specific to ACh to prevent ACh binding.
Neuromuscular Junctions
Synaptic Cleft between a presynaptic neurone and skeletal muscle cell
Always excitatory, always motor neurone.
ACh binds to receptors on muscle cell surface rather than a postsynaptic neurone.
The nerve impulse ends at the junction.
