5.1.3 Neuronal Communication Flashcards
role of sensory receptors
- respond to specific types of stimuli (eg. pressure Pacinian corpuscle)
- as transducers (convert diff stimulus types into nerve impulse- generator potential)
Pacinian corpuscle
what is it
- mechanoreceptor
- converts mechanical energy to electrical energy
- located deep in skin
- sensory neurone surrounded by layers of connective tissue, covered by gel
- neurone ending has stretch mediated Na⁺ channel. Permeability to Na⁺ ions changes when channels change shape
Pacinian corpuscle
how does it work
- in resting state, stretch mediated Na⁺ channels are too narrow to allow Na⁺ in. the neurone has a resting potential.
- when pressure is applied the corpuscle changes shape, causing the membrane surrounding the neurone to stretch. (Temporary gaps/spaces appear between the phospholipids in the bilayer.)
- Na⁺ channels widen and Na⁺ diffuses in
- membrane is depolarised by influx of Na⁺, resulting in generator potential
- this creates an action potential that passes along sensory neurone to CNS
structure and function of sensory neurones
one long dendron
cell body
one short axon
outside of CNS
transmits impulses from sensory receptor cell to relay neurone, motor neurone or brain
structure and function of relay neurones
many short dendrons
cell body
many short axons
transmit impulses between sensory and motor neurones
structure and function of motor neurones
many short dendrons
cell body
one long axon
axons
transmit impulses Away from the cell body
Axon=Away
can be myelinated
what is a myelin sheath?
schwann cells grow and wrap around axon many times, laying down double phospholipid layer each time
act as an insulating layer, allowing nerve impulses to be transmitted x100 faster
adaptations of cell body
- many endoplasmic reticulum
- many mitochondria
both involved in producing neurotransmitters
How is information about the strength & intensity of a stimulus communicated to the brain?
by the frequency of the action potentials sent. high freq = high intensity stimulus
why is there a difference in speed of conduction of an AP along myelinated vs non-myelinated neurone?
- depolarisation can only occur where voltage gated Na⁺ channels are present ie. at nodes of ranvier
- myelinated neurones have longer sections with no voltage gated Na⁺ channels
- longer circuits
- saltatory conduction: action potential jumps from node to node
how is a resting potential established?
Sodium potassium pump
- active transport: 3Na+ out for every 2K+ in
- as a result there is more Na+ outside the axon and more K+ inside. Therefore Na+ diffuse back into the axon down its electrochemical gradient and K+ diffuse out. This is by diffusion through channel proteins, NOT diff. across phospholipid bilayer
- However, most gated Na+ channels are closed and most K+ channels are open so more K+ diffuses out than Na+ in, so there is a resting potential across the membrane of -70mV (inside more -ve than outside)
describe saltatory conduction
depolarisation of the axon can only occur at the nodes of ranvier where no myelin is present as Na⁺ ions can pass through protein channels in membrane.
this creates longer localised circuits between adjacent nodes. the action potential ‘jumps’ from node to node ‘saltatory conduction’. much faster than wave of depolarisation along whole length of axon as it takes time for channels to open and ions to move.
repolarisation also uses ATP in the Na pump so reducing repolarisation means saltatory conduction is much more efficient
how is a resting potential established?
Sodium potassium pump
- active transport: 3Na⁺ out for every 2K⁺ in
- as a result there is more Na⁺ outside the axon and more K⁺ inside. Therefore Na⁺ diffuse back into the axon down its electrochemical gradient and K⁺ diffuse out. This is by diffusion through channel proteins, NOT diff. across phospholipid bilayer
- However, most gated Na⁺ channels are closed and most K⁺ channels are open so more K⁺ diffuses out than Na⁺ in, so there is a resting potential across the membrane of -70mV (inside more -ve than outside)
at resting potential, the inside of the axon has loads of …….. ions
K⁺
at resting potential, outside the axon has loads of …….. ions
Na⁺
how is an action potential generated?
- energy of stimulus triggers opening of Na⁺ channels. Na⁺ diffuses into axon and down electrochemical grad, depolarising the membrane
- this change causes voltage gated Na⁺ channels to open (positive feedback) so more Na⁺ floods in
- When potential difference reaches +40mV, Na⁺ voltage gated channels close and K⁺ voltage gated channels open. K⁺ diffuses out of the axon, reducing charge and repolarising axon membrane.
- There is a delay in the closing of K⁺ channels so membrane hyperpolarises
- Na⁺/K⁺ pump restores resting potential
- After an AP there is a short refractory period where the axon can’t be excited again. Voltage gated Na⁺ channels remain closed to stop Na⁺ depolarising membrane. –> prevents propagation of AP backwards along axon. Also ensures APs don’t overlap.
UNIDIRECTIONAL & DISCRETE
what are the 3 factors that affect the speed of an action potential?
- presence of myelin sheath
- axon diameter
- -> the bigger the diameter the faster the impulse is transmitted as there is less resistance to flow of ions in the cytoplasm
- temperature
- -> higher temp = faster impulse as diffusion of ions is faster at higher temperatures. However above 40C proteins start to denature
what is the refractory period and its purpose?
period where axon can’t be excited
voltage gated Na⁺ channels remain closed to stop Na⁺ depolarising membrane
ensures APs are UNIDIRECTIONAL and DISCRETE
all or nothing principle
if stimulus surpasses threshold value an AP is generated, no matter how large the stimulus. Stronger stimuli create more frequent APs but of the same size.
role of synapse
- Allows neurones to communicate
- Ensures transmission between neurones is UNIDIRECTIONAL -> neurotransmitter receptors are on postsynaptic membrane only
- Allows convergence/ impulses from more than one neurone to be passed to a single neurone
- Allows divergence/ impulses from a single neurone to be passed to more than one neurone
- Filters out background/low level stimuli or ensures only stimulation that is strong enough will be passed on
- Prevents fatigue/over-stimulation
- Allows many low level stimuli to be amplified
- Presence of inhibitory and stimulatory synapses allow impulses to follow specific path
- Permits memory/learning/ decision making
adaptations of synapse for function
- many endoplasmic reticulum
- many mitochondria
both involved in producing neurotransmitters which are stored in vesicles
what are the two types of neurotransmitter
Excitatory
- results in the DEPOLARISATION of the postsynaptic membrane
- if the threshold is reached in the postsynaptic membrane, an AP is generated.
- increase the likelihood of a response
eg. Acetylcholine
Inhibitory
- results in HYPERPOLARISATION of the postsynaptic membrane
- prevents an AP being triggered
eg. GABA
how is a nerve impulse transmitted across a synapse?
- AP arrives at presynaptic neurone and depolarises membrane, causing Ca²⁺ channels to open and Ca²⁺ to diffuse into the presynaptic knob
- Infux of Ca²⁺ causes synaptic vesicles to fuse with the presynaptic membrane, releasing ACh into synaptic cleft by exocytosis
- ACh diffuses across synaptic cleft
- ACh binds to receptors on Na⁺ voltage gated channels on postsynaptic neurone, causing them to open and Na⁺ to diffuse in rapidly
- Influx of Na⁺ generates an AP in postsynaptic neurone
- Acetylcholinesterase hydrolyses ACh –> choline and ethanoic acid, which diffuse back across synaptic cleft into presynaptic neurone. Breakdown of ACh prevents it from continuously generating AP.
- ATP released from mitochondria is used to recombine choline and ethanoic acid to form ACh, which is stored in synaptic vesicles. Na⁺ channels close.
