Bioelectric Potentials, Neurones & Synapses

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.