Week 9 - Structure of nervous tissue, nerve conduction + peripheral nerves Flashcards
What glial cells are in the central nervous system?
Astrocyte
Oligodendrocyte
Microglia
Ependymal cell
What glial cells are in the peripheral nervous system?
Satellite cell
Schwann cell
What are the functions of astrocytes and satellite cells?
- Regulate the extracellular microclimate
- Remove waste products
- Scar formation
- Help to selectively control passage of molecules between the blood stream and the nervous tissue via the BBB
What are the functions of oligodendrocytes and schwann cells?
- Myelination โ provides insulation of the axon.
Prevents degradation of electrical signals and enhances conduction speeds.
NOT ALL AXONS MYELINATED
What is the function of microglia?
injest pathogens - provide immune support to the CNS
What is the function of ependymal cells?
Producing cerebrospinal fluid - protective cushion for brain and spinal cord
Functions of the brain?
๐ญ.๐๐๐๐ผ๐ป๐ผ๐บ๐ถ๐ฐ ๐ณ๐๐ป๐ฐ๐๐ถ๐ผ๐ป๐
heartbeat, breathing, digestion, body temp.
๐ฎ.๐๐ผ๐ด๐ป๐ถ๐๐ถ๐๐ฒ ๐๐ธ๐ถ๐น๐น๐
planning, thinking, emotions + behaviours, learning
๐ฏ.๐ฆ๐ฒ๐ป๐๐ผ๐ฟ๐ถ๐บ๐ผ๐๐ผ๐ฟ ๐ณ๐๐ป๐ฐ๐๐ถ๐ผ๐ป
sensation + movement
What two cellular components is neural tissue made of?
- neurons
- glial cells
Function of an axon
transmit messages away from cell body
Function of a dendrite
receive messages from other cells
What are neurones
โfunctional unit of the nervous system
โspecialised cells which transmit electrical impulses
โhave common structural features
Function of axon terminals
form junctions with other cells
Describe a multipolar neurone and example
Many branches coming from the soma.
e.g. Motor neurone
Describe a pseudo-unipolar neurone and an example
One process leaving the soma.
The axon has a peripheral branch and a central branch.
e.g. Sensory neurone
Describe a bipolar neurone and an example of itโs location
Have one dendritic process and one axonal process with the soma in the middle.
E.g. Retina (vision) or olfactory system (smell)
Why is an oligodendrocyte different to a schwann cell?
In an oligodendrocyte, each cell provides for a number of different axonal segments on different axons.
How does the astrocyte impact the BBB
It secretes growth factors - stimulate endothelial cells to create an increased number of tight junctions (see a picture).
The more tight junctions, the more effective the permeability of the BBB is to remain selective.
What is the resting membrane potential?
Generated by an unequal distribution of Sodium (Na+) Potassium (K+) and Chloride (Cl-) ions.
At rest, this is around -70mV
This is due to the extracellular environment having a net +ve charge and the intracellular environment having a net -ve charge.
What is the sodium/potassium pump?
โ3 sodium ions bind to the pump.
โA phosphate from ATP is donated to the pump (energy used)
โPump changes shape and releases sodium ions outside of the cell.
โ2 potassium ions bind to the pump and are transferred into the cell.
โPhosphate group is released and pump returns to its original shape.
How is an action potential generated?
โStimulus triggers influx of positively charged ions, changing voltage across membrane from its resting value (-70mV) to a positive value (+30/40mV)
โDEPOLARISATION โ transient rapid influx of Na+ ions followed by repolarisation.
โREPOLARISATION โ delayed, sustained efflux of K+ions
Why is an action potential an โall or nothingโ response?
An action potential cannot be generated of different strengths, it is either generated or isnโt, depending on if the threshold stimulus is met.
What is a sub-threshold stimulus?
A stimulus that is too small to cause an influx of sodium ions and therefore produce an action potential.
What is hyperpolarisation?
Following repolarisation.
A phase where some potassium channels remain open and sodium channels reset.
A period of increased potassium permeability results in excessive potassium efflux before the potassium channels close.
New AP canโt be generated until RMP is restored.
What is propagation?
The AP is generated at the axon hillock and then propagates as a wave of depolarisation along the axon.
Voltage-gated Na+ channels produce regenerative current so AP retains amplitude with distance as subsequent patches of membrane are activated.
What propagation velocity?
The speed at which an AP moves along axon. (range = 0.1m/sec - 100m/sec).
How does myelin improve propagation velocity?
โIt is a fatty substance that increases the resistance of the path across the membrane - prevents ions โleakingโ out.
โIn myelinated axons - voltage gated Na+ ions concentrated at Nodes of Ranvier.
โAP โjumpsโ from node to node in a process called SALTATORY CONDUCTION
What factors (besides myelin) influence conduction velocities?
diameter of the axon
temperature
(larger diameters and higher temps. generally increase conduction velocity)
Describe the process of synaptic transmission
- AP reaches axon terminal
- Calcium channels open
- Ca2+ causes synaptic vesicles to release a neurotransmitter
- The neurotransmitter diffuses across the synaptic cleft
- Before binding to post synaptic receptors (receptor specificity)
- Activation of receptors causes Na+ channels to open in post synaptic cell โ if post synaptic potential reaches threshold, new AP generated in 2nd neurone.
Active re-uptake of remaining neurotransmitter from synaptic cleft.
What is a neurotransmitter?
A chemical messenger that transmits signals across a chemical synapse from one neuron to another (dopamine, serotonin, acetylcholine, GABBA, glutamate, noradrenaline etc)
What is summation?
The post synaptic potentials (PSPโs) produced by each active synapse can sum together - in space and/or time - to determine the behaviour of the postsynaptic neuron.
What is spatial summation?
The addition of simultaneous stimuli from several conducting fibres.
Excitatory potentials from many neurons trigger threshold point.
What is wave summation?
Many excitatory potentials from one neuron triggers threshold point.
What constitutes the CNS?
The brain and spinal cord
(integration and command centre)
What are the divisions of the peripheral nervous system?
Autonomic nervous system โ regulates internal environment
(sympathetic + parasympathetic division)
Somatic nervous system โ enables us to interact with the external environment
(involves sensory and motor nerves)
How does the CNS interact with the ANS?
The ANS uses autonomic sensory neurones to transmit information from internal organs and feed-back to the CNS.
The CNS processes this information and produces a motor command for the autonomic motor neurones which transmit signals from the CNS to innervate internal organs and glands.
(involuntary control)
