9.5 - Nervous Transmission Flashcards
What do motor neurones do?
Carry impulses from the central nervous system to muscles or glands
What is the function of sensory neurones?
Transmit impulses/ action potentials via a dendron to the central nervous system from sense organs or receptors
Where are intermediate/relay neurones located?
Found in the central nervous system relaying impulses between neurones
what is the function of relay neurones
Carry action potentials
- between neurones
- within the CNS
- involved in reflexes
Carry action potentials from and to numerous other neurones
Where is the cell body of a motor neurone situated? and what are its characteristics?
In the central nervous system (CNS)
is rich in rough endoplasmic reticulum (RER) and mitochondria
Contains the nucleus
has many thin cytoplasmic processes extending from the cell body
The CNS includes the brain and spinal cord.
What do dendrites do?
Conduct impulses towards the cell body
Dendrites receive signals from other neurons.
What is the function of the axon in a motor neurone?
Carries impulses away from the cell body
The axon transmits electrical signals to other neurons or muscles.
What is at the end of an axon?
Branches that terminate in a muscle or gland; terminal buttons (knobs) contain many vesicles with transmitter substance
These branches are crucial for transmitting signals to effectors.
what is the structure of a sensory neurone
- Sensory neurones have a LONG DENDRON (by definition this is the process which takes impulse towards the cell body)
- relatively SHORT AXON (by definition this is the process which takes the impulse away from the cell body)
- The cell body is found in a swelling or the DORSAL ROOT GANGLIAN close to the CNS (just outside the spinal cord)
- Dendron and axon may be myelinated
what is the structure of a relay neurone?
- Cell bodies and cytoplasmic prosesses IN BRAIN/ SPINAL CORD
None of the processes are myelinated
what does the speed of transmission depend on? 2 THINGS
Diameter of nerve fibre
Presence of myelin sheath
how to investigate nerve impulses?
- in an axon put an internal electrode through the CSM which is made from a glass pipette and filled with an electrically conducting soln
- connect this by wire to computer
- keep an electrode outside the axon as well to detect difference in voltage in an unstimykated neurone
- amplify small difference
- display on oscilloscope screen
- constant Dif of -70mV bw out and in is resting potential
what is the resting potential?
- REMEMEBR: even resting neurone = v active
= when NOT conducting nerve impulse} inside always negatively charged wrt outside (~ -65/70mV)
= difference in charge bw out and in = resting potential
how is RP maintained? 3 things
- by the sodium-potassium pump in plasma membrane
- for every 3Na+ ions pumped out, 2K+ ions are imported
- this is against conc gradient so req ATP for AT
- ALSO (3) facilitated diffusion of K+ ions out + NA+ back in BUT (2) membrane more permeable for K+ ions so more K+ flows out then Na+ back in
- as more +ve ions pumped out and more K+ ions flowing out -> excess of +ve ions out then in -> polarised membrane = RESTING POTENTIAL
how is an AP formed?
1) The membrane starts in its resting state - polarised with the inside of the cell being
-60 mV compared to the outside.
2) Neurone stimulated and some sodium ion channels open and some sodium ions diffuse into the cell. - diffusion as electrochemical gradient
3) The membrane depolarises - it becomes less negative with respect to the outside and reaches the threshold value of -50 mV.
4) Once threshold value reached, Voltage-gated sodium ion channels open and many sodium ions diffuse in down electrochemical gradient. As more sodium ions enter, the cell becomes positively charged inside compared with outside. and membrane is further depolarised causing more VG Na+ channels to open (+ve feedback)
5) The potential difference across the plasma membrane reaches +40 mV. The inside of
the cell is positive compared with the outside. +40mV = ACTION POTENTIAL generated
6) The V-G sodium ion channels close (smaller gradient so Na+ stops diffusing in) and depolarisation of membrane causes V-G potassium channels to open.
7) Potassium ions diffuse out of the cell and excess Na+ ions pumped put by active Na+ pumps bringing the potential difference back to negative inside compared with outside - this is called repolarisation.
8) there is a delay in closing V-G K+ channels so the potential difference overshoots slightly, making the cell hyperpolarised.
9) The original potential difference is restored by Na-K pump so that the cell returns to its resting state.
what is the refractory period?
- recovery time of an axon - time taken fro an area of axon membrane to recover after an AP
- = HYPERPOLARISATION
what are two parts to refractory period?
- absolute refractory period - V-G Na+ channels LOCKED
- relative refractory period - hyperpolarised so harder to reach threshold potential
what is the importance of the refractory period
- makes sure depolarisation cannot take place as ion channels locked and harder to reach threshold potential - means further AP can’t be generated
- so LIMITS rate at which impulses may flow along fibre
- ensures impulses only flow in ONE direction
how is an AP propagated along an axon?
- A nerve impulse is a wave of depolarisation that moves along an axon
- Na+ entering creates an area of positive charge inside the axon
- A flow of current is set up in a local current between the active area and the negatively charged resting region immediately ahead
- The current reduces the membrane potential in the resting region and this depolarisation causes an increase in Na+ permeability and an action potential in the region
- Once an action potential has been started it becomes self-generating
- This is because the depolarisation in one region of the axon causes a depolarisation in a neighbouring section which then causes depolarisation in a neighbouring section… and so on
what is a local current
movement of ions along neurone; flow of ions is caused by increase in conc at one point which leads to diffusion away from this high conc region
why do nerve impulses only travel in one direction?
- local currents set up bw regions where there is an AP + regions in RP
- APs only travel in one direction away from cell body in a motor neurone
- the area of neurone that has AP has j come from area that is unable to generate another for some time - ONE DIRECTION
- so APs only move forward - AWAY FROM SOURCE of OG AP
what is the all/nothing response?
- if depolarisation of membrane reaches threshold - AP generated + nerve impulse transmutted
- impulse has same AP no matter size of stimulus
- can only have greater freq of AP - not bigger AP
what happens when its a stronger stimulus?
- more neurones stimulated - higher freq of APs
- brain can interpret freq of APs + no. neurones carrying APs to get info on strength
what is saltatory conduction?
- ions can only pass in and out of axon freely at nodes of ranvier -> speeds up transmission as ionic movements associated w AP r less frequent so take less time
what are myelinated nerve fibres?
- type of cell = Schwann cell} wraps around axon repeatedly (one each then gap (= nodes of ranvier)) forming fatty layer known as myelin sheath
role of myelinated nerve fibres? 2
- protect nerve from damage
- speed up transmission of nerve impulses
what is the structure of a synapse?
axon of presynapticc nerve, in the presynaptic neurone: synaptic knob, vesicles containing neurotransmitter, mitochondrion, presynaptic membrane, V-G Ca2+ ion channels
then synaptic cleft in bw
post synaptic neurone, w post-synaptic membrane and on it = receptor for neurotransmitter, ligand V-G Na+ channels
how is a nerve impulse passed across 2 neurones?
- depolarisation occurs along axon at each node of Ranvier
- leads to V-G Ca2+ channels to open and Ca ions diffuse in
- this causes vesicles to move to presynaptic membrane
- vesicles fuse q presynaptic membrane and release neurotransmitters (acetylcholine) into synaptic cleft
- acetylcholine diffuses across SC to postsynaptic membrane and binds to specific receptors in PS membrane
- stimulates ligand V-G na ion channels to open
- then PS membrane depolarised and if reaches threshold potential an AP produced
- once neurotransmitter had effect - it is destroyed so receptors can be emptied and ready for next impulse
why are synapses necessary? why not just on long nerve (4)
- ensures one way transmission - only pre contains vesicles + only post has receptors
- filters out low level stimulation - if low - only small EPSP produced so threshold not reached
- can be many Pre converging to one Post (several Dif stimuli warning of danger) or the other way (reflex arc)
- acclimatisation - after repeated stimulus; synapse = fatigued - so NS no longer responds to stimulus -> prevents over stimulation of effector
what is the formation and effect of excitatory post synaptic potentials?
- formed when na ion channels opened in post s membrane -> influx of na + ions so change in pd across membrane = EPSP set up
- effect: if sufficient no of EPSPs - +ve charge in PS cell exceeds threshold level -> AP!
what is the formation and effect of inhibitory post synaptic potentials?
- some times neurotransmitter has opp effect and opens neg ion channels (cl-) -> makes inside more neg so less likely to reach threshold
- effect: IPSP = less likely for AP to occur
what are the two neurotransmitters and where r they typically found?
- ACETYLCHOLINE = all motor neurones, parasympathetic nervous system, cholinergic synapses in CNS
- NORADRENALINE = sympathetic nervous system, adrenergic nerves in brain
