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)
How does the CNS interact with the SoNS?
Somatic sensory neurones transmit information from the sense organs and feed-back to the CNS.
The CNS processes this information and produces a motor command. Somatic motor neurons transmit signals from the CNS to innervate skeletal muscles.
(voluntary control)
Spinal cord structure?
โ long, delicate cord of nervous tissue
โ about 45cm long and between 6mm - 13mm in diameter
โ protected by bony structures in the vertebral column
Spinal cord location?
โ lies in vertebral canal
โ pair of spinal nerves emerge from each spinal vertebrae level via the intervertebral foramen (easily squashed)
Spinal cord function?
Responsible for transmitting nerve signals to and from brain
How many cervical vertebrae are there?
7
How many thoracic vertebrae are there?
12
How many lumbar vertebrae are there?
5
How many sacral vertebrae are there?
5 fused vertebrae
How many vertebrae are in the coccyx?
4 fused vertebrae
How many pairs of spinal nerves are there?
31
How many pairs of cervical spinal nerves are there?
8
How many pairs of thoracic spinal nerves are there?
12
How many pairs of lumbar spinal nerves are there?
5
How many pairs of sacral spinal nerves are there?
5
How many pairs of coccygeal spinal nerves are there?
1
What does grey matter within the spinal cord consist of?
Cell bodies of neurones within the spinal cord
What does white matter within the spinal cord consist of?
myelinated axons
What is the dorsal root?
Afferent (sensory) fibres
(all sensory info comes into spinal cord at the back)
What is the ventral root?
Efferent (motor) fibres
(all motor info leaves spinal cord at the front)
What is the dorsal root ganglion?
Cluster of cell bodies - sensory neurones
What are the grey and white rami responsible for?
Making connections with the ANS via the sympathetic ganglion
What is a mixed spinal nerve?
Refers to the part of the nerves where both afferent (sensory) and efferent (motor) nerves are housed in the same nerve epineurium to go through the intervertebral foramen.
What is the ventral ramus?
โBranches from the mixed spinal nerve.
โLarger than dorsal ramus.
โ Carries sensory and motor info
โSupplies the anterolateral parts of the trunk and limbs.
What is the dorsal ramus?
โBranches from mixed spinal nerve
โCarries sensory and motor info
โPass posteriorly to supply the skin and deep muscles of the back
What is a dermatome?
The area of skin that is supplied by a specific spinal nerve and itโs afferent (sensory) fibres.
These fibres are responsible for telling the brain what is going on externally.
What is a myotome?
A group of muscles innervated by a specific spinal nerve and itโs efferent (motor) fibres.
How may a spinal root/nerve issue be found through dermatomes?
Impaired sensation at specific dermatome region.
Indicates an issue with spinal nerve/root that controls that area.
How may a spinal root/nerve issue be found through dermatomes?
Changes in muscle strength within a particular myotome may indicate damage to a particular spinal nerve root.
What movement has the strongest association with C2?
Neck flexion
What movement has the strongest association with C3?
Lateral flexion of the neck
What movement has the strongest association with C4?
Shoulder elevation?
What movement has the strongest association with C5?
Shoulder abduction
What movement has the strongest association with C6?
Elbow flexion with wrist extension
What movement has the strongest association with C7?
Elbow extension
What movement has the strongest association with C8?
Finger flexion
What movement has the strongest association with T1?
Finger abduction
What movement has the strongest association with L2?
Hip flexion
What movement has the strongest association with L3?
Knee extension
What movement has the strongest association with L4?
Ankle dorsiflexion
What movement has the strongest association with L5?
Great toe extension
What movement has the strongest association with S1?
Ankle plantar flexion
What are plexus?
A network of nerves that enables fibres from different spinal levels to come together to form named peripheral nerves.
These named peripheral nerves go on to supply the muscles and skin in periphery.
Cervical plexus info?
Spinal nerves C1-C4
Creates named peripheral nerves that innervate the back of the head, the neck, the shoulders and the diaphragm.
E.G. PHRENIC NERVE
Brachial plexus info?
Spinal nerves C5-T1
Creates named peripheral nerves that innervate the upper limbs.
E.G. MEDIAN NERVE / RADIAL NERVE / ULNAR NERVE
Lumbar plexus info?
Spinal nerves L1-L4
Creates named peripheral nerves that innervate anterior + lateral aspects of the thigh.
E.G. FEMORAL NERVE
Sacral plexus info?
Spinal nerves L4-S4
Creates named peripheral nerves that innervate posterior aspect of the thigh and everything below the knee.
E.G. SCIATIC NERVE
Nerve root origins of the femoral nerve?
L2, 3, 4
Nerve root origins of the sciatic nerve?
L4, 5
S1, 2 , 3
What is the epineurium?
Outermost layer of dense irregular connective tissue surrounding a peripheral nerve.
Provides protective surface.
What is a fascicle?
A group of axons.
Fascicles group together axons according to their function e.g. sensory axons together or motor axons together within a peripheral nerve.
What is perineurium?
(around one fascicle)
Holds a bundle of axons of a similar type together within fascicle.
What is the endoneurium?
Protective tissue sleeve around each axon
