Neuromuscular and spinal cord Flashcards
What is meant by a synapse
Synapse - Greek word for “contact or junction”
Allows for contact from neurone to muscle or from neurone to neurone.
Describe the structure of synapses throughout the nervous system
Basic structure is similar throughout the nervous system.
Arrangements can be simple or complex.
Contact ratio – ranges from 1:1 for muscle to 103:1 in the CNS
How can the membrane potential of the post-synaptic neurone be altered
The membrane potential of the post synaptic neurone can be altered in two directions by inputs.
The membrane potential can ultimately become more negative or more positive
Graded effects = SUMMATION- will receive input from multiple stimulatory and inhibitory stimuli- final potential is the sum of these stimuli.
Describe an excitatory post-synaptic potential (EPSP)
It can be made less negative – i.e. be brought closer to threshold for firing; this is an excitatory post synaptic potential (EPSP)
Describe an inhibitory post-synaptic potential
It can be made more negative – i.e. be brought further away from threshold for firing; this is an inhibitory post synaptic potentials (IPSP)
What does the ability of the post-synaptic neurone to reach its threshold potential depend on
EPSPs & IPSPs can also SUMMATE.
The degree of summation will
determine how readily a neuron
can reach threshold to produce an action potential.
What is meant by the NMJ
A specialised synapse between the motor neuron and the motor end plate, the muscle fibre cell membrane
What is meant by a graded effect
You get graded effects – whether the neurone fires or not is dependent on the summation of inputs
What happens when an action potential arrives at the terminal bouton of an NMJ
When an action potential arrives at the MNJ, Ca2+ influx causes ACh release. ACh binds to receptors on motor end plate.
Ion channel opens – Na+ influx causes action potential in muscle fibre.
What proteins are involved with the release of ACh at synapses
SNARE proteins
Describe ACh release at rest
At rest, individual vesicles release ACh at a very low rate causing miniature end-plate potentials (mEPP)
What is the key difference between intrafusal and extrafusal muscle fibres
Intrafusal – these are skeletal muscle fibres that serve as sensory organs (proprioceptors) that detect the amount and rate of change of length of a muscle
Extrafusal – standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting, thereby allowing for skeletal muscle movement
What are the alpha motor neurones
The lower motor neurones of the brainstem and ventral horn of the spinal cord
Describe the role of the alpha motor neurones
They innervate the (extrafusal) muscle fibres of the skeletal muscles
Their activation causes muscle contraction
What is meant by a motor pool
The motor neuron pool contains all alpha motor neurons innervating a single muscle
Describe the somatotopic arrangement of alpha motor neurones in the spinal cord
Flexors more dorsally
Extensors more ventrally
Proximal muscles more medially
Distal muscles more laterally.
Define the term 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.
What is an important rule to remember about the connections between alpha motor neurones and muscle fibres
One motor neurone can innervate several muscle fibres
But every muscle fibre can only be innervated by one motor neuron
Describe some important facts to remember about the motor unit
§ Each muscle fibre is only innervated by one distinct motor neuron – i.e. two different motor neurons cannot innervate the same muscle fibre.
§ This competition for innervation of muscle fibres occurs around birth.
§ The motor unit is the smallest functional unit that creates force.
§ On average, each motor neuron supplies ~600 muscle fibres and stimulation of one motor unit causes contraction of all the muscle fibres in that unit.
§ Humans have ~420,000 motor neurons and 250m skeletal muscle fibres.
What can happen to the innervation of muscle fibres in pathological conditions
Under pathological conditions (e.g. severed nerve), the axonal regeneration can result in the innervation of muscle fibres that are already innervated
What are the three types of motor unit
Slow (Type 1)
Fast fatigue-resistant (Type 2A)
Fast fatiguable (Type 2B)
Describe the slow fibres
Slow fibres have: · Smallest diameter cell bodies · Small dendritic trees · Thinnest axons · Slowest conduction velocity
Describe the key properties of the fast fibres
· Larger diameter cell bodies
· Large dendritic cells
· Thicker axons
· Faster conduction velocity
What is important to remember about the different types of motor unit
The 3 types of motor units are NOT discretely located but spread out as can be seen in the picture to the left.
Can be seen histologically by the use of an ATP- myosin stain- looks for ATPase in the different fibre types.
Describe the basis of classification for the 3 different motor unit types
The 3 different motor unit types are classified by the amount of tension generated, speed of contraction and fatiguability of the motor unit.
Type 2b has high tension- recruited first- but fatigues quickly
Type 2a- moderate tension- fatigue resistant and recruited quickly
slow twitch- recruited slowly- fatigue resistant -low tension
Fast twitch fibres make a greater (and faster) percentage contribution to the tension generated by the musle fibre.
Describe and explain the different innervation ratio of different muscles in the body
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)
Describe the two mechanisms that the brain uses to regulate the force that a single muscle can produce
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.
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.
Describe the order of motor unit recruitment for increasing force generation
Slow à Fast Fatigue-Resistant à Fast Fatiguable
What are neurotrophic factors
Factors produced within the nerves and are transported throughout the nerve to maintain the nerves integrity and function.
They are a type of growth factor that prevents neuronal death and promotes the growth of neurones after injur
Describe the effects of neurotrophic factors on the properties of the motor unit
Motor unit and fibre characteristics are dependent on the nerve which innervates them.
If a fast twitch muscle and a slow muscle are cross innervated, the soleus becomes fast and the FDL becomes slow.
The motor neurone has some effect on the properties of the muscle fibres which it.
What is the function of the muscle fibre dependent on
The characteristics of the motor unit are dependent on the nerve that innervates them – i.e. if a fast twitch muscle and slow muscle are cross innervated, the latter becomes fast twitch and the former becomes slow! (Look left at SOL and FDL).
The action potentials can’t be the only thing being delivered to the muscles by the nerves.
Describe normal innervation and crossed innervation
Normal innervation: motor unit and fibre characteristics depend on nerve innervating them, so normally slow units are slow, while fast are rapid
Crossed innervation: if fast and slow innervation are swapped, slow units become faster and fast slower e.g. Soleus becomes faster and FDL becomes slower
Describe the plasticity of the motor unit
Fibre types can change properties under many different conditions.
Type IIB to IIA most common following training
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 is the loss of muscle called in ageing
sarcopenia
What tract is responsible for voluntary movements
Pyramidal/Corticospinal tract
Lateral corticospinal tract - 1a
Anterior corticospinal tract- 1b
§ There is some semblance of somatotopic representation in 1a – sacral, lumbar, thoracic and cervical (S, L, Th, C).
What is the general role of the extra-pyramidal tract
It is responsible for automatic movements in response to stimuli (these are movements that your body makes without you being aware of it)
Describe the rubrospinal tract
Red nucleus in midbrain – spinal cord (2a)
Extra-pyramidal
automatic movements of arm in response to posture/balance changes - e.g when we fall over
Describe the reticulospinal tract
Reticular formation – spinal cord (2b)
Extra-pyramidal
coordinate automated movements of locomotion and posture (e.g. to painful stimuli
Describe the vestibulospinal tract
Vestibular nuclei — spinal cord (2c)
Extra-pyramidal
regulates posture to maintain balance, and facilitates mainly α motoneurones of the postural, anti-gravity (extensor) muscles
name another extra-pyramidal tract
Olviospinal tract
2d
What is meant by 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.
What happens to reflexes in upper motor neurone lesions
In upper motor neuron lesions, there is a loss of descending inhibition, causing an increase in drive and therefore brisk reflexes.
What triggers that contraction in a reflex
An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli (i,e pain, temperature)
“…….whose magnitude and timing are determined respectively by the intensity and onset of the stimulus”.
How do reflexes differ from voluntary movement
If the biceps is tapped, the reflex occurs quickly and is related in size to how hard the biceps was hit. Reflexes differ from voluntary movements in that once they are released, they can’t be stopped.
Summarise the key components of a basic reflex arc
Sensory receptor- responds to a stimulus by producing a generator or receptor potential
Sensory neurone- axon conducts impulses from receptor to integrating centre
Integrating centre- one or more regions in the CNS that relay impulses from sensory to motor neurones (interneurons)
Motor neurones- axons conducting impulses from integrating centre to effector
Effector- muscle or gland that responds to nerve impulses.
What is critical for reflexes
Afferents
No reflex produced when the DRG is cut
What is the name given to the sensory receptors in muscle that feedback to the CNS and allow an excitatory reflex to be generated?
Spindles- stretching causes Ia sensory neurones to fire - causing the excitation of alpha motor neurones.
GMNs innervate the contractile portions at the two ends of the intrafusal muscle fibres. Their function is to cause the contraction of the two poles of the intrafusal fibre when the extrafusal fibres contract. This happens so that when the extrafusal fibres contract the spindle does not become slack, reducing feedback from the Ia sensory neurons and, therefore, providing no information about muscle length.
Describe, experimentally, how we can infer how many synapses there are in a reflex arc
§ You can do an experiment to see how many synapses there are within a reflex arc (a volley = an AP travelling past recording equipment).
§ The difference in time from the afferent volley to the intracellular efferent recording gives experimenters an idea on the number of synapses in the reflex arc pathway – in increments of about 0.7ms.
Summarise the monosynaptic stretch reflex
Monosynaptic stretch reflexes: stretching stimulates sensory neurone in muscle which synapses to
Motor neurone to same muscle, causing contract to relieve stretching
Inhibitory interneuron, which inhibits motor neurone to antagonistic muscle, causing relaxation
What two signals 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)
Describe the time difference in the signals travelling to the quadriceps and the hamstrings
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.
How did Hoffman plan on standardising the reflex test
He considered bypassing the stretch that is caused by the tendon hammer on the patellar ligament and directly stimulating the nerve, which has sensory and motor fibres.
This would mean that the stimulus is identical every time (in duration and amplitude) and the magnitude of the reflex elicited will not be due to variations in input (how hard you tap the patellar tendon/where you tap it)v
What are the two twitches seen when you stimulate the nerve behind the knee
M wave – twitch due to the direct conduction of the impulse down the motor neurone to the muscle fibre
H wave – due to the action potential passing down the sensory neurone back to the spinal cord and then coming out via a motor neurone to stimulate the muscle
o The H-wave activates at lower stimuli strengths but is slower to fire. (sensory neurones are larger)
o H-wave decreases at higher stimuli strength because at higher stimuli, motor cells get activated that fire against the normal flow and cancel out the AP.
Summarise the polysynaptic flexion withdrawal reflex
Stepping on painful stimulus stimulates sensory receptor and hence neurone
Sensory neurone activates interneurons in several spinal cord segments within integrating centre (spinal cord)
Interneurons as/descend several levels and stimulate motor neurones from several levels to contract flexor muscles to move limb (leg) - multiple motor units and spinal nerves innervating the same muscle
Crossed extensor: interneurons can cross cord and stimulate effectors on opposite limb to contract and extend the other limb
Describe the ‘Jendrassik’ maneovure
Traditionally we think of reflexes as being automatic (knee jerk) and stereotyped behaviours (sneeze, cough) in response to stimulation of peripheral receptors.
BUT can they be influenced?
Try clenching teeth or making a fist when having patellar tendon tapped.
What happens? The response elicited is 2-3 times greater.
This is the “Jendrassik” manoeuvre.
How else are we aware of the supraspial control of reflexes
Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex.
Inhibitory control dominates in normal conditions (N).
Decerebration reveals the excitatory control from supraspinal areas (D). (reduced inhibiton from CNS)
Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.
If you decerebrate an animal (but keep them alive) and test their reflexes, what would you expect to observe?
Hyperreflexia
increased muscle tone
How can higher centres influence reflexes
Higher centres can influence reflexes by activating: § Alpha motor neurons. § Inhibitory interneurons. § Propriospinal neurons. § Gamma motor neurons. § Terminals of afferent fibres.
Describe some of the higher pathways and centres involved in the supraspinal control of reflexes
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).
When are gamma motor neurones activated
They are activated in response to stretching of the muscle- detected by splindles (which are sensory nerves)
Describe 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
Describe the plantar reflex
The plantar reflex is elicited when the plantar surface of the foot is stroked from heel to toe, causing reflex plantar flexion of the toes in normal individuals. However, in infants (whose corticospinal tract is not yet fully myelinated) and in patients with damage to the motor cortex or corticospinal tract (an upper motor neuron lesion), dorsiflexion of the toes is elicited. This is known as positive Babinski’s sign.
What can cause a hyper-reflexia
Due to e.g. a stroke
Loss of descending inhibition
Upper motor neurone lesion
May also cause a clonus
What can often be seen in stroke patients
§ In stroke patients, there is a loss of descending inhibition and so hyperreflexia ensues.
§ Also in stroke patients, there can be clonus due to continuous stretch and contraction until voluntary stopping.
What are hypo-reflexias normally due to
Below normal or absent reflexes
Mostly associated with lower motor neuron diseases
