nEUROMUScular and spinal cord 9. BLASTIN Flashcards
What is a synapse and describe its functions and features
Contact/junction (greek word)
Allows for contact neuron to muscle or from neuron to neuron
Basic structure similar throughout entire NS
Arrangement can be simple or complex
Contact ratios range from 1:1 for muscle to 10^3 : 1 in CNS
What is the distance of the synaptic cleft?
10-50 nM
What are the two directions the post synaptic membrane potential can be altered ?
It can be made less negative i.e. closer to threshold for generation of an action potential EXCITATORY POST SYNAPTIC POTENTIAL (EPSP)
More negative, further away from threshold for an AP, INHIBITORY POST SYNAPTIC POTENTIALS (IPSP)
What are graded effects?
Whether the neuron fires or not is dependent on summation of inputs.
EPSPs and IPSPs can also summate, degree of summation will determine how readily threshold for AP can be reached
Define a neuromuscular junction
It is a specialised synapse between a pre-synaptic motor neuron and a motor end plate i.e. MUSCLE FIBRE MEMBRANE
Describe the activation of a neuromuscular junction
When an AP arrives at NMJ membrane, ACh released from presynaptic cell when the SNARE proteins interact with membrane, Ca2+ influx triggers ACh release.
ACh binds to receptors on motor end plate, ion channels open and there is an Na+ influx causing an AP in the muscle fibre.
What would you expect to see at rest in NMJ?
Miniature end-plate potentials (mEPPs)
At rest, individuals release ACh at very low levels/rate causing small changes in membrane potential that are inconsequential.
Vesicles are constantly dumping their contents into the cleft.
Summarise the alpha motor neuron
Also called ventral horn cells, anterior horn cells or LMN
These are the lower motor neurons of the brainstem and the spinal cord
They innervate the (extrafusal) muscle fibres of the skeletal muscles
Their activation causes muscle contraction
The motor neuron pool contains all alpha motor neurons innervating a single muscle
What is extrafusal and intrafusal muscle?
Extrafusal = standard skeletal muscle fibres, innervated by alpha, generate tension by contracting, allow skeletal muscle movement
Intrafusal = Serve as specialised sensory organs that detect amount and rate of change in length of a muscle , also skeletal
What are spindles? NOT IN PP
Coiled spring like sensory receptors in muscles, when stretched feedback to CNS and allows excitatory reflex, patella ligament when hit by hammer!
Describe the arrangement of alpha motor neurones within the ventral horn.
Dorsal – flexors
Ventral – extensors
Medial – proximal
Lateral - distal
What is a motor unit?
This is the name given to a single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force.
Humans have approximately 420,000 motor neurons and 250 million skeletal muscle fibres.
On average each motor neuron supplies about 600 muscle fibres.
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.
What is an important rule to remember regarding the connections between alpha motor neurons and muscle fibres?
One motor neurone can innervate several muscle fibres
But every muscle fibre can only be innervated by one motor neuron
What are the 3 types of motor unit?
Slow (S type 1)
Fast, fatigue resistant (FR type 2a)
Fast, fatiguable (FF type 2b)
Describe the Slow motor unit
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
Describe the fast fatigue resistant motor unit
2a larger diameter cell bodies larger dendritic trees thicker axons faster conduction velocity
Describe the fast fatiguable motor unit
2b larger diameter cell bodies larger dendritic trees thicker axons faster conduction velocity SAME AS 2A
Discuss distribution of fibre types
Not bunched together based on type, fairly spread out
Postural muscles are SLOW muscles e.g. soleus
What is the classification criteria TING, how have they been classified?
amount of tension generated
speed of contraction
fatiguability
Describe and explain the difference in innervation ratio across different muscles in the body using examples. CONTEXT NOT IN PP
Muscles that require very fine control (e.g. extrinsic eye muscles) have a low innervation ratio (few fibres innervated by a single neurone)
Muscle that are required to generate a lot of power have a high innervation ratio because when the motor unit fires, it will cause the contraction of a large mass of muscle fibres thus generating power (e.g. quadriceps)
What are the two mechanisms by which the brain regulates the force a single muscle can produce?
Recruitment (more motor units) Rate coding (increase freq. of APs travelling down nerves of muscle fibre)
Describe recruitment
Motor units are not randomly recruited. There is an order to this.
Governed by the “Size Principle”. Smaller units are recruited first (these are generally the slow twitch units).
As more force is required, more units are recruited.
This allows fine control (e.g. when writing), under which low force levels are required.
Slow then Fast Fatigue-Resistant then Fast Fatiguable
Describe rate-coding
A motor unit can fire at a range of frequencies. Slow units fire at a lower frequency.
As the firing rate increases, the force produced by the unit increases.
Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
What are neurotrophic factors?
A type of growth factor that prevents neuronal death and promotes growth after injury
Factors produced within the nerves and are transported throughout the nerve to maintain the nerves integrity and function.
Why doesn’t CNS regenerate like peripheral nerves?
You have millions of axons in CNS as opposed to a few 1000s so consequences of incorrect re-wiring outweigh benefit
What are effects of neurotrophic factors
Fibre characteristics are dependent on the type motor unit that innervates it
E.G. fast twitch muscle and slow twitch muscle are cross innervated, soleus becomes fast-twitch and FDL becomes slow
Discuss the plasticity of alpha motor neurons
Fibre types can change properties under many different conditions.
Type IIB to IIA most common following training
Usually no way of converting
Type I to II possible in cases of severe deconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types
Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle (evidence from slower contraction times).
What tract is responsible for voluntary movements?
Pyramidal/Corticospinal tract
only 2 nerves from motor cortex to big toe, one synapse to lower motor neuron in ventral horn at spinal level
What is the role of extrapyramidal tracts?
It is responsible for automatic movements in response to stimuli (these are movements that your body makes without you being aware of it)
Function of vestibulospinal tract?
regulates posture to maintain balance, and facilitates mainly α motoneurones of the postural, anti-gravity (extensor) muscles
What is a reflex ?
An automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness.
An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.
“…….whose magnitude and timing are determined respectively by the intensity and onset of the stimulus”.
Components of a reflex arc SIMPLE
afferent signal sensory receptor sensory neuron interneuron/relay neuron integrating centre motor neuron effector
What are the two signals that are generated when the patellar ligament is tapped?
There is an excitatory signal going to the quadriceps
There is also an inhibitory signal going to the hamstrings (antagonist)
Why is there a difference in the time taken for these signals to reach the relevant muscles?
The signal going to the quadriceps only has one synapse (monosynaptic) whereas the signal to the hamstrings goes via an inhibitory interneurone so there are two synapses.
This means that the signal to the quadriceps arrives slightly faster than the signal to the hamstrings.
Function of reticulospinal tract
coordinate automated movements of locomotion and posture (e.g. to painful stimuli)
Discuss the Hoffmann test
Can’t rely on knee jerk as reflex dependent on which part of the tendon is hit
IDENTICAL STIMULUS EVERY TIME
Bypass physical stretch muscle
Deliver an impulse to a nerve containing sensory and motor fibres, carry it via reflex arc back to muscle
What two twitches will you get when you stimulate nerve at back of knee?
M wave: direct motor response, motor neuron that has been stimulated directly affecting muscle
H wave: caused by AP going sensory, SC then motor, seen SHORT time later
Sensory nerves more amendable as larger get H wave at lower stimulus intensity
Discuss polysynaptic reflexes and flexion withdrawal
Lots of polysynpatic reflexes that go up and down SC to innervate groups of muscles on same side
There are also reflexes that cross SC such that other limbs keep us upright etc.
FLEXION WITHDRAWAL AND CROSSED EXTENSOR
L00K AT IMAGES TO HELP
are there Supraspinal control of reflexes?
There is some descending control of reflexes
Describe Jendrassik’s Manoeuvre
Testing knee jerk with clenched teeth reflex is 2/3 times greater
Shows large inhibitory control over reflexes when removed
What is seen in a decerebated animal?
Meaning they’ve had cerebral cortex and nuclei removed
Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.
HYPERREFLEXIA
how do Higher centres influence reflexes
Activating alpha motor neurons Activating inhibitory interneurons Activating propriospinal neurons Activating gamma motor neurons Activating terminals of afferent fibres
What is the gamma reflex loop?
It shortens the spindles in muscle to maintain its sensitivity
There is also facilitation from higher centres, which increase the sensitivity of the motor neurone to afferent input
Higher centres & pathways involved are:
Cortex – corticospinal (fine control of limb movements, body adjustments)
Red nucleus – rubrospinal (automatic movements of arm in response to posture/balance changes)
Vestibular nuclei – vestibulospinal (altering posture to maintain balance)
Tectum – tectospinal (head movements in response to visual information).
What signs are seen with upper motor neurone lesions?
stroke is an example
Hyperreflexia
Clonus – muscular spasm repeated, often rhythmic, contractions
Babinski’s Sign - plantar extension if you stroke bottom of foot, toes fan out
What is hypo-reflexia associated with?
LMN lesions
BELOW OR ABSENT REFLEXES
