5.1.3 Neuronal Communication Flashcards
(21 cards)
Synapse
Junction between a neuron and another neuron or between another neuron and effector cell.
AP transmitted as neurotransmitters that diffuse across the synapse
Process of synaptic transmission
- An impulse arrives at the end of the presynaptic neurone.
- Vesicles move towards + fuse with the presynaptic membrane - releases neurotransmitters into the synaptic cleft.
- The neurotransmitters diffuse across the synaptic cleft (down a conc gradient)
- Neurotransmitters attach to specific receptors on the postsynaptic membrane.
- This triggers an impulse which travels along the postsynaptic neurone.
- The neurotransmitters are recycled or destroyed once an impulse is sent.
More in depth process of synaptic transmission
- Action potential triggers Calcium influx:
An action potential arrives at the synaptic knob of the presynaptic neurone.
The AP stimulates voltage-gated calcium ion channels in presynaptic neurone to open.
Calcium ions diffuse into the synaptic knob. - Calcium influx causes neurotransmitter release:
Influx of Ca ions into synaptic knob causes synaptic vesicles to move to the presynaptic membrane + fuse.
The vesicles release the neurotransmitter into the synaptic cleft by exocytosis. - Neurotransmitter triggers AP in post synaptic neurone:
Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane.
Causes Na ion channels in postsynaptic neurone to open.
Influx of Na ions into postsynaptic membrane causes depolarisation.
AP on postsynaptic membrane is generated if threshold is reached - allowing impulse to continue down axon.
Neurotransmitter removed from synaptic cleft so response doesn’t keep happening.
Unidirectional impulses
Synapses ensure one-way transmission of impulses - neurotransmitter released on one side, its receptors are on the other - cannot occur in the opposite direction.
Cholinergic synapse
One of the key neurotransmitters used throughout the nervous system is acetylcholine (ACh)
Synapses that use this neurotransmitter are known as cholinergic synapses.
3 main features in neurones
Cell body - contains organelles found in typical animal cell, proteins + neurotransmitter chemicals made here
Dendrons: carry action potentials to surrounding cells
Axon: conductive, long fibre that carries the nervous impulse along the motor neurone
Myelin Sheath features
- Myelinated neurones have Schwann cells - wrap around the axon to form myelin sheath (lipid, doesn’t allow charged ions through)
- Gaps between myelin sheath - nodes of Ranvier
- Action potential jumps from node to node (saltatory conduction) - AP travels along axon faster, doesn’t have to generate an AP along the entire length, only at nodes of Ranvier
Sensory neurones
- Carry electrical impulse from sensory receptor to relay neurone (or to motor + brain)
- Long dendron that carries impulse from sensory receptor cell to cell body of neurone, then an axon to carry the impulse from the cell body to the next neurone
Motor neurone
- Carry impulse from relay or sensory neurone to effector (muscle or gland)
- Have one long axon + multiple short dendrites
Relay neurones
- Carry impulses between sensory + motor neurones
- Have multiple short axons + dendrons
Sensory receptors
- Detect a stimulus
- Transducer cells - convert different types of stimuli into electrical nerve impulses
Photoreceptors (rods + cone cells): light stimulus
Thermoreceptors (skin): heat stimulus
Mechanoreceptor (Pacinian corpuscle in skin): pressure stimulus
Pacinian corpuscle
- Pressure receptor located deep in skin - fingers + feet
- Sensory neurone in Pc has stretch-mediated sodium channels in plasma membrane
- Open + allow Na+ to enter sensory neurone only when stretched + deformed
- Pressure applied, deforms neurone plasma membrane, widens Na+ channels so Na+ diffuses in > leads to establishment of generator potential
Resting potential
- When neurone not conducting an impulse - difference between electrical charge inside + outside of neurone
- More positive ions (Na+, K+) outside compared to inside - more negative -70mV
Establishing a resting potential
- RP maintained by sodium-potassium pump (ATP involved)
- Pump moves 2 K+ in and 3 Na+ out
- Creates electrochemical gradient causing K+ to diffuse out & Na+ to diffuse in
- Membrane more permeable to K+ - more moved out resulting in -70mV
Action potential
- When the neurone’s voltage increases beyond a set point from RP - generates nervous impulse
- Increase in voltage (depolarisation) due to neurone membrane becoming more permeable to Na+ at -55mV
- Action potential generated - moves along axon like a Mexican wave
- At +40mV, Na+ channels close > repolarisation
- K+ pump opens, ions leave > hyperpolarisation, refractory period (cannot generate another AP)
All-or-nothing principle
- If depolarisation does not exceed -55mV, AP + impulse not produced
- Stimulus that triggers depolarisation to -55mV always peaks at the same maximum voltage - bigger stimuli increase frequency of AP
- Makes sure animals only respond to large enough stimuli, rather than responding to every slight change in environment
Refractory period
- After AP generated, membrane enters a refractory period, can’t be stimulated - Na+ channels are recovering + can’t be opened
1. Ensures that discrete impulses are produced. AP cannot be generated immediately after another, makes sure that each is separate
2. Ensures AP travel in one direction - stops AP from spreading out in two directions that would prevent a response
3. Limits number of impulse transmission - prevent over reaction to a stimulus
Summation
Rapid build-up of neurotransmitters in synapse to help generate an AP by two methods:
Spatial summation
Temporal summation
Spatial summation
Many different neurones collectively trigger a new action potential by combining the neurotransmitter they release to exceed the threshold value
Temporal summation
One neurone releases neurotransmitter repeatedly over a short period of time to add up to enough to exceed the threshold value
Inhibitory synapse
- Cause chloride ions to move into the postsynaptic neurone + potassium ions out
- Makes membrane potential decrease to -80mV - hyperpolarisation
- AP is highly unlikely