Topic 6B - Nervous Coordination Flashcards
What happens to neurone cell membranes?
In a neurone’s resting state (when it’s not being stimulated), the outside of the membrane is positively changed compared to the inside. This is because there are more positive ions outside the cell than inside. So the membrane is polarised - there’s a difference in charge (called a potential difference or voltage) across it. The voltage across the membrane when it’s at rest is called the resting potential - it’s about -70mV mini volts.
The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone’s membrane:
The sodium-potassium pumps move sodium ions out of the neurone, so they can’t diffuse back in. This creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside. The sodium-potassium pumps also move potassium ions in to the neurone, but the membrane is permeable to potassium ions, so they diffuse back out through potassium ion channels. This makes the outside of the cell positively charged compared to the inside. These channels are all types of transport proteins. Sodium-potassium pump - these pumps use active transport to move three sodium ions out of the neurone for every two positions moved in. ATP is needed to do this.
Potassium ion channel - these channel allow facilitated diffusion of potassium ions out of the neurone, down their concentration gradient.
What happens to neurone cell membranes when they’re stimulated?
A stimulus triggers other ion channels, called sodium ion channels, to open. If the stimulus is big enough, it’ll trigger a rapid change in potential difference. The sequence of events is known as an action potential:
1) stimulus - this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
2) Depolarisation - if the potential difference reaches the threshold (around -55 mV), more sodium ion channels open. More sodium ions diffuse rapidly into the neurone.
3) Repolarisarion - at a potential difference of around +30mV the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium 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.
4) hyperpolarisation - potassium ion channels are slow to close so there’s a slight “overshoot” where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (less than -70mV).
5) resting potential - the ion channel are reset. The sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane’s excited by another stimulus. After an action potential, the neurone cell membrane can’t be excited again straight away. This is because the ion channels are recovering and they can’t be made to open - sodium ion channels are closed during repolarisarion. This period of recovery is called the refractory period.
How does the action potential move?
When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways. The causes sodi 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 on the refractory period because these parts can’t fire an action potential.
What does the refractory period produce? What is the nature of action potentials?
During the refractory period, ion channels are recovering and can’t be opened. So the refractory period acts as a time delay between one action potential and the next. This means that:
Action potentials don’t overlap, but pass along as discrete (separate) impulses. There’s a limit to the frequency at which the nerve impulses can be transmitted. Action potentials are unidirectional (only travel in one direction).
Once the threshold is reached, an action potentials will always fire with the same change in voltage, no matter how big the stimulus is. If the threshold isn’t reached, an action potential won’t fire. This is the all-or nothing nature. A bigger stimulus won’t cause a bigger action potential, but will cause them to fire more frequently.
What are the three factors that affect the speed of conduction of action potentials?
Myelination: some neurones have a myelin sheath. The myelin sheath is an electrical insulator. In the peripheral nervous system, the sheath is made up of a type of cell called schwann cell. Between the Schwaan cells are tiny patches of bare membrane called the nodes of ranvier. Sodium ion channels are concentrated at the nodes. In a myelinated neurone, depolarisation only happens at the nodes of ranvier (where sodium ions can get through the membrane). The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse jumps from node to node. This is called saltatory conduction and it’s really fast. In a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane (so you get depolarisation along the whole length of the membrane). This is slower than saltatory conduction.
Axon diameter: 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.
3) The speed of conduction increases as the temperature increases too, because ions diffuse faster. The speed only increases up to around 40°C though - after that the proteins begin to denature and the speed decreases.
What is a synapse?
A synapse is the junction between a neurone and another neurone, or between a neurone and an effector cell e.g a muscle or gland cell. The tiny gap between the cells at a synapse is called the synaptic cleft. The presymaptic neurons (the one before the synapse) has a swelling called a sypnatic knob. This contains synaptic vesicles filled with chemicals called neurotransmitters. When an action potential reaches the end of a neurone it causes neurotransmitters to be released into the synaptic cleft. They diffuse across to the post synaptic membrane and bind to specific receptors. When neurotransmitters bind to receptors they might trigger an action potential (in a neurone), causing muscle contraction, or cause a hormone to be secreted (from a gland cell). Because the receptors are only on the postsynaptic membranes, synapses make sure impulses are unidirectional - the impulse can only travel in one direction. Neurotransmitters are removed from the cleft so the response doesn’t keep happening, e.g they’re taken back into the presynaptic neurone or they’re broken down by enzymes (and the products are taken into the neurone). There are many different neurotransmitters, e.g acetylcholine (ACh) and noradrenaline. Synapses that use ACh are called cholinergic synapses. Their structure is exactly the same as in the diagram above.
