Nervous Communication Flashcards
Describe the resting membrane potential
Ina neurones resting state, the outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside, so the membrane is polarised. The potential difference across the membrane when it’s at rest is about -70mV and is called the resting potential
How is the resting potential of a membrane created
Sodium-potassium pumps use active transport to move three sodium ions out of the neurone for every two potassium ions that moved in. This requires ATP. Potassium ion channels allow facilitated diffusion of potassium ions out of the neurone down their concentration gradient. The sodium-potassium pumps move sodium ions out of the neurone but they are unable to diffuse back in, creating a sodium ion electrochemical gradient (more positive sodium outside than inside). The sodium-potassium pumps also move potassium ions in to the neurone. When the cell is at rest, most potassium ion channels are open. This means that the membrane is permeable to potassium ions so they diffuse back out through the potassium ion channels
Action potential sequence
The stimulus excites the neurone cell membrane, causing the sodium ion channels to open. The membrane becomes more permeable to sodium ions so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative. If the potential difference reaches the threshold, more sodium ions channels open causing more sodium ions to diffuse rapidly into the neurone. The cell become depolarised. At a potential difference of +30mV, the sodium ions channels close and potassium ion channels open. The membrane is more permeable to potassium ions so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential by repolarising it. However, potassium ion channels are slow to close meaning more potassium ions diffuse out of the neurone than they should. This is known as hyperpolarisation and makes the inside of the cell more negative than its resting potential. Once the ion channels are reset, the sodium potassium pump returns the membrane back to its resting potential.
What is the refractory period?
The period of recovery for the membrane. Period of time where the cell membrane cannot be excited as the ion channels are recovering and cannot open
How does the action potential move along a neurone
When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways. This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part. This causes a wave of depolarisation to travel along the neurone. The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential
What is the role of the refractory period
Acts as a time delay to ensure:
- action potentials don’t overlap but pass along as discrete impulses
- there’s a limit to the frequency at which nerve impulses can be transmitted
- action potentials are unidirectional
What is the action potential all or nothing theory
Threshold reached, action potential will occur - bigger stimulus doesn’t mean bigger action potential but causes action potentials to fire more frequently
Threshold not reached then no action potential
Structure of myelinated motor neurone
Myelin sheath surrounding the axon made up of Schwann cells. In between Schwann cells are nodes of Ranvier. Sodium ion channels are concentrated at the nodes of Ranvier
How are impulses passed along a myelinated neurone
The neurones cytoplasm conducts enough electrical charge to depolarise the next node of Ranvier so the impulse jumps from node of Ranvier to node of Ranvier. This is called saltatory conduction and is very fast
How are impulses passed along a non myelinated neurone
The impulse travels as a wave along the whole length of the axon membrane
How does axon diameter affect action potential conductivity?
Action potentials are conducted quicker along axons with bigger diameters because there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
How does temperature affect action potential conductivity?
The speed of conduction increases as the temperature increases too, because ions diffuse faster. The speed only increases up to 40 degrees Celsius- after that the proteins begin to denature and the speed decreases
What is a synapse
Junction between 2 neurones or between a neurone and an effector cell
Structure of a synapse
Presynaptic membrane contains a synaptic knob. Synaptic knob contains vesicles filled with neurotransmitters. Postsynaptic membrane has receptors on its surface. Synaptic cleft between presynaptic membrane and postsynaptic membrane
How do action potentials travel across synapses
When an action potential reaches the end of a neurone is causes the neurotransmitters inside the vesicles to be released into the synaptic cleft. The diffuse across to the postsynaptic membrane and bind to specific receptors. This causes an action potential. Neurotransmitters and e removed from the cleft so the response doesn’t keep occurring
How do synapses ensure impulses are unidirectional
Receptors are only found on the postsynaptic knob
