Human Physiology: 6.5 Neurones and Synapses Flashcards
Define neurones and nervous system
Neurons are specialised cells that function to transmit electrical impulses within the nervous system
The nervous system converts sensory information into electrical impulses in order to rapidly detect and respond to stimuli
While neurons may differ according to role (sensory, relay or motor), most share three basic components:
Dendrites – Short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals
Axon – An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors
Soma – A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival
What is myelin and sheath and its purpose?
In some neurons, the axon may be surrounded by an insulating layer known as a myelin sheath
The myelin sheath improves the conduction speed of electrical impulses along the axon, but require additional space and energy
What is the meaning of membrane potential?
Neurons generate and conduct electrical signals by pumping positively charged ions (Na+ and K+) across their membrane
The unequal distribution of ions on different sides of the membrane creates a charge difference called a membrane potential
What is the resting potential in a neuron?
A resting potential is the difference in charge across the membrane when a neuron is not firing
In a typical resting potential, the inside of the neuron is more negative relative to the outside (approximately –70 mV)
How is a resting potential generated and what kind of process it is?
The maintenance of a resting potential is an active process (i.e. ATP dependent) that is controlled by sodium-potassium pumps
The sodium-potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions (antiport)
It expels 3 Na+ ions for every 2 K+ ions admitted (additionally, some K+ ions will then leak back out of the cell)
This creates an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment (as there are more positively charged ions outside of the cell and more negatively charged ions inside the cell)
The exchange of sodium and potassium ions requires the hydrolysis of ATP (it is an energy-dependent process)
What is action potential
Action potentials are the rapid changes in charge across the membrane that occur when a neuron is firing
Action potentials occur in three main stages: depolarization, repolarization and a refractory period
Describe the first stage of action potential
Depolarisation refers to a sudden change in membrane potential – usually from a (relatively) negative to positive internal charge
In response to a signal initiated at a dendrite, sodium channels open within the membrane of the axon
As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of sodium
The influx of sodium ions causes the membrane potential to become more positive (+30 mV) (depolarisation)
Describe the second stage of action potential
Repolarisation refers to the restoration of a membrane potential following depolarisation (i.e. restoring a negative internal charge)
Following an influx of sodium, potassium channels open within the membrane of the axon
As K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium
The efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarisation) (-80mV)
Describe the third stage of action potential
The refractory period refers to the period of time following a nerve impulse before the neuron is able to fire again
In a normal resting state, sodium ions are predominantly outside the neuron and potassium ions mainly inside (resting potential) Following depolarisation (sodium influx) and repolarisation (potassium efflux), this ionic distribution is largely reversed Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump
How are nerve impulses transmitted/how do they travel?
Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation
Depolarisation occurs when ion channels open and cause a change in membrane potential
The ion channels that occupy the length of the axon are voltage-gated (open in response to changes in membrane potential)
Hence, depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon
This causes depolarisation to spread along the length of the axon as a unidirectional ‘wave’
Explain the all or none principle to generate action potentials
Action potentials are generated within the axon according to the all-or-none principle
An action potential of the same magnitude will always occur provided a minimum electrical stimulus is generated
This minimum stimulus – known as the threshold potential (–55 mV) – is the level required to open voltage-gated ion channels
If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire
Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimum level of depolarisation
If the overall depolarisation from the dendrites is sufficient to activate voltage-gated ion channels in one section of the axon, the resulting displacement of ions should be sufficient to trigger the activation of voltage-gated ion channels in the next axon section
Describe the main purpose of myelin sheath
The main purpose of the myelin sheath is to increase the speed of electrical transmissions via saltatory conduction
Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation
In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin sheath called the nodes of Ranvier
This results in an increase in the speed of electrical conduction by a factor of up to 100-fold
Advantages and disadvantage of myelination of axon
Not all neurons within the nervous system are insulated with a myelin sheath
The advantage of myelination is that it improves the speed of electrical transmission via saltatory conduction
The disadvantage of myelination is that it takes up significant space within an enclosed environment
Describe the whole synaptic process
Electrical impulses cannot ‘jump’ across the synaptic cleft
When an electrical impulse arrives at the end of the axon on the presynaptic neurone, the membrane of the presynaptic neurone becomes depolarised, triggering an influx of calcium ions into the presynaptic cell via calcium ion channels in the membrane
The calcium ions cause vesicles in the presynaptic neurone to move towards the presynaptic membrane where they fuse with it and release chemical messengers via exocytosis called neurotransmitters into the synaptic cleft
The neurotransmitters diffuse across the synaptic cleft and bind with receptor molecules on the postsynaptic membrane; this causes associated sodium ion channels on the postsynaptic membrane to open, allowing sodium ions to diffuse into the postsynaptic cell
If enough neurotransmitter molecules bind with receptors on the postsynaptic membrane then an action potential is generated, which then travels down the axon of the postsynaptic neurone
The neurotransmitters are then broken down to prevent continued stimulation of the postsynaptic neurone
