Neuronal communication Flashcards
What does the sensory neuron do?
transmit impulses from receptors to the CNS
What does the motor neuron do?
transmit impulses from the CNS to the effectors
What does the relay neuron do?
transmit impulses between the other neurons
What do dendrites and dendrons do?
carry impulses to the cell body
What does the axon do?
carry impulses away from the cell body
Briefly describe neuronal communication
- Stimulus is detected by receptor cells and a nerve impulse is sent along the sensory neuron
- when it reaches a synapse, neurotransmitters are transmitted to the relay neurons which send a nerve impulse
- the CNS processes the info, decides what to do, and sends impulses along motor neurons to the effector
How do sensory neurons act as transducers?
they convert the energy of a stimulus into electrical energy
When the nervous system isn’t stimulated, what is the inside of the cell like?
it has a negative charge relative to the outside and there’s a potential difference across the membrane. this is the resting potential being generated.
What is the resting potential generated by?
ion pumps and ion channels
What happens when a stimulus is detected?
the membrane is excited and becomes more permeable, allowing more ions in and out, altering the PD.
-this is the generator potential
When is an action potential triggered?
if the generator potential is big enough and reaches the threshold level
What are Pacinian corpuscles?
they are mechanoreceptors- detect mechanical stimulus found in the skin
What is the basic structure of PCs?
they have sensory nerve endings, wrapped in layers of connective tissue called lamellae
What happens when a PC is stimulated?
the lamellae are deformed and press on the sensory nerve endings which causes a deformation of stretch-mediated Na+ channels in the sensory neuron’s membrane.
- Na+ channels are open so they diffuse into the cell, creating a generator potential
- if it reaches the threshold level, it triggers an AP
What is the membrane like in the resting potential?
it is polarised as the outside is more positively charged
How is the resting potential maintained by Na-K pumps?
- the pumps use active transport to move 3Na ions out of the neuron for every 2K ion moved in.
- K+ channels allow facilitated diffusion of K+ out of the neuron down the conc. grad.
Process of maintaining the resting potential
1-Na-K pumps move Na out but the membrane isn’t permeable to it so it can’t diffuse back in. This creates a Na+ electrochemical gradient.
2-Na-K pumps move K+ in
3- When the cell rests, most K+ channels are open. the membrane is permeable to K+ ions so some diffuse back out through K+ channels
When a neuron is stimulated, are the Na channels open or closed?
open
What are the stages of the nervous impulse?
- stimulus
- depolarisation
- repolarisation
- hyperpolarisation
- resting potential
Stimulus stage
- this excites the membrane
- Na channels are open
- membrane becomes more permeable to Na+ so they diffuse in, down the Na electrochemical con. grad.
- the inside is now less negative
Depolarisation stage
-if the PD reaches the threshold (55 mV), the V-G Na+ channels open and more Na+ diffuse in the neuron by positive feedback
Repolarisation stage
- at a PD of 30 mV, the Na channels close and the V-G K+ channels open
- membrane becomes more permeable to K+ so they diffuse out down their electrochemical conc. grad
- this starts getting the membrane back to its resting potential, this is negative feedback
Hyperpolarisation stage
- K+ channels are slow to close so there’s a slight overshoot where too many K+ diffuse out
- the PD becomes more negative than the resting potential ( less than 70 mV)
Resting potential stage
- the ion channels are reset
- Na-K pumps return the membrane to its RP by pumping Na+ in and K+ out and maintains it until the membranes excited again
What is a refractory period?
a period after an AP where the membrane can’t be excited again straight away. the ion channels are recovering and can’t open.
What does the refractory period make sure of?
that APs don’t overlap and are unidirectional
How does a wave of depolarisation travel along a neuron?
when an AP happens, some Na+ diffuse sideways which causes Na channels in the next region of the neuron to open and the Na+ diffuses into it. this causes the wave of depolarisation.
What does the all or nothing principle state?
if the threshold isn’t reached, an AP won’t fire.
How does a bigger stimulus affect the frequency of APs?
makes them fire more frequently
What 3 factors affect the speed of conduction?
- myelination
- axon diameter
- temperature
What is the myelin sheath in the PNS made out of?
Schwann cells that are wrapped around the axon
When in a myelinated neuron are Na+ channels concentrated?
nodes of Ranvier
What is saltatory conduction?
when the impulses jump from node to node in a myelinated neuron. this is fast.
What causes saltatory conduction?
the cytoplasm of the neuron conducts enough electrical charge to depolarise the next node
How does impulse travel in a non-myelinated neuron?
as a wave along the neuron. this is slow.
How does a bigger axon diameter affect the speed of conduction?
APs are conducted quicker as there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon. with less resistance, depolarisation reaches other parts of the neuron quicker.
How does a high temp affect the speed of conduction?
speed increases as ions diffuse faster. only increases up to 40 degrees as proteins denature and speed decreases.
What is a synaptic cleft?
tiny gaps between the cells of a synapse
What is a synaptic knob?
swelling on the postsynaptic neuron. it contains synaptic vesicles containing neurotransmitters.
What happens when an AP reaches the end of a neuron?
- neurotransmitters are released into the synaptic cleft
- they diffuse into the postsynaptic membrane and bind to specific receptors
- this can cause an AP, a muscle contraction or hormone secretion
- neurotransmitters are removed from the cleft so the response stops. this is done by being taken back into the presynaptic neuron or being broken down by enzymes
What are cholinergic synapses?
synapses that use ACh neurotransmitter to bind to cholinergic receptors
Stages of synaptic transmission
1- the arrival of an AP
2-fusion of the vesicles
3-diffusion fo ACh
Arrival of an AP stage
- arrives at synaptic knob of pre
- stimulates V-G Ca2+ channels to open
- Ca2+ ions diffuse into the synaptic knob
Fusion of the vesicles stage
- influx of Ca ions in the synaptic knob causes synaptic vesicles to move to the pre membrane and fuse with it
- the vesicles release ACh into the synaptic cleft by exocytosis
Diffusion of ACh stage
- it binds to specific cholinergic receptors
- causes Na+ channels in the post membrane to open
- influx of Na causes depolarisation
- an AP is only reached in the post if the threshold is reaches
- ACh removed to stop the response
How is ACh removed?
it is broken down by acetylcholinesterase and products are reabsorbed into the pre and used to make more ACh
Why can synapses be disrupted by chemicals like drugs and toxins?
they use chemical transmission
What can cause disruption to synaptic transmission?
1- some mimic the same shape as neurotransmitters so bind to receptors e.g. nicotine mimics ACh
2- some block receptors so fewer are activated e.g. Curare blocks cholinergic receptors at neuromuscular junctions
3-some inhibit enzymes that break down neurotransmitters so more is found in the synaptic cleft e.g. nerve gases stop ACh breakdown
4-some inhibit the release of neurotransmitters from the pre neuron so fewer receptors are stimulated
e.g. opioids block Ca channels in the pre neurone so fewer vesicles fuse with the pre membrane so less neurotransmitter is released
What effects does Curare blocking cholinergic receptors have?
muscles cells aren’t stimulated and muscles are paralyzed
What effect do nerve gases stopping ACh breakdown have?
can lead to loss of muscle control
What do excitatory neurotransmitters do?
depolarise the post membrane, firing an AP if the threshold is reached
What do inhibitory neurotransmitters do?
hyperpolarise the post membrane, preventing the firing of an AP
What is an excitatory synapse?
a synapse where ex-NTs are released
What is an inhibitory synapse?
a synapse where in-NTs are released
What is synaptic convergence?
when many neurons connect to one and info is amplified
What is synaptic divergence?
when one neuron connects to many neurons and info is dispersed to diff. parts of the body
What is summation?
where the effect of NTs released from many neurons is added together. there are 2 types.
Briefly describe spatial summation
-2 or more neurons converge and release NTs at the same time into the post neuron
If some neurons release in-NTs, what happens to the summation?
it might be no action potential
Briefly describe temporal summation
- 2 or more nerve impulses arrive in quick succession from the same pre neuron.
- makes an AP more likely as more NTs are released into the synaptic cleft
How do synapses make sure nerve impulses are unidirectional?
NTs are only released from the pre neurons and receptors are only on the post membranes
