Neurones WAL An overview of neurones Some Most

Neurones WAL: An overview of neurones Some Most All What are the different types of neurone? What is the structure of a myelinated motor neurone? What is a neurone? Starter – Use the following words to explain tropisms – IAA. Light, elongation

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Neurones: Nerves are bundles of neurones Coordination & the Nervous system

Coordination & the Nervous system

Schwann cells form myelin sheath Coordination & the Nervous system

Structure of neurones Cell body – nucleus and large amounts of RER associated with production of proteins and neurotransmitter. Dendrons (dendrites) – carry nerve impulses towards the cell body Axon – single long fibre that carries nerve impulses away from the cell body Schwann cells – surround axon by wrapping around many times, protecting it and providing electrical insulation. Phagocytosis and nerve regeneration.

Structure of neurones Myelin sheath – forms covering of axon and made of membranes of the Schwann cells. Rich in a lipid known as myelin. Can be myelinated or unmyelinated. Myelinated neurones transmit nerve impulses faster. Nodes of Ranvier – gaps between adjacent Schwann cells where there is no myelin sheath. Gaps 2 -3 um and occur every 1 -3 mm

The structure of a motor neurone

Transverse section through an axon showing the myelin sheath

Neurones WAL: An overview of neurones Some Most All What are the different types of neurone? What is the structure of a myelinated motor neurone? What is a neurone? Plenary– Draw and label a picture of a neurone

Neurones WAL: An overview of how a nerve impulse works Some Most All What is a resting potential? How is resting potential established in a neurone? What is a resting potential? Starter– Draw and label a picture of a neurone

Nerve impulse A nerve impulse is a self propagating wave of electrical disturbance that travels along the surface of the axon membrane. It is not, however, an electrical current, but a temporary reversal of the electrical potential difference across the axon membrane. This reversal is between 2 states called the resting potential and the action potential.

Resting potential The inside of an axon is negatively charged related to the outside. This is known as the resting potential and ranges from -50 -90 m. V, but is usually -65 m. V. In this condition the axon is said to be polarised.

Resting potential The establishment of the potential difference (the difference between the inside and the outside of the axon) is due to the following: 1. Na ions are actively transported out of the axon by the Na-K pump. 2. K ions are actively transported into of the axon by the Na-K pump.

Resting potential 3. The active transport of Na ions is greater than that of K ions, 3 Na move out for every 2 K ions that move in. 4. Although Na and K ions are both positive, the result of the movement is that there are more Na ions in the tissue fluid surrounding the axon than in the cytoplasm, and more K ions in the cytoplasm than in the tissue fluid, thus creating a chemical gradient.

Resting potential 5. The Na ions begin to diffuse back naturally into the axon whilst the K ions diffuse out of the axon. 6. However, most of the gates in the channels that allow the K ions to move through are open, while most of the gates in the channels that allow Na ions to move through are closed.

Resting potential 7. As a result the axon membrane is 100 x more permeable to K ions, which diffuse back out of the axon faster than the Na ions back in. This increases the potential difference (difference in charge). 8. There is also an electrical gradient. As more and more K ions diffuse out of the axon, so the outside of the axon becomes more and more positive.

Resting potential 9. Further outward movement of K ions therefore becomes difficult because, being positively charged, they are attracted by the overall negative state inside the axon, which compels them to move into the axon, and repelled by the overall positive state of the surrounding fluid, which prevents them from moving out of the axon. 10. An equilibrium is established in which the chemical and electrical gradients are balanced and there is no net movement of ions.

The action potential When a stimulus is received by a receptor or nerve ending, its energy causes a temporary reversal of the charge on the axon membrane. As a result, 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 in this condition the membrane is said to be depolarised.

Resting potential Na channels open Na ions rush into cytoplasm Membrane is depolarised K channels open K ions rush out Membrane re-polarised Wave of depolarisation continues along membrane Ion pump re-establishes normal resting potential Refractory period lasts as long as it takes to return to resting potential

Action potential: 1. Na+ gated channels open; 2. Na+ flood into axon; 3. Potential difference reversed 4. Na+ gates close; 5. K+ gated channels open; 6. K+ flood out of axon; 7. Inside axon returns to negative; 8. Resting potential restored.

The action potential 1) At resting potential some K voltage-gated channels are open but Na channels are closed. 2) Energy of stimulus causes some Na voltage channel in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels, along their electrochemical gradient. As they are + charged, they trigger a reversal in the PD across the membrane.

The action potential 3) As the Na+ diffuse into the axon, more Na channels open, increasing the Na+ influx by diffusion. 4) Once the action potential of around +40 m. V has been established, the voltage gates on the sodium ion channels close and the voltage gates on the K+ channels open.

The action potential 5) With some K voltage-gated channels open, the electrical gradient that was preventing further outward movement of K ions is now reversed, causing more K ion channels to open. This means more K ions diffuse out, causing repolarisation of the axon. 6) The outward diffusion of these K ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative (relative to the outside) than usual (hyperpolarisation)

The action potential 7) The gates on the K+ channels now close and the activities of the Na-K pumps once again cause Na+ to be pumped out and K+ to be pumped in. The resting potential of -65 m. V is re-established and the axon is said to be repolarised.

The squid Loligo vulgaris

Coordination & the Nervous system

Coordination & the Nervous system

Neurones WAL: An overview of how a nerve impulse works Some Most All What is a resting potential? How is resting potential established in a neurone? What is a resting potential? Plenary– Refections: What did you learn? What do you want to find out? How might you find this out? What skills did you use? How did your group function? What worked and what didn’t? What connections did you make? How was your thinking pushed? Why did you choose the approach you did? What did you enjoy and why? How could you have done it differently?