Nervous system Flashcards
What is the structure of a nerve cell?
Soma (cell body)
Dendrites
Axon
Axon terminals
What are afferent neurons?
Neurons that come from tissues/ organs
Cell body outside of CNS
What are efferent neurons?
Neurons that come from CNS to effector organs
Cell body and dendrites within the CNS
What is included in the reflex arc?
Sensory receptor - Sensory neuron - Relay neuron - Motor neuron (interneuron) - Effector
What is an effector?
A muscle or gland that responds to motor impulses
What is Acetylcholine and what does it do?
A neurotransmitter found in the brain and neuromuscular junctions.
Synthesised from choline
It acts as a chemical message .that is released by neurons and allows them to communicate with one another and other specialized cells
Broken down by the enzyme acetylcholinesterase.
What is a neuromuscular junction?
A synapse between an axon and a muscle cell
What is hyperpolarisation?
When the membrane is more negative than its resting value
What is repolarisation?
When membrane potential is back at its resting value, Na+ channels are closed
What is the refractory period?
The minimum amount of time after an action potential has occurred before another can happen.
What is saltatory conduction?
When the impulse jumps along the nodes of ranvier. This occurs in myelinated neurons. It allows conduction of action potential to be faster the impulse is not going through axon walls.
What can harm the myelin sheath? (Demyelination)
- Stroke
- Inflammation
- Immune disorders
- Nutritional deficiencies
- Metabolic disorders
- Excess drug and alcohol use
What initiates action potential?
Outside the body stimulus e.g. touch
Inside the body stimulus e.g. change in concentration of carbon dioxide in the blood
What is an excitatory neurotransmitter?
A neurotransmitter with stimulating effect
Increases chance of action potential
e.g. epinepherine
What is an inhibitory neurotransmitter?
A neurotransmitter than inhibits neurones
Decreases likelihood of action potential
E.g. GABA, endorphins
What are neuromodulators?
Neurotransmitters that effect a large number of neurons at one time and can influence other neurotransmitters
They don’t activate receptors of neurons but work with neurotransmitters to enhance their effect.
How is an impulse passed through a synapse?
Chemical synapses
- Action potential arrives at synaptic knob, causing calcium voltage gated channels to open.
- Calcium flows into the synaptic knob and allows vesicles to fuse with the cell membrane releasing the neurotransmitters, via exocytosis, across the synaptic cleft
- The neurotransmitters bind to the receptors on postsynaptic synapse
- Sodium channels open so sodium diffuses in
- Depolarisation occurs and action potential on post synaptic neurone is generated if threshold is reached
What is the mechanism of Na-K ATPase pump
- 3x Na bind to a transporter protein with high affinity
- The binding activates enzyme activity of ATP
- Phosphorylation occurs on the surface of the protein and ADP is released
- This causes a conformational change exposing Na to the extracellular fluid
- This then reduces the affinity for Na and Na is released
- It now has a high affinity for K, 2 bind to the extracellular surface of the transporter protein
- Dephosphorylation occurs
- Transporter protein returns to its original shape
- It now has a low affinity for K, so K is released into the intracellular fluid
Describe resting potential
Inside the cell is more negatively charged than the outside.
There are more K in the cell and more Na outside the cell
Na-K atpase pump actively transports 3 sodium ions out for every 2 potassium in
Some K channels are open whilst Na channels are closed so K ions diffuse back out decreasing the potential further
Describe how an action potential is generated
- The cell is at resting state, all Na channels are closed, some K channels are open
- A stimulus causes some Na channels to open and are released down their electrical and concentration gradient.
- If the potential difference threshold of -55mv is reached more Na channels open causing a higher influx of Na ions
- K ion channels open and leave the cell down its concentration gradient however is slowed by its electrical gradient
- Once the potential difference of around 40mv is reached, Na ion channels close
- K ions continue to leave bring the membrane down to its resting potential level. K is driven by both electrical and concentration gradients
- Hyperpolarisation occurs as K channels are slow to close and the resting potential level is overshot as too many K ions diffuse out
- K channels close and Na-K ATPase returns membrane to its resting potential
What is saltatory conduction?
Instead of the current going through the axon walls, the myelin creates insulation which means that the currents has to jump along the nodes of ranvier. This causes the conduction of action potential to be much faster.
What is the refractory period?
Minimum amount of time after an AP has occurred before another one can happen. The shorter the period the quicker the the frequency of the signal generated by the axon.
After this there is a relative refractory period which requires a larger signal to generate an action potential.
Absolute refractory period is when an action potential is reached no matter the size of the signal immediately after an action potential. Na channels closed.
How can temperature effect conduction speed?
It can increase it if its up to 40’C as the ions diffuse faster
But too high and the enzymes denature and the speeds decreases
Explain how acetylcholine is reabsorbed
- Acetylcholinsterase hydrolyses acetylcholine into acetyl and choline, which then diffuse back into the presynaptic neuron.
- Mitochondrial ATP recombines acetyl and choline into acetylcholine which is then stored in vesicles
- Na channels close in postsynaptic neuron receptor sites now there is no longer acetylcholine
How can drugs effect synapses?
They can cause stimulation of the nervous system by creating more action potentials in the post synaptic neurone:
- When the drug is the same shape as the neurotransmitter they can mimic their action at the receptor and activate them, causing an AP
- They can stimulate the release of neurotransmitter they then bind and activate receptors of the the PostNS
- They could inhibit enzymes which break down neurotransmitters so there are more to bind to the receptors and they stay there for longer
They can inhibit nervous system by causing fewer action potentials in the PostSN
- Block receptors so they cant be activated
- Inhibit release of neurotransmitters from preSN so fewer receptors are activated
