neuromuscular physiology Flashcards
muscular system functions:
*Body movement
*Move bones, speak, breathe, swallow
*Maintenance of posture, stabilize joints
*Respiration
*Production of body heat; maintains
temperature
*Package internal organs and hold them in
place
*Constriction of organs and vessels; regulates
entering and exiting of material
*Heart beat
properties of muscle:
- Contractility
- Ability to shorten with force (when filaments slide past each other)
- It DOES NOT produce force by lengthening/pushing!
- Conductivity
- Transmit/propagate action potentials along the sarcolemma
(similar to AP propagation along an axon) - Excitability
- Respond to a stimulus (neurotransmitters) by changing
electrical membrane potential (and producing action potentials) - Extensibility
- Ability to be stretched
- Elasticity
- Ability of to recoil to original resting length after lengthening or shortening
skeletal muscle:
- Long cylindrical cells
- Many nuclei per cell
- Striated (shortens
during contraction
under “voluntary”
control) - Voluntary
- Rapid contractions
‘built’ for speed (structure minimises diffusional distance of neurotransmitters, Ca++)
smooth muscle:
- Non-striated muscle; associated with ‘tubular’ visceral organs, vasculature
- Forms syncytium; mononucleate cells; bulges on contraction
- Typically involuntary (intrinsic activity), some with voluntary control
- Controlled by autonomic nervous system & endocrine responsive
(NE, Ach, NO/ IP3, cAMP, cGMP) - Slowest muscle
cardiac muscle:
- Striated muscle: shortens during contraction
- Forms syncytium through gap junctions at intercalated disks
(electrotonic connection) - Involuntary control: intrinsic (pacemaker) activity
- Autonomic nervous system and endocrine control (eg., NE,
Ach, NO/ IP3, cAMP, cGMP) - Medium speed
phrenic nuclei:
motor neurone that controls diaphragm (c pr ça slide 11 = when degeneration touches it mortality follows)
striated muscles = innervated by:
Striated skeletal muscles are innervated by motor neurone pools (pools are in the brainstem for cranial nerves, in the ventral horn for spinal nerves)
spinal cord pools for:
brainstem pools for:
trunk & limb muscles
head muscles
upper motor neurones (def + contact + pathway)
- originate in the brain, primarily in the motor cortex, and send signals down through the brainstem and spinal cord to control voluntary muscle movements. These neurons do not directly contact muscles; instead, they communicate with lower motor neurons (LMNs), which in turn innervate muscles
- examples of UMN pathways include the corticospinal tract, which controls movement of the limbs, and the corticobulbar tract, which governs movements of the face and neck
- are in the cerebral cortex or brainstem ❑ There are multiple descending motor tracts from these supraspinal neurones
(slide 13) - ❑ UMNs synapse on LMNs (some), or on interneurons to LMNs (most)
❑ LMN is the final common pathway for UMN and spinal reflex net input
(slide 14)
LMN lesion:
▪ UMN input ineffective
▪ reduced or absent tone (hypotonia-atonia)
▪ muscle can’t contract as a result of nerve firing (flaccid paralysis)
▪ no voluntary or involuntary control
▪ reduces or eliminates reflexes (hyporeflexia-areflexia)
▪ disuses and loss of neurotrophic effect leads to atrophy
(slide 15)
(ex: polyomyelitis slide 16)
muscle can only:
push, never pull
UMN lesion:
▪ Sensory and extrapyramidal input intact
▪ LMN can still fire to contract muscle, but not voluntarily (spastic paralysis)
- Initially, LMNs are in “spinal shock” and paralysis is flaccid, but eventually
they recover to become hypersensitive to input, especially reflexes
(hyperreflexia)
▪ Neurotrophic effect intact - no atrophy
(ex: cerebral palsy: slide 18)
Renshaw interneurones:
type of inhibitory interneuron found in the spinal cord. They play a key role in regulating motor neuron activity and preventing excessive excitation in the spinal motor circuits
how does motor of sensory input work
❑ Total motor and sensory input to a ventral horn motor neuron is very large
❑ Interneurons mediate much of the regulation, integration and coordination
❑ Large alpha motoneurons are embedded in neuropil of smaller cells and fibers
❑ Many of the neuropil small cells are interneurons
❑ α motor neuron integrates all direct and indirect synaptic input from UMNs,
interneurons, and sensory afferents
(slide 20)
❑ Inhibitory Renshaw interneurons are involved in recurrent feedback loops
❑ Loops prevent excessive contraction in certain circumstances
❑ Renshaws themselves can be regulated from above to allow sustained contraction
(slide 21)
tetanus neurotoxin: (game)
❑ Tetanus neurotoxin (from Clostridium) preferentially inhibits transmitter
release from inhibitory interneurons to LMNs’in spinal cord and brainstem.
Uninhibited alpha motor neurons all fire. Larger muscle groups win the tug-of-war between agonist – antagonist pairs
tetanus neurotoxin (def)
- produced by the bacterium Clostridium tetani, is a potent toxin that affects the nervous system, leading to severe muscle spasms and rigidity, a condition known as tetanus.
- Tetanus neurotoxin specifically targets inhibitory interneurons in the spinal cord and brainstem, such as Renshaw cells and other neurons that release inhibitory neurotransmitters like glycine and GABA (gamma-aminobutyric acid).
(slide 23)
sensory input to alphas (def + what it does)
- sensory information transmitted to alpha motor neurons, which are responsible for initiating the contraction of skeletal muscles. Alpha motor neurons receive sensory input through reflex pathways, allowing for rapid and coordinated responses to stimuli, such as maintaining posture or reacting to pain.
- ❑ Sensory input to alphas is from same level and/or other levels (via propriospinal
tract); may be ipsilateral and/or contralateral
❑ A typical peripheral muscular nerve br. is only 25% efferent (alpha + gamma)
Golgi tendon organs:
❑ Golgi tendon organs (GTO) provide feedback about tension-force
▪ located at the muscle-tendon junction (in series with muscle)
muscle spindle:
❑ Muscle spindles provide feedback about stretch (length and change of length)
▪ distributed within the muscle belly (in parallel with muscle fibres)
❑ Muscle spindle stretch (length) receptors are scattered throughout skeletal mm.
❑ Intrafusal muscle fibers are innervated by gamma motor neurons
❑ Intrafusal contraction keeps receptor in range of highest sensitivity to change
❑ A significant number of neurons in the ventral horn are gamma motor neurons
❑ Spindles function such that, when extrafusals contract, intrafusals contract
❑ Therefore, alphas and gammas are usually coactivated
alpha and gamma neurones role:
- alpha= direct controlling of extrafusal muscle fibers (force and movement)
- gamma = intrafusal muscle fibers (adjust sensitivity of muscle spindle to ensure feedback to CNS about muscle length (stretch and position))
= they come from ventral horn of spinal cord et innervate skeletal muscles and play distinct roles in controlling muscle contraction and muscle spindle sensitivity.
Efferent nerve fiber classification:
slide 28 !! (put it)
muscle stretch reflex
❑ On stretch, muscle spindle afferents stimulate 2 types of neurons:
▪ α’s of homonymous (same) muscle + synergists , via 1 synapse (monosynaptic)
▪ inhibitory interneurons to α’s of antagonists (polysynaptic)
(slide 29)
how muscle sretch reflex works:
- Muscle Spindles: Specialized sensory receptors located within the muscle that detect the degree of stretch or lengthening of the muscle.
- Sensory (Afferent) Neurons: Nerve fibers that carry the sensory information from the muscle spindles to the spinal cord.
- Alpha Motor Neurons: Motor neurons in the spinal cord that receive the sensory input and send signals back to the muscle to cause contraction.
- Effector Muscle (Extrafusal Fibers): The muscle fibers that contract in response to stimulation by the alpha motor neurons.
flexor-crossed extensor reflex:
❑ flexor-crossed extensor reflex is a hard-wired polysynaptic withdrawal response
❑ Peripheral nociceptor inputs via polysynaptic networks of stimulatory or inhibitory
interneurons to ipsi- and contralateral synergists and antagonists
▪ flexion on stimulus side, extension on opposite side
(The muscle stretch reflex (also known as the myotatic reflex) is a simple, fast, and automatic response that helps maintain muscle tone and posture by resisting changes in muscle length when the muscle is stretched. It involves both sensory and motor components and is a monosynaptic reflex, meaning there is only one synapse between the sensory neuron and motor neuron, allowing for a quick response.) = muscles contract to pull the limb away from the harmful stimulus (e.g., bending the knee to withdraw the foot)
“Babinski” sign:
❑ “Babinski” sign is an age-normality dependent sign for pyramidal tract health
▪ in the normal infant (< 2yrs): corticospinal axons are unmyelinated, and for some reason, stroking the outer lateral sole causes the big toe to extend and the other toes to flare; this is normal
▪ in the normal child/adult: the normal response to lateral sole stroking is plantar
flexion of the digits (grasping reflex)
▪ in CST UMN lesion, hallucal dorsiflexion and toe fanning is elicited; this positive Babinski sign indicates the abnormality
(slide 31)
