Week 6 - lower motor neurons Flashcards
Which part o the reticular formation produces noradrenaline?
noradrenaline
What neurotransmitter is produced by the substantial nigra?
dopamine
What neurotransmitter is produced by Ralph nuclea ?
serotonin
what produces movement?
spatial and temporal patterns of muscular contractions, which are orchestrated by neural circuits in the brain and spinal cord
What do neural circuits make
simple movements eg. knee-jerk reflex
complex motor movements eg. playing musical instrument
ascending
always sensory
descending
always motor
what motor neurons originate in the motor cortex
voluntary movement
what motor neuorns originate in the brain stem?
involuntary movement
Lower motor neuron
spinal cord to muscle
upper motor neuron
brain to spinal cord
interneurons
found only in CNS
connector between sensory/ motor system in real time
found in both spinal cord+ Brain
example of a pathway
receptor -> dorsal root ganglion on sensory -> interneurone at lateral horn -> motor
local interneuron
short axon, only connects with nearby neurones, a lot in reflex chains
relay interneurons
long axons, tracts connect separate regions of brain for large scale processing, most common in Brian
are interneurons multi or unipolar?
always multi
what are interneurons important for?
learning, decision making (relay) and synchronised muscle movement (local)
can release excitatory glutamate or inhibitory GABA as neurotransmitters
where do upper neurons cell bodies lie ?
brainstem or cerebral cortex with axons descending to synapse with local circuit neurons or more rarely lower motor neurones directly
What are upper motor neurons that originate in the motor cortex responsible for?
initiation of voluntary movements
What are upper motor neurones that originate in brainstem responsible for?
basic navigational movements and regulating posture
What is the basal ganglia
upper motor neurons in cortex will relay through it, checkpoint, ensure its the right response, can inhibit constant excitation eg. seizure
cerebellum
coordination of movement
coordinates Which types of neuron primarily inputs to LMNs ?
inhibitory interneurones -> need to stop incorrect movement
What are lower motor neurons?
motor neurons in cord and brainstem that directly innervate SKELETAL muscle contraction and bind to motor synaptic junction always ACETYLCHOLINE
Name the two types of lower motor neuron found in pools of the ventral horn
alpha and gamma
Alpha motor nurons
- innervate extrafusal muscle fibres
- largest, widest axons, prevent inhibition of organelles as get in the way, more myelin
What are gamma motor neurones
- smaller, less myelinated
- innervate intramural muscle fibres
What do extrafusal muscles do?
- generate force
- on outside
What do intramural muscles do?
- on inside
- measures stretch also assists in contraction
What are motor units
small functional motor unit in nervous system
made up of an individual alpha motor neuron and extrafusal fibres it innervates
in femur
lot of muscle fibres innervated by single motor neuron, the bigger the motor units, the more muscle fibres innervated at once
motor unit subtypes
- slow-contracting, fatigue-resistant MUs (s) - needed for sustained, low effort ie. posture eg. cardiac muscle
- fast-contracting, fatigue resistant MUs (FR) - intermediate ie. walking
- fast-cot rating but fatiguable units (FF) - brief exertion ie sprinting
What do distinctions between motor units help explain?
- how the Ns produces movements appropriate diff circumstances
- strucutal differences between muscle groups
What is the best example of a large motor unit
in the calf (gastronemius) around 100 muscle fibres in each one
which of these is proprioceptive
LMN intramural
how is muscle contraction force generated?
- recruitment of motor units
- impulse frequency
recruitment of motor units
increasing or decreasing number of motor units active at any one time changes the amount of force produced by a muscle
-> size principle
impulse frequency
frequency of impulse contributes to twitch strength - nr of units recruited
- at low frequency stimulation, each AP in motor neuron results in single twin of recruited muscle fibres
- at higher frequency, twitches sum to produce a force greater than that produced by single twitches
tetanus
at higher frequency o stimulation, the force produced is greater, but individual twitches are still apparent - unfused tetanus
- highest rate of motor neuron activation, individual twitches are no longer apparent, the response is referred to as fused tetanus
-> can’t see individual muscle contractions as it is constant
which motor units are recruited to gastrocnemius to allow a massive jump
S, FR, FF
reflex arc
- spinal pathway in which a motor response is generated in response to a sensory stimulus
- local interneurons within the spinal cord initiate sensory motor reflex actions
simples reflex arc
entails sensory response to muscle stretch, which provides direct excitatory input to the lower motor neuones innervating muscle that has been stretched
-> sensory signal for stretch reflex originates in muscle spindle
muscle spindle
- highly specialised organs found in most skeletal muscles that provide mechanosensory information about muscle length
how are muscle spindles organised?
8-10 intramural fibres arranged in parallel with the extrafusal fibres that make up bulk of muscle
what are the two structural classes of intramural fibres?
- nuclear bad fibres and nuclear chain fibres that differ in sensitivity to stretch
How many nuclear bag fibre and nuclear chain fibres are contained in most muscle spindles?
3 bag
twice as many nuclear chain fibres
muscle spindle Ia
afferents are sensory icons that coil around central part of each class of intramural fibre
group 2 afferents
form secondary endings mainly on nuclear chain fibres
what does the stretch imposed on muscle fo
deforms intramural muscle fibres, which turn initiate action potential by activating mechanically gated ion channels in afferent axons innervating spindle
what do muscle spindles do
detect muscle length
- fire action potential spontaneously at steady rate at resting muscle lengths
- when stretched action potential firing rate briefly increases
- muscle spindle can therefore be viewed as a muscle length detector
How can we detect and measure action potentials in real time?
EMG electromycrogram
stretch reflex circuitry
what is the highest level of afferent stretch circuitry
spinal local interneurons - brain not involved in reflex
gamma motor neurons
- innervate intramural fibres
- in stretch: work in tandem with alpha motor neurons
- alpha simulated alone, response of Ia fibre decreases muscle contracts, when both alpha and g-motor in Ia firing during muscle shortening, thus g-motor neurones regulate gain of muscle spindles so they can operate efficiently at any muscle length
Golgi tendon organ
encapsulated afferent nerve endings located at junction of muscle and a tendon at the junction o muscle and tendon that provide mechanosensory information about sucre tension
gogli tendon negative feedback - not in exam
what afferents are sensory axons that could around central part of each class of intramural fibre ?
group Ia
organisation of descending motor control
UMN pathways differ in where they terminate in SC
innervate different muscles
white matter tracts want to be as short as possible for efficient
brodmann’s areas
- developed in 1909
- defined via cytoarchitecture
- distinct under microscope
Area 9 and 46
prefrontal cortex - planning future movements
Area 6 and 8
muscle contractions - premotor area
area 4
primary motor cortex - execution fo simple motor programs
area 1 and 3
somatosensory cortex
dorsolateral prefrontal cortex
- located in area 9 and 46
- executive functions
- highest cortical area involved with motor planning
pre-motor cortex
- sensory input form inferior (visual) and superior (touch) parietal lobes
- prefrontal areas in frontal lobe have input regarding motivation
primary motor cortex
- located in pre central gyrus in area 4
- strong axonal connections to LMNs (thick axons)
Types of PMC cells
- pyramidal cells of cortical layer 5 are upper motor neuons of primary motor cortex
- Among these neurons, Betz cells, which are the largest neurons in the human CNS but only account for -5% of projections to spinal cord
posture is controlled via which region fo white matter?
medial
corticobulbar spinal tract
- axons leave tract at appropriate level of brainstem:
1. cranial nerve nuclei eg. hypoglossal nucleus for motor supply of tongue nucleus
2. reticular formation - important wakefulness - corticobulbar axons can also terminate among nuclei at base of pons
Tracts
- axons of LMNs descend into corticobulbar and corticospinal tracts (axons that terminate in brainstem or spinal cord respectively)
- internal capsule > cerebral peduncle> pons > pontine fibres
lateral corticospinal tract
large majority of axons -90% cross caudal part of medulla to form lateral corticospinal tracts ins spinal cord
ventral corticospinal tract
remaining corticospinal axons -10% that do not cross in caudal part of medulla terminate bilateraly to ventral corticospinal tract
what are lateral axons involved in
innervating muscles of forearm and hand
where do ventral axons arise form primarily?
from dorsal and medial motor cortex and serve axial and proximal limb muscles
where are spinal lock interneurones found?
intermediate horn
What happens when motor systems are damaged?
- lower - 99% axon - can repair, cell bodes can’t
- > spinal cord/brain much worse
symptoms of lower
- weakness or paralysis
decreased superficial reflexes
hypoactive deep reflexes
decreased tone
fasciculations and fibrillations
severe muscle atrophy
symptoms of upper
weakness
spasticity
increased tone
hyperactive deep refills
clonus
babinskis sign
loss of fine voluntary movements
what is decussation?
axons crossing the contralateral side across the midline
