5.1.1 Communication and Homeostasis Flashcards
Sensory Neurones
They have short dendrites and one long dendron to carry impulse from receptor cells to the cell body. Also has one short axon that carries impulse from the body to the CNS.
Motor Neurones
Motor neurones have many short dendrites that carry nerve impulses from the CNS to the cell body. Then has one long axon that carries nerve impulses from the cell body to the effector cells.
Relay Neurones
Relay neurones have many short dendrites that carry nerve impulses from the sensory neurone to the cell body. It also has one axon that carries the impulse from the cell body to the motor neurones.
Neurone Cell Membrane
++++++++ outside cell
Resting potential =-70mV
maintained by Na+ and K+ pumps and K+ channels
Makes outside the cell positive
- inside cell
Stimulus
excited neurone cell membrane causing Na+ channels to open. Membrane becomes more permeable to Na+ so they diffuse into the neurone. Makes the inside of the neurone less negative
Depolarisation
If potential difference reaches the threshold (around -55mV), voltage gates Na+ channels open. More Na+ diffuses into the neurone. This is positive feedback.
Repolarisation
At around 30mV the Na+ channels close and the voltage gates K+ channels open. Cell membrane is more permeable to K+ ions. So K+ diffuses out of the neurone. This is negative feedback.
Hyperpolarisation
K+ channels are slow to close so often too many diffuse out. The potential gets more negative (below -70mV)
Resting Potential
Ion channels reset and potential difference returns to -70mV.
All or Nothing
Once the threshold is reached, an action potential 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. A bigger stimulus won’t cause a bigger action potential but will cause them to fire more frequently.
Myelinated Neurones
The Myelin Sheath is an electric insulator
In the PNS, the myelin sheath is made from Schwann cells
Between Schwann cells, there are areas of bare membrane called nodes of Ranvier. Na+ channels are concentrated here.
Depolarisation only happens are the nodes of ranvier where Na+ can get through the membrane.
Neurones cytoplasm conducts enough charge to depolarise the next node. This is called salatory conduction.
Non-Myelinated Neurones
In non-myelinated neurones, impulse travels as a wave along the whole length of the axon membrane.
Synapses part 1
An AP arrives at the pre-synaptic neurone and moves to the synaptic knob. The AP stimulates the voltage gates calcium ion channels in the presynaptic neurone to open. The calcium ions diffuse into the synaptic knob and they will be pumped back out when the AP has finished via active transport.
Synapses part 2
The influx of calcium ions into the synaptic knob causes the synaptic knob causes the synoptic vesicles to fuse with the presynaptic membrane. Vesicles release acetylcholine by exocytosis
Synapses part 3
ACh diffuses across the synaptic cleft and binds to cholinergic receptors. Causes Na+ channels in post neurone to open. Influx of Na+ causes depolarisation. AP generated if threshold is reached