5.1.3 Neuronal Communication Flashcards
What is a neurone?
a specialised nerve cell that transmits electrical impulses rapidly around the body
What is the function of a sensory neurone?
to transmit electrical impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain
What is the structure of a sensory neurone?
- one dendron (transmits impulse to cell body)
- cell body
- one axon (transmits impulse from cell body)
- dendron and axon have a myelin sheath
What is the function of a relay neurone?
to transmit electrical impulses between neurones
What is the structure of a relay neurone?
- cell body
- have many short dendrons and axons
- non-myelinated
What is the function of a motor neurone?
to transmit electrical impulses from a sensory or relay neurone to an effector (aka muscle or gland)
What is the structure of a motor neurone?
- cell body
- lots of short dendrons (transmit impulses to cell body)
- one long myelinated axon (transmits impulses from cell body)
What is a cell body of a neurone?
- made up of nucleus, endoplasmic reticulum and mitochondria in cytoplasm
- produces neurotransmitters
What is an axon?
- a singular, elongated nerve fibre
- transmits impulse away from cell body
What are dendrons?
- short extensions that come out of the cell body
- split into smaller dendrites
- transmit impulses towards cell body
What is a myelin sheath?
layers of plasma membrane around the axon
- produced by Schwann cells growing around the axon
- act as an insulating layer so that impulse can be transmitted quicker
What are nodes of Ranvier?
small gaps between each Schwann cell where the impulse jumps from one node to the next
What cells produce the myelin sheath?
Schwann cells
Why are impulses transmitted quicker in myelinated neurones?
at every node of Ranvier, the impulse jumps from one node to the next, which means it is transmitted quicker
What are sensory receptors?
groups of specialised cells which detect a change and convert this detected stimulus into a nerve impulse
-are usually located in sense organs
What are the main types of sensory receptors?
- photoreceptor
- thermoreceptor
- chemoreceptor
- mechanoreceptor
What is the stimulus of a thermoreceptor?
heat
What is the stimulus of a photoreceptor?
light
What is the stimulus of a chemoreceptor?
chemicals
What is the stimulus of a mechanoreceptor?
pressure and movement
Name an example of a thermoreceptor
end-bulbs of Krause
-located in the tongue
Name an example of a photoreceptor
cone cell
-located in eye
Name an example of a chemoreceptor
olfactory receptor
-located in nose
Name an example of a mechanoreceptor
Pacinian corpuscle
-located in skin
Baroreceptor
-located in blood vessels near heart
How does the Pacinian corpuscle convert its stimulus into an impulse?
- in resting state, stretch-mediated Na+ channels are too narrow for Na+ to pass through (resting potential)
- when pressure is applied, the corpuscle changes shape so the axon stretches so its membrane is deformed, causing the Na+ channels to widen
- Na+ diffuse in
- this makes the membrane potential becomes +ve (is depolarised) which is known as a generator potential
- this generator potential creates an action potential so the impulse passes along the neurone
What is resting potential?
potential difference across membrane when it’s not transmitting an impulse (aka it is at rest)
- membrane is polarised
- potential difference is about -70mV
How is resting potential established?
- sodium/potassium pump actively transports 3Na+ out for every 2K+ it transports in
- Na+ channels are closed so Na+ can’t move back into the axon but K+ channels are open so K+ can diffuse out of the axon
- this means there are more +ve ions outside the axon so the axon is more -ve so reaches a resting potential of -70mV
What is the potential difference at resting potential?
-70mV
What is an action potential?
when an electrical impulse is created by the depolarisation in the membrane of a neurone
How is an action potential generated?
- neurone is at resting potential (-70mV) where some K+ channels are open and Na+ channels are closed
- energy of the stimulus causes voltage-gated Na+ channels to open so Na+ diffuse out of the axon, making it less -ve
- by positive feedback, more voltage-gated Na+ channels open so more Na+ leave and the inside of the axon becomes more +ve (it is depolarised)
- when pd reaches +40mV, voltage-gated Na+ channels close and voltage-gated K+ channels open
- K+ diffuse out of axon, making the inside of the axon more -ve (it is repolarised)
- as lots of K+ diffuse out, axon becomes more -ve than at normal rest (it is hyperpolarised)
- voltage-gated K+ channels close and sodium/potassium pump actively transports Na+ in and K+ out, bringing the axon back to resting potential (-70mV)
What is the potential difference of the axon once it is depolarised?
+40mV
What is a synapse?
the gap between two neurones or a neurone and an effector
How is an impulse transmitted across a synapse?
- action potential (Na+) reaches the end of the pre-synaptic neurone
- the depolarisation of the pre-synaptic neurone causes Ca2+ channels to open
- Ca2+ diffuse in
- this causes vesicles containing neurotransmitters t move across the pre-synaptic knob and to fuse with the pre-synaptic membrane so that the neurotransmitters are released into the synapse by exocytosis
- the neurotransmitters diffuse across the synapse and bind to receptors on Na+ channels on the post-synaptic membrane, causing these channels to open
- Na+ diffuses into post-synaptic knob and along the neurone, causing an action potential (aka impulse) to be generated
What neurotransmitter do cholinergic synapses use?
acetylcholine
What is acetylcholine broken down into after an impulse is transmitted across a cholinergic synapse?
acetyl and choline
-by acetylcholinesterase
How do synapses ensure impulses are only transmitted in one direction?
receptors for neurotransmitters are only on the post-synaptic membranes
What is an inhibitory synapse?
a synapse where when neurotransmitters bind to receptors on the post-synaptic membrane, the membrane is hyperpolarised (pd becomes more -ve) which prevents an action potential from being fired
What is an excitatory synapse?
a synapse where when neurotransmitters bind to receptors on the post-synaptic membrane, the membrane is depolarised (pd becomes less -ve) which causes an action potential to be fired if the threshold is reached
What is synaptic divergence?
when one neurone connects to several neurones so the information can be dispersed to different parts of the body
What is synaptic convergence?
when multiple neurones connect to one neurone so information can be amplified
What is summation?
the process where whether a new action potential is generated after the synapse is controlled based on the conc of neurotransmitter released by the pre-synaptic neurone
-an action potential is only generated when the neurotransmitters reach a threshold
What is spatial summation?
when neurones converge and multiple pre-synaptic neurones release neurotransmitters which diffuse across the synapse to the post-synaptic neurone
-if only one of the pre-synaptic neurones receives the signal (and releases neurotransmitters) no action potential would be generated on the post-synaptic knob
What is temporal summation?
occurs when there is only one pre and post-synaptic neurone and higher frequencies of action potentials in the pre-synaptic neurone cause higher concs of neurotransmitters to be released so lots bind to the receptors on the post-synaptic neurone and cause an action potential/impulse to be triggered
-if there is only one action potential/impulse received, only low concs of neurotransmitters could be released so not enough bind to the post-synaptic neurone to trigger an action potential
What is the threshold when an action potential is being created?
-50mV
