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Lecture 3
Divisions of the Nervous System: (2)
Central Nervous System: Brain and spinal chord.
Peripheral Nervous System: nerves outside the CNS.
If a signal does not reach the cortex ?
If a signal does not reach the cortex, then we are not aware of it.
2 main cell types:
1) Neurons (nerves)
2) Neuroglia (support cells)
3 main parts of neurons
1) Cell body ( centre containing nucleus)
2) Dendrites (branches)
3) Axon (conduct impulse in one direction)
What does myelin sheath do ? What are the three different types of cells ?
Myelin Sheath helps make an impulse faster.
Sensory/Afferent - carries impulse from the body to the CNS.
Motor/Efferent - carries the impulse from the CNS to the body.
Interneurons - link up the sensory and the motor neurons.
Neuroglia: What are they ? What are the divisions of the CNS (2) and the PNS 94), list of their functions:
Neuroglia: are support cells, help nerves to conduct electrical impulse.
PNS: schwann cells: produce myelin sheath, satellite cells: support cell bodies
CNS: oligodendrocytes: make myelin sheath, microglia: defence and housekeeping, astrocytes: regulate the external environment of the neurons in the CNS, BBB( blood brain barrier protects brain - not everything that circulates is passed into the blood). Ependymal cells- epithelial
What is neurolemma ?
All axons in the CNS are covered in a continuous sheath, cover of schwann.
Myelin Sheath: what makes it ? Function ?
The oligodendrocytes in the CNS and the schwann cells in the PNS. Make a nerve impulse faster.
What is RMP, and PD. What is a nerve at rest ?
Resting Membrane Potential (RMP): Inside nerves are negative, and outside is positive. Potential Difference (PD). Nerve at rest is -70mV.
Why ? (2)
1) Because mostly negatively charged species are on the inside of a nerve and then mostly positively charged species are on the outside of a nerve.
2) Plasma membranes are permeable to K+, cross membranes via ion channels ( some leaky, some voltage charged)
RMP is mostly due to K+, Why ??
Equilibrium potential. PD balances ion concentration. K+= -85mV (mostly intracellular material) , Na+=65mV (mostly extracellular material).
What is Depolarisation ? Hyperpolarisation ? Repolarisation ?
Depolarisation; +ve charge in the nerve. Hyperpolarisation: more -ve charge in the nerve Repolarisation: Back to RMP.
Action Potential (AP): How nerve impulses work
Stimulus causes Na+ - cells via voltage gated Na+ channels.
Local anaesthetic:
Local anaesthetic blocks the Na+ pumps, so impulse does not reach the cortex.
At peak, What happens the Na+ channels ?
What affects stimulus strength ?
At peak, the Na+ channels close, the K+ channels open and the K= leaves. Stimulus strength depends on frequency not amplitude. So the more action potential, the stronger the stimulus.
Refractory period: What is it ? 2 types?
Reason why AP is unidirectional. Na+ is inactive. 2 types: absolute: no amount of stimulus can open them. Refined: strong stimulus can trigger AP.
Saltatory Conduction (Myelin):
Na+ channels are very concentrated at the Nodes Of Ranvier ( 1-2mm apart). It jumps chunks of membrane.
How does a signal transfer between two nerves ?
Neurotransmitters
What is the EPSP ? and the IPSP ?
Excitatory post-synaptic potential: graded depolarisation, when the membrane of the postsynaptic membrane becomes less negative
Inhibitory post-synaptic potential: when the inside of the postsynaptic membrane becomes more negative.
What occurs after the absolute refractory period ?
The relative refractory period occurs after the absolute refractory period.
What triggers the release of neurotransmitters ?
The entering of calcium ions into the synapse triggers the release of neurotransmitters.
Does myelin sheath surrounds all axons in the PNS but not in the CNS ?
The axons in the PNS are surrounded by a continuous sheath, layer of Schwann. Some of the axons in the CNS are covered but not all by a layer of Schwann.
