Neurophysiology Flashcards
What part of the neuron generates the action potential?
Axon hillock
Describe the generation of an action potential.
- Resting membrane potential = -70mV
- Passively depolarises until it reaches ‘threshold’ = -60mV
- Na and K channels open
- Sodium flows into cell first causing upstroke and then Na channel closes
- K channel still open, K flows out repolarising the neuron
Why is the neuron ‘leaky’ for current?
What does the body do to overcome this?
Neuron membrane has a low resistance
Envelop neuron in myelin sheath
What cell produces myelin?
Oligodendrocytes in CNS
Schwann cells in PNS
Describe the flow of the action potential through a normal neuron.
Saltatory conduction
Action potential jumps from rode of ranvier to node of ranvier
Describe neurotransmission.
- Precursor synthesised to neurotransmitter and stored in vesicles.
- Action potential depolarises cell which opens Ca channels causing Ca influx
- Ca influx releases neurotransmitter by exocytosis
- Neurotransmitter in synaptic cleft acts on post-synaptic receptor
- Neurotransmitter inactivated, recycled into presynaptic neuron or diffuses away
What features of the pre and post synaptic neuron aid neurotransmission?
Active zone: terminal ends of presynaptic neuron have high concentration of neurotransmiter vesicles
Post-synaptic density: post-synaptic neuron have high concentration of receptors
What is the excitatory neurotransmitter?
How does it cause excitation?
Glutamate
Activates post-synaptic cation receptors which depolarise the cell and contribute to the action potential
What is the inhibitory neurotransmitter?
How does it cause inhibiton?
GABA
Activates post-synaptic anion receptors which polarise the cell and inhibit the action potential
What do EPSP and IPSP stand for and what are they?
EPSP = excitatory post synaptic potential = positive change in membrane potential (depolarise) caused by influx of cations (Na)
IPSP = inhibitory post synaptic potential = negative change in membrane potential caused by influx of anions (Cl)/efflux of cation (K)
Describe the two processes used by the cell body to integrate the inputs.
Which of these is more important?
Spatial summation: number of inputs determines output
Temporal summation: frequency of inputs determines output
Both are used together and are of equal importance
Polarisation of the neuron membrane generates an action potential.
T/F?
False
Depolarisation of the action potential by influx of Na generates an action potential
Depolarisation of the neuron membrane generates an action potential.
T/F?
True
Depolarisation of the action potential by influx of Na generates an action potential
Which of these would be inhibitory and which would be excitatory?
- Sodium channel agonist
- Sodium channel antagonist
- Potassium channel agonist
- Potassium channel antagonist
- Excitatory
- Inhibitory
- Inhibitory
- Excitatory
Sodium channels cause influx of sodium which depolarises the cell and contributes to the action potential
Potassium channels cause efflux of potassium which polarises the cell and inhibits the action potential
What are the two main classes of neurotransmitter receptor?
What are their main differences?
Ionotropic (receptor part of channel it controls, aka ligand gated ion channels)
Metabotropic (receptor distinct from channel it controls, aka G-protein coupled receptor)
Ionotropic receptors are faster and generally excitatory by generating action potentials
Metabotropic receptors are slower and generally inhibitory and moduate neuronal activity
What are the two classes of glutamate receptor?
What are their main differences?
non-NMDA: simple Na/K transfer, fast transmission
NMDA: Na/K/Ca and other ion transfer, slower
What neurotransmitter receptor is the most important in neurotoxicity?
Why?
NMDA
It allows Ca influx. Ca influx contributes to neurotoxicity
What are the two classes of GABA receptor?
What are their main differences?
GABA-A: controls ionotropic Cl channel
GABA-B: controls metabotropic K channel
Where is the neurotransmitter glycine found?
Is it inhibitory or excitatory?
Found in interneurons in spinal cord
Inhibitory (inhibits antagonist muscle contraction during flexion)
What is graded potential?
Changes in membrane potential which vary in size (not all or none as with action potentials), they are the summation of the impulses coming in through the dendrites
Where are the soma of upper and lower motor neurons and where do they synapse?
What is the basic function of these neurons?
Upper motor neuron: soma in brain and extend to ventral horn where they synapse
Lower motor neuron: soma in ventral horn and extend to muscles
Lower motor neuron contracts the muscles and upper motor neuron modulates contraction
What are the two types of motor neuron?
What are their functions?
aMN - contract extrafusal muscle fibres and generate force
yMN - contract the intrafusal muscle fibres in the muscle spindle in parallel with aMN which prevents them going slack as the extrafusal muscle fibres shorten
Where in the ventral horn would you find a neuron which supplies an
- axial muscle
- flexor muscle
- extensor muscle
- distal muscle?
Axial: medial
Distal: lateral
Extensor: ventral (posterior)
Flexor: dorsal (anterior)
What is a motor unit?
aMN + muscle it innervates
Where would you find a motor unit which innervates few muscle fibres?
Where would you find a motor unit which innervates many fibres?
What is the significance of this?
Few: extraocular eye muscle
Many: antigravity/leg muscles
The fewer muscle fibres a motor unit innervates, the more precise the movement is, but the movement is weak (extraocular eye muscles)
The more muscle fibres a single motor unit innervates strengthens the movement but it doesn’t allow fine movement (leg muscles)
What are the 3 types of muscle fibre?
What are their main differences?
Type I
- energy from oxidative phosphorylation
- appear dark due to myoglobin
- slow contraction
- fatigue resistant (good for marathons)
Type IIa
- energy from oxidative phosphorylation
- appear red
- fast contraction
- fatigue resistant (not as much as T1)
Tupe IIb
- energy from glycolysis
- appear pale as poorly vascularised
- fast contraction
- faitgue easily (but good for sprinting)
What is the muscle spindle?
Sensory organ contained in the muscle which sense stretch of the muscle through its intrafusal muscle fibres
Describe the myotatic reflex.
Stretch of intrafusal fibres in muscle spindle
Sensed by sensory afferents in muscle spindle which activate an aMN in the spinal cord
aMN contracts the muscle preventing further stretch
What is the Golgi tendon organ?
Sensory structure at muscle-tendon junction which assesses muscle tension
Pain is produced when a sensory neuron is overloaded by a strong stimulus.
T/F?
False
Pain is produced when a strong and potentially damaging stimulus is strong enough to activate high threshold units (HTU)
It is not produced by increased low threshold unit (LTU) activity
What are the 4 main types of sensory neuron?
Describe their functions and how myelinated they are.
Aa - proprioception - lots of myelin/very fast conduction
Aß - mechanoreceptor - some myelination/fast conduction
Aδ - temperature and acute pain (e.g. stab) - little myelination/moderate conduction
C - slow pain (e.g. ache) - unmyelinated
What is the receptive field?
Area over which one neuron can sense a stimulus
Where on the body might you find neurons with large/small receptive fields?
Small: areas with lots of sensory neurons (e.g. finger)
Large: areas with few sensoty neurons (e.g. back)
Name 3 low threshold mechanoreceptors and their function.
Merkel cell: mechanoreceptor
Meissner corpsucle: vibration
Pacinian corpsucle: pressure
What sensory nerve fibres are nociceptors?
What sort of pain do they cause?
Aδ - acute pain (stab, prick)
C - slow pain (ache, throbbing)
Where in the spinal cord does
- pain enter
- touch enter
- proprioception enter?
Pain: anterior dorsal horn
Touch: posterior dorsal horn
Proprioception: ventral horn
What information does the dorsal column medial lemniscus (DCML) transmit?
Fine touch and conscious proprioception
What information does the spinothalamic tract transmit?
Pain, temperature, pressure
Describe the path of an action potential through the dorsal column medial lemniscus (DCML).
First order neuron: skin > medulla
Second order neuron: medulla (crosses midline) > thalamus
Third order neuron: thalamus > somatosensory cortex
Describe the path of an action potential through the spinothalamic tract.
First order neuron: skin > dorsal horn
Second order neuron: dorsal horn (crosses midline) > thalamus
Third order neuron: thalamus > somatosensory cortex
What tract have the fasciculus cuneatus and fasciculus gracilis?
What are these?
Where are they in relation to each other?
Dorsal column medial lemniscus (DCML)
Fasciculus cuneatus: carries sensory information from upper limb through DCML (lateral to gracilis)
Fasciculus gracilis: carries sensory information from lower limb through DCML (medial to cuneatus)
Describe the path of an action potential through the trigeminal system.
First order neuron: skin > trigeminal ganglion
Second order neuron: trigeminal ganglion (thourgh trigeminal lemniscus) to thalamus
Third order neuron: thalamus to somatosensory cortex
Where on the trigeminal ganglion do pain/temperature and touch synapse?
pain/temperature: spinal nucleus
touch: sensory nucleus
What happens to the region of the somatosensory cortex supplying an amputated arm?
It is utilised by other sensory inputs (remapping)
What happens to sensory information after it is delivered to somatosensory cortex?
Integrated in the posterior parietal cortex
What helps integrate synergistic flexion and extension?
Interneurons
(e.g. extension of arm stretches flexor muscles which would induce the myotatic reflex. interneurons inhibit the myotatic reflex of flexor muscles during extension)
What is the flexor reflex?
What occurs in the other limb when this occurs?
Noxious stimuli causes flexion and inhibition of extension by excitatory interneurons (flexor reflex)
Contralteral limb is extended and flexion is inhibited to aid posture (crossed extensor reflex)
What information does the corticospinal transmit?
Fine motor movement
Describe the path of an action potential through the corticospinal tract.
UMN: motor cortex (cross midline at decussation of the pyramids) > anterior horn
LMN: anterior horn > muscle
Name the pyramidal tracts.
What are their functions?
Corticospinal: conduct impulses to the spinal cord
Corticobulbar: conduct motor impulses to the cranial nerves
Name the lateral descending tracts.
What are their general function?
Corticospinal tract
Rubrospinal tract
Cause movement
Name the ventromedial descending tracts.
What are their general function?
Reticulospinal tract
Vestibulospinal tract
Tectospinal tract
Modulatory, control movement
Function of reticulospinal tract.
Enhances antigravity muscles and posture
Function of rubrospinal tract.
Help control limb flexor muscles
Soma in red nucleus
Function of vestibulospinal tract.
Inputs from vestibular labyrinth and cerebellum
Controls posture and head movements
Function of tectospinal tract.
Has inputs from retina
Influence neck/upper tunk muscles in response to visual stimulus
Which sensory fibre has both afferent and efferent functions?
What is its efferent function?
Some C fibres
Release proinflammatory mediatiors (substance P, CGRP) with pain
