Topic 8: Grey Matter Flashcards
8.1 Explain some common features of all neurones.
All neurones have a cell body, containing a nucleus and other cell organelles within the cytoplasm. There are two typed of thin extension from the cell body: dendrites, which conduct impulses towards the cell body, and axons, which conduct impulses away from the cell body.
8.1 What are the structure and function of the three different neurones?
Sensory neurones - one long dendron carries nerve impulses from receptor cells to the cell body, which is located in the middle of the neurone. One short axon carried nerve impulses from the cell body to the central nervous system.
Relay (or connector) neurone - multiple short dendrites carry nerve impulses from sensory neurones to the cell body. An axon carries nerve impulses from the cell body to motor neurones.
Motor (or effector) neurones - the cell body is always situated within the CNS. Multiple short dendrites will carry nerve impulses from the CNS to the cell body, and one long axon will carry nerve impulses from the cell body to effector cells.
8.1 What is the role of myelination?
Some neurones are myelinated - they have myelin sheath, made up of Schwann cells, which acts as an electrical insulator (preventing any flow of ions across the membrane). Gaps known as the nodes of Ranvier occur in the myelin sheath at regular intervals - in myelinated neurones, depolarisation only takes place here. As ions flow across the membrane in one node, it forces those inside the axon to diffuse down the electrical gradient. This rapid conduction of electrical signal reaches the next node and creates another action potential. This is saltatory conduction, and is much faster than the movement of an impulse down the whole axon membrane of a non-myelinated neurone.
8.2 i) Explain how a reflex arc works.
Reflexes are rapid, involuntary responses. When a receptor (in a sense organ) detects a stimulus, it generates a nerve impulse. Sensory neurones conduct the nerve impulses to the CNS along a sensory pathway. Neurotransmitters across a synapse allows the nerve impulses to be carried from the sensory neurone, to the relay neurone (in the CNS, bypassing the conscious areas of the brain), to the motor neurone. The motor neurone carries impulses to an effector (either a gland or a muscle) which produces a response.
8.2 ii) Explain how the pupil constricts and dilates.
The iris controls the size of the pupil - it contains a pair of antagonistic muscles: radial and circular muscles. These are both controlled by the autonomic nervous system.
- High levels of light are detected by photoreceptors in the retina, causing impulses to travel along sensory neurones to relay neurones in the CNS, followed by motor neurones to circular muscles of the iris, causing them to contract. The radial muscles simultaneously relax. This constricts the pupil, reducing the amount of light entering the light (a parasympathetic reflex).
- Low levels of light are detected by photoreceptors in the retina, causing impulses to travel along sensory neurones to relay neurones in the CNS, followed by motor neurones to radial muscles of the iris, causing them to contract. The circular muscles simultaneously relax. This dilutes the pupil, increasing the amount of light entering the light (a sympathetic reflex).
8.3 How is the resting potential across a neurone is created and maintained?
A neurones resting potential is -70mV: the outside of the membrane is more positively charged than the inside (so it’s polarised, and there’s a difference in charge). The resting potential is created and maintained by two processes:
- Sodium-potassium pumps, acting against the concentration gradient, use active transport (and ATP) to move three sodium ions out of the neurone for every two potassium ions moved in. The membrane isn’t permeable to sodium ions, so they can’t diffuse back in, creating a sodium ion electrochemical gradient, with more positive sodium ions outside the cell.
- The membrane is permeable to potassium ions. Potassium ion channels allow facilitated diffusion of potassium ions from their high concentration inside the cell, to the lower concentration outside the cell - this is the chemical gradient. However, the increased negative charge inside the cell will cause potassium ions to diffuse back into the cell - this is the electrical gradient. At -70mV potential difference, the chemical and electrical gradients counteract each other, so there is no net movement of potassium ions. (There are other voltage-dependent sodium ions and potassium ions channels that remain closed at resting potential).
8.3 Explain the process of depolarisation.
When a neurone is stimulated, slight depolarisation occurs: some voltage-dependent sodium ion channels open and the membrane becomes more permeable to sodium ions. They diffuse into the neurone down the sodium ion electrochemical gradient, causing depolarisation to increase and more sodium ion channels to open (positive feedback) once a threshold is reached (-55vM). The potential difference across the membrane reached +40mV after depolarisation.
8.3 Explain the process of repolarisation.
For repolarisation to begin, the voltage-dependent sodium ion channels close, and the voltage-dependent potassium ion channels open. The membrane is no longer permeable to sodium ions, although is permeable to potassium ions now: they therefore move out of the axon, down a potassium ion concentration gradient - this results in the inside of the cell becoming negative once again.
8.3 Explain the process of hyperpolarisation.
The membrane is now highly permeable to potassium ions, and more ions move out than occurs at resting potential, causing the potential difference to be more negative than at resting potential - this is hyperpolarisation. The resting potential is re-established by closing the voltage-dependent potassium ion channels, with the potassium ions also diffusing back into the axon.
8.3 Explain how the process and function of a refractory period?
A new action potential cannot be generated until all the voltage-dependent sodium and potassium channels have returned to their closed resting state, and the resting potential is restored - this is known as the refractory period, where ion channels can’t be opened. The sodium-potassium pump restores the original ion concentration across the cell membrane. This ensure that action potentials pass along as discrete impulses, without overlap, and are unidirectional (travel in one direction).
8.3 How does the size or strength of a stimulus affect the action potential?
The size or strength of the stimulus, once the threshold is reached, has no affect on the size or strength of the action potential. However, a stronger stimulus does cause more frequent action potentials (as well as increasing the number of neurones that are conducting impulses).
