Neurones and nervous coordination Flashcards
What six things is a mammalian (motor) neurone made up of
- Cell body
- Dendrons
- Axon
- Schwann cells
- myelin sheath
- Nodes of ranvier
What are neurones
Specialised cells adapted to rapidly carry electrochemical changes (nerve impulses) from one part of the body to another.
What is the function of the cell body of a neurone
- The cell body contains all of the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum.
- This is associated with the production of proteins and neurotransmitters.
What is the function of the dendrons in a neurone
Dendrons are extensions of the cell body which subdivide into smaller branched fibres called dendrites, that carry nerve impulses towards the cell body.
What is the function of the axon of a neurone
The axon is a single long fibre that carries nerve impulses away from the cell body.
What are the Schwann cells of a neurone and what is their function
- Schwann cells are cells that surround the axon, protecting it and providing electrical insulation.
- They also carry out phagocytosis 9the removal of cell debris) and play a part in nerve regeneration.
- Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it.
What is the myelin sheath of a neurone and what is its function
- The myelin sheath forms a covering to the axon and is made up of the membranes of the Schwann cells.
- These membranes are rich in the lipid myelin.
- Neurones with a myelin sheath are known as myelinated neurones.
What are the nodes of Ranvier of a neurone
- The nodes of Ranvier are contractions between adjacent Schwann cells where there is no myelin sheath.
- The constrictions are 2-3um long and occur every 1-3mm in humans.
Describe the function of a sensory neurone and how this relates to its structure
- Sensory neurones transmit nerve impulses from a receptor to an intermediate or motor neurone.
- They have one dendron that is often very long.
- It carries the impulse towards the cell body and one axon carries this impulse away from the cell body.
Describe the function of a motor neurone and how this relates to its structure
- Motor neurones transmit nerve impulses from an intermediate or relay neurone to an effector such as a gland or muscle.
- Motor neurones have a long axon and many short dendrites.
Describe the function of intermediate/relay neurones and how this relates to their structure
- Intermediate or relay neurones transmit impulses between neurones.
- They have numerous, short processes.
What is the definition of a nerve impulse
- A nerve impulse is a self-propagating wave of electrical activity that travels along the axon membrane.
- It is the temporary reversal of the electrical potential difference across the axon membrane.
- This reversal is between two states, called the resting potential and the action potential.
Describe the ways in which the movement of ions (Na+ and K+) across the axon membrane is controlled
- The phospholipid bilayer do the axon plasma membrane prevents sodium and potassium ions diffusing across it.
- Channel proteins span the phospholipid membrane which have ion channels with gates that can be opened or closed so that sodium and potassium ions can only travel through them at specific times.
- Some channels remain opened at all times so that sodium and potassium ions move unhindered through them by facilitated diffusion.
- Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon- this is the sodium-potassium pump.
Explain what the resting potential of an axon is
- The inside of an axon is negatively charged relative to the outside- this is called the resting potential.
- This ranges from 50 to 90 millivolts (mV) but is usually 65 mV humans
What is the resting potential of an axon in humans
-65 mV
Describe how the resting potential of an axon is established and maintained
- Sodium ions are actively transported out of the axon by sodium-potassium pumps.
- Potassium ions are actively transported into the axon by the sodium potassium pumps.
- The active transport of sodium ions is greater than that of potassium ions- 3 sodium ions move out for every two potassium ions that move in.
- Although both sodium and potassium ions are positive, the outward movement of sodium ions being greater than the inward movement of potassium ions means that there are more sodium ions in the tissue fluid surrounding the axon that in the cytoplasm.
- There are also more potassium ions in the cytoplasm than in the tissue fluid, thus creating an electrochemical gradient.
- The sodium ions begin to diffuse back naturally into the axon while the potassium ions begin to diffuse back out of the axon.
- However, most of the gates in the channels that allow the potassium ions to move through are open, while most of the gates in the channels that allow the sodium ions to move through are closed.
What is the membrane of the axon said to be when at resting potential
Polarised
Explain what an action potential is
- When a stimulus of sufficient size is detected by a receptor in the nervous system, its energy causes a temporary reversal of the charges either side of this part of the axon membrane.
- If the stimulus is great enough, the negative charge of -65 mV inside the membrane becomes a positive charge of around +40 mV.
- This is known as the action potential and this part of the axon membrane is said to be depolarised.
What key property of the axon membrane allows depolarisation to occur
Depolarisation occurs because the channels in the axon membrane change shape, and hence open or close, depending on the voltage across the membrane (they are therefore called voltage-gated channels).
Describe how an action potential is formed
- At resting potential some voltage-gated channels are open (those that are always open) but the sodium volatage-gated channels are closed.
- The energy of the stimulus causes some sodium voltage-gated channels in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient.
- Being positively charged, they trigger a reversal in the potential difference across the membrane.
- As the sodium ions diffuse into the axon, more sodium channels open, causing an even greater influx of sodium ions by diffusion.
- Once the action potential of around +40 mV has been established, the voltage gates on the sodium ion channels close, thus preventing the further influx of sodium ions, and the voltage gates on the potassium ion channels begin to open.
- With some potassium voltage-gated now open, the electrical gradient that was preventing further outward movement of potassium ions is now reversed, causing more potassium ion channels to open.
- This means that yet more potassium ions diffuse out, starting repolarisation of the axon
- The outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative (relative to the outside) than usual.
- This is hyperpolarisaiton and the period is called the refractory period.
- The closable gates on the potassium ion channels now close and the activities of the sodium-potassium pumps once again cause sodium ions to be pumped out and potassium ions in.
- The resting potential of -65 mV is re-established and the axon is said to be repolarised.