2.2.3 Neuromuscular and Myotendinous Junctions Flashcards
What is the safety factor?
The safety factor is very high. Every action potential in the motor neuron results in an action potential in the muscle fiber.
What is Gower’s sign?
Sign of DMD
This is a diagram depicting Gower’s sign. Since the leg muscles and hip muscles are weak the patient has difficulty in standing up from a resting position and compensate by ‘walking’ up their legs.
Describe muscle type grouping
This is determined by the motor neuron that is innervating the muscle fiber
This muscle has been stained with an antibody to myosin isoforms. The dark fibers are fast twitch fibers, and the lightly stained fibers are slow twitch. The pattern of muscle fibers reveals that the muscle fiber types are equally distributed throughout the muscle.
What is the role of the muscle spindle?
Muscle spindles are found throughout the muscle. Force and velocity of muscle movement are translated by the spindle to inform the CNS of the force and velocity produced by the muscle. Stretch of the muscle spindle deforms the afferent Sensory endings. This mechanical deformation of the sensory ending leads to the generation of an action potential in the sensory ending. The action potential then goes from the muscle to the central nervous system via the intramuscular nerves. The cell bodies of the sensory fibers are found in the dorsal root ganglion. The gamma motor neurons act like regular neuromuscular junctions. They release Ach to make the intrafusal muscle fibers contract. The contraction of the intrafusal muscle fibers changes the response of the sensory nerve endings.
What is the role of the satellite cell?
The satellite cell is found under the basal lamina that wraps the muscle cell. The satellite cell moves up and down the muscle fiber, and will be attracted to regions of muscle fiber damage.
Muscle fibers are repaired by satellite cells.
Muscle satellite cells are myoblasts that did not fuse with the other myoblasts during development.
Describe the current dystrophin model
This is the current view of how dystrophin functions at the muscle cell membrane. Dystrophin binds to actin filaments at it’s N-terminus and with a dystroglycan complex at it’s c-terminus. The dystroglycan complex spans the plasma membrane and dystroglycan complex binds to laminin in the extracellular matrix. In this way dystrophin and the dystroglycan complex provides a structural link between the cytoskeletal actin filaments and the extracellular matrix. It is believed that this link is required in order to prevent muscle cell membrane damage during contraction.
What are the characteristics of a monosynaptic stretch reflex?
This is the knee jerk reflex. You tap the patellar tendon to stretch the quadriceps muscle. The stretch causes an action potential to be generated at the sensory endings at the muscle spindle. The action potential travels into the spinal cord, where the sensory neuron synapses upon the alpha motor neuron in the spinal cord. This synapse is excitatory, and results in an action potential in the alpha motor neuron. The action potential travels down the axon, to the nerve terminals. This then depolarizes the muscle cells in the quadriceps muscle, and the muscle contracts. This then causes the leg to move forward.
The amount of movement of the leg can give you information on the type of neuromuscular problems in a patient. In certain diseases the reflex is stronger than normal (hyperreflexia), and in others it is much weaker than normal (hyporeflexia).
?What is the process of exercise induced hypertrophy
Describe the process of neuromuscular transmission in the muscle.
This diagram has more info on the sequence of events shown in the previous slide. The important feature is that Choline is taken back up into the nerve terminal and recycled. Acetylcholinesterase is found in the synaptic cleft. It is a very stable enzyme that efficiently cleaves Ach to remove Ach from the synapse.
Describe the process of fiber splitting.
Following regeneration it is very common for the muscle fibers to show a feature called fiber splitting. This results when multiple myotubes form within the injury area. Each of the myotubes will join the damage muscle fragments together, and each will make a new basal lamina. The observance of fiber splitting is a strong indicator of recurrent muscle damage.
Describe the process of muscle development.
Muscle develops from mesenchyme cells which migrate from the somites. The cells differentiate into myoblasts, and these cells proliferate. The myoblasts will align into developing myotubes, and the membrane between the myoblasts will fuse thus resulting in a multinucleated cell. Once the myoblasts fuse, they no longer divide. The myotubes produce actin and myosin, and the mature muscle cells are multinucleated with the nuclei located at the periphery of the cell.
Describe reflex grading
What is this an image of?
These are biopsy samples from 2 boys with Duchenne muscular dystrophy. Note the increased mononuclear cell infiltrates. Increased connective tissue, the fat cells are labeled with *. The muscle cells that are still alive are clearly weakened and atrophied.
What is this an image of?
fiber splitting
What is this an image of?
EM of myotendinous junction
This is a cross section through the myotendinous junction. The junction is primarily a place where the force produced by the actin and myosin are transferred to the collagen fibrils that make up the tendon. The top panel shows sarcomeres, and the invagination of the membrane. The myotendinous junction is very similar to the fascia adherens zone in the intercalated disk in the heart. This is an area where the actin filaments are bound to a protein complex that transfers the force produced by that actin filament across the plasma membrane, to the tendon. The lower panel shows a higher mag view of the junction and the muscle is on the right, and the collagen is on the left.
