Neurones

• What are nervous impulses?

  Electrical charges transmitted along neurone. Created by movement of sodium and potassium ions.
• What is the resting membrane potential?

  Outside membrane positively charged compared to inside. More positive ions outside cell than inside (membrane polarised) as there's difference in charge across it (potential difference). Resting potential is -70mv.
• How is the resting potential created and maintained?

  Na-K Pumps and potassium ion channels in neurone's membrane. Pumps use active transport to move 3 Na+ out neurone for every 2K+ moved in (ATP used). K+ channels allow facilitated diffusion of K+ out neurone, down their conc. gradient.
• How is an Na+ electrochemical gradient created?

  Na/K pumps move Na+ out neurone, but membrane isn't permeable to Na+, so they can't diffuse back in. Creates gradient as there's more Na+ outside cell than inside.
• What happens when the neurone cell is at rest?

  Most K+ channels open. Membrane is permeable to K+, so some diffuse back out through K+ ion channels.
• How do action potentials happen?


  When neurone stimulated, other ion channels in cell membrane (Na+ channels which are voltage-gated) open. If stimulus big enough, it'll trigger rapid change in potential difference. Causes cell membrane to become depolarised, and action potential happens.
• What happens when a stimulus excites a neurone cell membrane?

  Na+ channels open. Membrane becomes more permeable to Na+, so Na+ diffuses into neurone down Na+ electrochemical gradient. Makes inside of neurone less negative.
• What happens during depolarisation?

  If potential difference reaches threshold (around 55mv), more Na+ channels open. More Na+ diffuse into neurone.
• What happens during repolarisation?

  At potential difference around 30mv, Na+ channels close and K+ channels open. Membrane more permeable to K, so K+ diffuse out neurone down K+ conc. gradient. Starts to get membrane back to resting potential.
• What happens during hyperpolarisation?

  K+ channels slow to close so there's slight overshoot where too many K+ diffuse out neurone. Potential difference becomes more negative than resting potential (less than -70mv).
• What is the recovery/refractory period?

  After action potential, neurone cell membrane can't be excited again straight away- ion channels recovering and can't open. Time delay between action potentials so they don't overlap. Means there's a limit to frequency at which nerve impulses can be transmitted and action potentials are unidirectional.
• What is the all-or-nothing principle?

  Neuron either fires completely or does not fire at all in response to a stimulus. Once threshold reached, action potential will always fire with same change in voltage no matter how big stimulus is. Threshold not reached, action potential won't fire. Bigger stimulus won't cause bigger action potential, but will cause them to fire more frequently.
• What are waves of depolarisation?

  When action potential happens, some Na+ that enter neurone diffuse sideways. Causes Na+ channels in next region of neurone to open and Na+ diffuse into that part. Causes wave of depolarisation to travel along neurone. Wave moves away from the parts of membrane in refractory period as these parts can't fire action potentials.
• What is myelination?

  Some neurones, including motor, are myelinated ( have electrical insulated myelin sheath). In peripheral nervous system, sheath made of Schwann cell. Tiny patches of bare membrane between Schwann cells (nodes of Ranvier). Na+ channels are concentrated at nodes of Ranvier.
• What is saltatory conduction?

  In myelinated neurone, depolarisation only happens at nodes of Ranvier (where Na+ can get through membrane). Neurone's cytoplasm conducts enough electrical charge to depolarise next node, so impulse jumps from node to node. Very fast.
• How does an electrical impulse travel along a non-myelinated neurone?

  Wave along whole length of axon membrane. Slower.
• How does axon diameter affect the speed of conduction of action potentials?

  Action potentials conducted quicker along bigger axon diameters as there's less resistance to flow of ions than in cytoplasm of smaller axon. With less resistance, depolarisation reaches other parts of neurone cell membrane quicker.
• How does temperature affect the speed of conduction of action potentials?

  Speed increases as temperature increases, as ions diffuse faster. Speed only increases up to around 40C, after that proteins begin to denature and speed decreases.