Topic 8: Grey Matter Flashcards
Resting Membrane Potential of a neuron
- The outside of the membrane is positively charged compared to the inside because there are more sodium ions outside the cell than there are on the inside.
- This means the membrane is polarised (has a potential difference/voltage). The voltage of the membrane in about -70.
- The resting membrane potential is primarily established by a sodium-potassium pump which works by active transport (uses ATP) to move three sodium ions (3Na+) outside of the neuron for every 2 potassium ions (2K+) which move into the neurons.
This process sets up 2 concentration gradients:
1. There is an inward concentration gradient for sodium - high conc. outside, low conc. inside. Sodium ion channels are primarily shut at this stage, which maintains this gradient.
2. There is an outward concentration gradient for potassium - high con. on inside, low conc. on outside. Potassium ion channels are mainly shut a this stage, which maintains this gradient.
Describe what happens when an action potential reaches a neuron
- A stimulus to the neuron triggers the opening of sodium ion channels. If the stimulus is above the threshold for that neuron then it will trigger more sodium ion channels to open (positive feedback).
Depolarisation:
When the sodium ion channels open, sodium ions diffuse into the cell down their electrochemical gradient (+ve to -ve). and concentration (high to low) gradients. This makes the inside of the neuron less negative.
Repolarisation:
At a potential difference of around +40mV*, the voltage-gated sodium ion channels close and voltage-gated potassium channels open. Potassium will now diffuse out of the neuron down the electrochemical ion concentration gradient. The inside of the cell becomes more negative again.
Hyperpolarisation
Potassium ion channels are slow to close so there’s a slight ‘overshoot’, in which the neuron becomes more negative than the resting potential because too many K+ ions leave the cell.
Refractory Stage
During repolaristaion and hyperpolarisaton the neuron cannot receive another stimulus (sodium ion channels cannot open again until the resting potential has been re-established)
The resting membrane potential & concentration gradients of Na+ & K+ are re-established using the sodium-potassium pump.
What is an action potential
An action potential is a wave of positive charge which moves along the axon. It is an all-or-nothing response because it is only one size (+40mV). If the body wants to communicate a stronger message then it needs to either:
- Stimulate more neurons
- Send the message more frequently
Explain how an action potential moves along an axon
The action potential travels along the neuron as a wave of depolarisation - some sodium ions that enter the neuron diffuse sideways, which causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part changing the potential differnece, thus creating local currents - thus stimulating another action potenial.
Why is the refractory stage important
- Allows time for the neurone to reset.
- Ensures that action potentials don’t overlap.
- Ensures that action potentials are unidirectional (can pass in one direction only).
Motor Neurones
Structure:
- Many short dendrites. Cell body at one end. One long axon carries nerve impulses from the cell body to effector cells.
Function
- Involved in transmitting electrical impulses from the CNS to muscles & glands in the body
Sensory Neurones
Structure:
- One long dendron carries nerve impulses from receptor cells to the cell body, which is in the middle of the neurone. One short axon carries nerve impulses for the cell body to the CNS.
Function:
- Transmits impulses from receptors to the CNS
Relay neurones
Structure:
- Many short dendrites carry impulses from sesnory neurones to the cell body. An axon carries nerve impulses from the cell body to motor neurones.
No myelin sheath
Function:
- Located within the CNS, involved in transmitting the electrical impulses from sensory neurones to motor neurons.
General structure & function of neurones
Neurones help co-ordinate communication within the nervous system
They all have a cell body composed of a nucleus as well as organelles such as mitochondria in the cytoplasm. These provide energy (in the form of ATP) needed for active transport of ions into & out of the cell in an impulse. They also contain dendrites (extensions involved into conducting imulses towards the cell body), and axons (which conduct impulsess away from the cell body).
Detail the role of myelination?
Myelination is a layer of fatty substance/lipid formed from schwann cells wrapped around the neurone - can increase the speed of impulses by acting as an electrical insulator, and allowing impulses to travel by saltatory conduction.
What is tho role of myelination in saltatory conduction
Myelination plays a key role in saltatory conduction byinsulating axons and allowing action potentials to “jump” from one node of Ranvier to the next, which speeds up the transmission of nerve impulses.
Describe the process of synaptic transmission
- An action potential triggers a calcium influx:
When an AP arrives at the presynaptic membrane the depolarisation stimulates voltage-gated calcium ion channels to open. Calcium ions enter the neurone -
Calcium influx causes Neurotransmitter Release:
The influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic memberane. They then fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft via exocytosis. - The Neurotransmitters trigger an Action potential in the postsynaptic neurone:
The neurotransmitters diffuse across the synaptic cleft and binds to specific receptors of the postsynaptic membrane, stimulating the opening of sodium ion channels in the postsynaotic neurone. The influx of sodium ions into the postsynaptic membrane causes depolarisation and an action poetential is genereted on the postsynaptic membrane if the threshold is reach.
Synaptic transmission is contolled with the help of digestive enzymes in the synaptic cleft, which break down excess neurotransmitters to prevent overstimulation on the postsynaptic membrane. These are then taken up by the pre-synaptic membrane and reused. Re-uptake of NTs, the presence of receptors on one side of the synapse only & the refractory period serves to ensure that the AP is unidirectional.
Acetylcholine
A neurotransmitter involved in muscle contraction & the control of heart rate
The role of neurotransmitters: excitatory vs inhibitory
Excitatory neurones stimulate/the production of an action potential in the postsynaptic membrane by stimulating theopening sodium ion channels.
Inhibitory neurones cause chloride ions to open in the postsynaptic membrane, so negative chloride ions enter, thus causing hyperopolarisation of the postsynaptic membrane. Therefore triggering a new action potential becomes difficult.
Whether or not an action potential is triggered in the post-synaptic membrane depends on the summation of excitatory & inhibitory neurones and if voltage reaches the threshold for that neurone.
What is Synaptic divergence?
Impulse diverge in when one neurone connects to many neurones alllowing info to be dispersed to different parts of the body.