Nervous system- module 5 Flashcards
What dos the sensory neurone do
Transmit nerve impulses from receptors to CNS
what does the Relay neurone do
Transmits nerve impulses between sensory and motor neurones
role of motor neurone
Transmit nerve impulses from CNS to receptors
flow diagram of stimulus to response in nervous system
Stimulus -> receptor - sensory neurone - relay - motor - effector - response
what are Pacinian corpuscles
Mechanoreceptors. they detect mechanical stimuli, such as pressure and movement.
Found deep in the skin are abundant in fingers and soles of the feet
How do Pacinian corpuscles convert pressure into a nervous impulse? process
1) pressure applied to Pacinian corpuscle
2)Lamellae become deformed and press on sensory nerve ending
3)Stretch mediated sodium channels change shape (widen) in the sensory neurone cell
4)sodium ion channels open
5) sodium ions diffuse into cell
6)creates generator potential (if threshold met)
7) Leads to action potential (nerve impulse) down sensory neurone
structure of sensory neurone
Short dendrites & long dendron to carry impulses from receptor cells to cell body.
One short axon that carries impulses from cell body to CNS
structure of motor neurone
Short dendrites that carry nerve impulses from CNS to cell body (located in CNS).
One long axon that carries impulses from cell body to effectors.
structure of cell body of neurone
Cell body- contains nucleus and other organelles. Has large amounts of ER & mitochondria to make neurotransmitters.
where do dendrites and axons carry electrical impulses
Dendrites and dendrons carry electrical impulses towards cell body.
Axons- carry nerve impulses away from cell body.
How is resting potential maintained
1)when a neurone is transmitting an impulse, the potential difference across its membrane is called a resting potential
2)the potential difference across the membrane at rest is around -70mV
3)This resting potential is due to sodium-potassium pumps and potassium ion channels
4)The pumps use active transport to pump 3 sodium ions(Na^+) out of the neurone for every 2 potassium ions(K^+). This requires ATP
5) The membrane isn’t permeable to sodium ions, so they can’t move back into the neurone. This creates a sodium ion electrochemical gradient
6) Potassium ion channels are open when the neurone is at rest, so the membrane is permeable to potassium ions
7)Some potassium ions move by facillitated diffusion back outr of tge neurone down the concentration gradient
8)In total, more positive ion move out of the neurone rather than in, making the outside of the neurone more positively charged than the inside. The membrane is said to be polarised
What happens after a stimulus to a neurone
energy of stimulus triggers some sodium ion channels to open, making the membrane more permeable to Na+
Na+ ions diffuse into axon down electrochemical gradient. Makes inside of neurone less negative
what happens during depolarisation
potential difference reaches the threshold (around -55mV) which opens voltage-gated sodium ion channels -> more Na+ diffuse into neurone - this is positive feedback
what happens at a potential difference of +40mV
voltage-gated sodium ion channels close and voltage-gated potassium ion channels open.
Sodium ions can no longer enter the axon
Potassium ions diffuse out of the axon, causing the inside of the axon to become more negative than the outside
what happens during hyperpolarisation
as the voltage-gated potassium ion pumps are slow to close, too many potassium ions diffuse out of the neurone.
This causes the potential difference to become more negative than the the resting potential (less than -70mV)
What happens during repolarisation
the sodium potassium pumps returns the membrane to resting potential by pumping sodium ions out and potassium ions in
refractory period
what is it
what does it ensure
After an action potential, the neurone can’t be excited straight away
the ion channels need to recover and can’t be made to open.
This acts as a time delay and ensures that action potentials don’t overlap and pass as separate impulses.
It also ensures impulses are unidirectional -only travel in one direction
Wave of depolarisation
-when an action potential happens, some sodium ions diffuse sideways, opening channels in the next part of the neurone.
-they always move away from areas in the refractory period as the channels can’t open there- these are called localised currents
-This triggers the action potential in the next segment
this means a wave of depolarisation moves along the neurone - the electrical impulse is said to ‘propagate’ down the axon
Wave of depolarisation
-when an action potential happens, some sodium ions diffuse sideways, opening channels in the next part of the neurone.
-they always move away from areas in the refractory period as the channels can’t open there- these are called localised currents
-This triggers the action potential in the next segment
this means a wave of depolarisation moves along the neurone - the electrical impulse is said to ‘propagate’ down the axon
what effects the speed of conduction of action potentials
Myelination
Axon diameter
Temperature
how does axon diameter affect the speed of conduction of action potentials
The bigger the axon diameter, the faster the impulse is transmitted.
There is less resistance to the flow of ions in the cytoplasm, compared with that in a smaller axon
how does temperature affect the speed of conduction of action potentials
the higher the temperature, the faster the nerve impulse. Because ions diffuse faster at higher temperatures. Generally only occours up to 40oC because denature
Myelination
-myelin sheath = electrical insulator
it is made of schwann cell wrapped around the axon or dendron
-The bare parts between schwann cells are called the nodes of Ranvier
-Sodium ion channels are concentrrated at these nodes
-Myelinated axons transfer electrical impulses much faster than non-myelimnated axons (due to saltatory conduction)
Saltatory Conduction
-depolarisation can only occour at nodes of Ranvier where no myelin is present -> sodium ions can only pass through channels here
-Longer localized circuits arise between the adjacent nodes -> action potential ‘jumps’ from one node to another
-Quicker as less channels to open, so less ions to move
-energy efficient -> less ATP needed for repolarisation in Na+K+ pump
-Myelin not permeable to sodium or potassium ions
-sodium ions diffuse into axon during depolarisation, diffuse along axon, sets up local current - causes sodium to diffuse to next node
-Sodium moves towards more negative region
what neurotransmitter is used in a cholinergic synapse
acetylcholine ACh
what is a synapse
a junction between two neurones or a neurone and effector cell
-Ensures impulses are unidirectional
-Impulses are transmitted in synapses using neurotransmitters
Process of synaptic transmission
1)Action potential reach end of synaptic neurone
2) depolarisation causes voltage-gated Ca^2+ cahnnels to open
3)Ca^2+ diffuse into presynaptic knob
4)Vesicles containing ACh fuse with presynaptic membrane
5)ACh released into synaptic cleft by exocytosis
6)ACh diffuses across synaptic cleft and binds to cholinergic receptors on sodium ion channels
7)Sodium ion channels open and sodium ions diffuse into postsynaptic neurone
8)Action potential triggered if threshold is met
9)ACh molecules at receptors broken down by acetylcholinesterase
SYNAPTIC DIVERGENCE
when one neurone connects to many neurones.
information can be dispersed to many different parts of the body
synaptic convergence
When many neurones connect to one neurone
information can be amplified (make stronger)
higher in frequency
Summation
where the effect of neurotransmitters released is added together (the sum)
(If a stimulus is weak, only a small amount of neurotransmitter will be released which might not be enough to excite the post synaptic membrane to threshold level)
Spatial summation
where two or more presynaptic neurones converge and release their neurotransmitters at the same time -> this can be enough altogether to reach the threshold and trigger an action potential
It allows signals from multiple stimuli to be coordinated in a single response
Temporal summation
where two or more impulses arrive in quick succession from the same presynaptic neurone.
-Action potential is more likely as more Neurotransmitter is released into the synaptic cleft
Follow each other extremely quickly so they don’t get broken down
how can chemicals affect transmission of nerve impulses
-mimic shape of neurotransmitter
-Block receptors
-Inhibit release of neurotransmitters
-Inhibit enzyme that breaks down neurotransmitters
how can a chemical mimic shape of neurotransmitter affect transmission of nerve impulses
chemicals mimic NTs action at receptors e.g: nicotine mimics ACh so binds to same cholinergic receptors in the brain
how can a chemical that Blocks receptors affect transmission of nerve impulses
e.g:
block receptors so they can’t be activated by NTs -> results in muscles being paralysed
(e.g curare blocks the ACh so muscle cells can’t be stimulated)
how can a chemical Inhibiting the release of neurotransmitters affect transmission of nerve impulses
e.g:
can inhibit release of NTs from the presynaptic neurone, so fewer receptors are activated. E.g: opioids block calcium ion channels so fewer vesicles fuse with the presynaptic membrane and less NT is released
how can a chemical -Inhibit enzyme that breaks down neurotransmitters to affect transmission of nerve impulses
Some inhibit the enzyme that breaks down NTs -> more NTs in the synaptic cleft to bind to receptors and are therefore longer
e.g: nerve gas stops ACh from being broken down - can lead to loss of muscle control
what two types of nuerotransmitter are there
excitatory
inhibitory
what is a excitatory neurotransmitter
a neurotransmitter that results in the depolarisation of the post synaptic neurone. If the threshold is reached in the post synaptic membrane, an action potential is triggered
e.g: acetylcholine
what is an inhibitory neurotransmitter
a neurotransmitter which results in the hyperpolarisation of the post synaptic membrane. This prevents an action potential from being triggered.
e.g: GABA in the brain
why is the enzyme in the synaptic clefts necessary
-so that neurotransmitters are broken down and
releases them from the receptors
can be recycled
stimulus is not maintained, so another stimulus can arrive and affect the synapse
