Motor Systems Flashcards
What part of the nervous system innervates smooth muscle?
Autonomic nervous system
What are two types of striatal muscle?
Cardiac
- Contracts rhythmically, even in absence of any innervation
Skeletal
What does autonomic NS innervation of cardiac straital muscle do?
It functions to accelerate or slow down the heart rate.
Each muscle fibre (cell) is innervated by how many axon branches from the CNS?
A single axon branch for each muscle fibre
What is the somatic nervous system?
Muscles and the parts of the nervous system that controls them
What major muscle cause an arm flexion? What are these muscles categorically called?
brachialis, tendons insert into the humerus (upper arm) at one end and into the ulna (forearm) at the other.
The biceps brachii and coracobrachialis also cause flexion at this joint.
These muscles are called flexors of the elbow joint.
What are muscles that work together called?
Synergists
Flexors and extensors are called _______ to each other
antagonists
What are axial muscles? What do they do?
Muscles that are responsible for movement of the trunk.
Maintains posture
What are proximal muscles? What are they critical for?
Muscles that move the shoulder, elbow, pelvis and knee
AKA girdle muscles
Critical for locomotion
What are distal muscles? What are they specialized for?
Muscles that move the hands, feet and digits
Specialized for manipulation of objects
What neurons directly command muscle contractions? Where are they found?
Somatic motor neurons, AKA lower motor neurons, are found in the ventral horn of the spinal cord. Final common pathway for the control of behaviour.
Where does the ventral horn of the spinal cord appear swollen?
C3-TI (arm) and LI-S3 (leg musculature), this is to accommodate the abundance of motor neurons that control the arms and legs.
How are lower motor neurons distributed in the ventral horn?
Distributed depending on their function. The cells innervating the axial (trunk) are medial to those innervating the distal muscles. Cells innervating flexors are dorsal to those innervating extensors. That is, they make a topographic map of the muscles in which they innervate (eg. the arm).
What are two categories of lower motor neurons of the spinal cord?
Alpha motor neurons
Gamma motor neurons
What are alpha motor neurons?
Directly trigger the generation of force by muscles. One alpha motor neurons and all the muscle fibres it innervates collectively make up the elementary component of motor control. AKA the motor unit. Muscle contraction results from the individual and actions of these motor units.
What is a collection of alpha motor neurons that innervates a single muscle called?
A motor neuron pool
What is the result of varying firing rate of motor neurons?
Because of the high reliability of neuromuscular transmission, the ACh released in response to one presynaptic action potential causes an excitatory postysynaptic potential in the muscle fibre (aka an endplate potential) large enough to trigger one postsynaptic action potential, this causes a twitch. Continue firings causes a sustained contraction. Twitch summations increases the tension in the muscle fibres, and smoothes the contraction. The rate of firing of motor units is therefore one important way the CNS grades muscle contraction.
What are two ways that the CNS grades muscle contractions?
- By varying firing rate of motor neurons
- By recruiting additional synergistic motor units (strength depends on how many muscle fibres are in that unit)
How can quantity to size of motor units affect fine motor control?
Muscles with large numbers of small motor units can be more finely controlled by the CNS. Large motor units tend to control things like the leg and arm muscles, with an innervation ratio of more than 1000 muscle fibres per single alpha motor neuron.
What accounts for greater muscle control under light loads, as apposed to heavy loads?
Motor units are recruited in the order of smallest first, largest last. This orderly recruitment explains why finger control is possible when muscles are under light loads than when they are under greater loads.
Small motor units have small alpha motor neurons and large motor units have large alpha motor neurons. How does this explain orderly recruitment of motor units from small to large?
As a consequency of their geometry, small neurons might be more easily excited by signals descending from the brain. This is called the size principle.
What do alpha motor neurons excite?
Skeletal muscles
What are the three major inputs to alpha motor neurons?
- Dorsal root ganglion cellswith axons that innervate a muscle spindle. (feedback about muscle length)
- Upper motor neurons in the motor cortex and brain stem (Initiation and control of voluntary movement)
- Interneurons in the spinal cord (largest input, excitatory or inhibitory, is part of the circuitry that generates the spinal motor programs)
Why is dark meat dark (eg. dark muscles) and where is it found? What are these types of motor units called?
Dark muscles contain many mitochondria and enzymes for oxidative energy metabolism. Slow to contract but can sustain contraction for a long time.
Typically found in antigravity muscles of the legs and wings of birds.
Called slow motor fibres
What are fast motor fibres?
White meat. Rely mainly on anaerobic metabolism. These fibres contract rapidly and powerfully, but they fatigue rapidly as well. Typical in escape reflexes, eg. jumping muscles in frogs and arm muscles in humans.
Each motor unit only contains muscle fibres of a certain type, which two types of motor unit are these?
Fast motor units (fast fatiguing white fibres)
Slow motor units (slowly fatiguing red fibres)
What are characteristics of alpha motor neurons of fast motor units? Slow?
- Fast motor unit alpha motor neurons are generally bigger with larger diameter, faster conducting axons. They have high frequency firing bursts of action potentials.
Alpha motor neurons of slow motor units are generally smaller with smaller diameter, slower conducting axons. Relatively steady, low frequency activity.
What probably controls whether a muscle fibre is slow or fast? How do we know?
The synaptic connection from the alpha motor neuron. We know this because of the experiment by John Eccles, where neurons that normally innervate fast twitch motor fibres were switched to slow. The slow then became fast. This is a biochemical switch in phenotype. This change may also be induced by changing the firing rate of the motor neuron.
What is ALS? (disease)
Amyotrophic Lateral Sclerosis is a disease that causes muscle weakness, that continues to worsen until death. It is the degeneration of the large alpha motor neurons, the large neurons of the motor cortex are also affected but other neurons in the brain are spared (selective neuron loss)
What is excitation-contraction coupling?
The excitation (EPSP) of a muscle fibre from an alpha motor neuron causes an action potential that triggers the release of calcium from an organelle inside the muscle fibre, which leads to contraction of the fibre. Relaxation occurs when calcium levels are lowered by reuptake into the organelle.
How are muscle fibres formed?
By the fusion of myoblasts which are derived from mesoderm, leaving each cell with more than one nucleus, multiple fusions form a fibre
What is the sarcolemma?
The excitable cell membrane that encolses muscle fibres
What are myofibrils? What surrounds each one?
Cylindrical structures within muscle fibres, which contract in response to an action potential sweeping down the carcolemma. They are surrounded by sarcoplasmic reticulum.
What is sarcoplasmic reticulum? What
An extensive extracellular sac that stores Ca. Action potentials sweepeing along the sarcolemma gain access to the sarcoplasmic reticulum deep inside the fiber by way of a network of tunnels called T tubules.
How does a muscle fibre respond to a action potential?
The action potential sweeps down the sarcolemma. When it arrives at a T tubule membrane, a conformational change in the voltage-sensitive tetrad of calcium channels to open, realeasing calcium intro the tetrad channels and the calcium release channels (in the sarcoplasmic reticulum membrane). The resulting increase of Ca within the cytosol causes the myofibril to contract.
In a nutshell, calcium flows from outside the muscle fibre through the T tubule (in the membrane of the sarcolemma) and into the cytosol of the muscle fibre. Then through the calcium release channel and into the Sarcoplasmic reticulum lumen.
Myofibril vertically divided into two types of filaments? What divides these filaments and what are they?
The Z line divides thick and thin filaments
Thick filament = myosin
Thin filament = actin
What is a sarcomere?
A segment of myofibril composed two Z lines and the myofibril inbetween with three bands of thin filaments and two of thick filament. The functional unit of a muscle fibre.
How does a sarcomere contract?
When thin filaments slide along thick filaments, bringing adjacent Z lines towards one another. This happens by a conformational change that causes the myosin to pivot, causing the thick filament to move with respect to the thin filament.
ATP causes the myosin head to disengage and uncock, so that it can repeat the movement, effectively walking along the actin filament.
Why can’t myosin interact with actin when the muscle is at rest?
The protein troponin is covering myosin attachement sites on the actin. Calcium initiates muscle contraction by binding to troponin and exposing the sites where myosin binds to actin
What is the protein that convers myosin binding sites on actin?
Troponin
What conditions enable contraction to continue? And to stop?
Contraction continues for as long as there is Ca and ATP. It relaxes when the Ca is sequestered by the sarcoplasmic reticulum. The reuptake of calcium requires on the action of calcium pumps and therefore requires ATP
What are the 5 steps of excitation of muscle fibres?
- An action potential occurs in an alpha motor neuron axon
2/ ACh is released by the axon terminal of the alpha motor neuron at the neuromuscular junction - Nicotinic receptor channels in the sarcolemma open and the postsynaptic sarcolemma depolarizes (EPSP)
- Voltage gated sodium channels open, and an action potential is generated in the muscle fibre, which sweeps down the sarcolemma and into the T tubules
- Depolarization of the T tubules causes Ca release from the sarcoplasmic reticulum
What are the 6 steps of contraction of muscle fibres (after excitation)?
- Ca binds to troponin
- Myosin binding sites on actin are exposed
- Myosin heads bind to actin
- Myosin heads pivot
- Myosin heads disengage at the expense of ATP
- The cycle continues as long as Ca and ATP are present
What are the 3 steps of relaxation in a muscle fibre after excitation and contraction?
- As EPSPs end, the sarcolemma and T tubules return to their resting potentials
- Ca is sequestered by the sarcoplasmic reticulum by and ATP driven pump
- Myosin binding sites on actin are covered by troponin
Why does rigor mortis occur?
Starving muscles cells of ATP prevents the detachment of th emyosin head and leaves the myosin attachment sites on the actin filaments exposed for binding. The end result is the formation of permanent attachments between the thick and thin filaments.
What causes Duchenne Muscular Dystrophy?
An absence of the mRNA region coding for the protein dystrophin, leading to paralysis. Only affects boys, passed down on X chromosome, paralysis starts in legs.
What is the condition called where released ACh into neuromuscular junctions is far less effective than normal, leading to muscle weakness? What causes this condition?
Myasthenia gravis
- This is an autoimmune disease where the body’s own antibodies attach nicotinic ACh receptors, interfering with the normal actions of ACh at the neuromuscular junction. This also causes secondary changes in the structure of the neuromuscular junction that also make transmission much less efficient.
What is a good treatment for myasthenia gravis?
Admistering drugs that inhibit the enzyme acetylcholinesterase (AChE). This prevents breakdown of ACh in the synaptic cleft. This doesn’t last very long however, because the receptor is quickly desensitized to the extra transmitter.
Another treatment is suppression of the immune system by drugs or removal of the thymus gland.
What is a muscle spindle?
Also called a stretch receptor, consissts of several types of specialized skeletal muscle fibers contained in a fibrous capsule. The middle third of the capsule is swollen, giving the structure the shape for which it is named. In this middle equatorial region, group Ia sensory axons wrap around the muscle fibers of the spindle.
The muscle spindle is an example of a proprioceptor
What type of sensory axons wrap around the muscle fibres of muscle spindles?
group Ia sensory axons
These are the largest and fastest type of axons in the body.
What is proprioception?
Body sense, tells us how our body is positioned and moving in space
Group Ia sensory axons from from muscle spindles go where and innervate what two types of neurons?
Ia axons enter the spinal cord via the dorsal roots, branch repeatedly and form excitatory synapses upon both interneurons and alpha motor neurons of the ventral horns.
A single Ia axon synapses on virtually every alpha motor neuron in the pool innervating the same muscle that contains the spindle.
What is the reflex called where a muscle that is pulls on pulls back (contracts).
A myotatic reflex
- Happens when sensory feedback from the muscle spindles Ia axons increases (when muscle is stretched). and decreased when it is shortened.
When do Ia axons from muscle spindles fire most?
When the muscle is stretched. This causes them to also excite alpha motor neurons, leading to contraction of the muscle. This is called a myotatic reflex.
What happens to muscles if the dorsal roots (sensory information going from muscle) of the muscle are cut?
Eliminated myotatic reflex and caused a loss of muscle tone
What is the name for the pathway involving Ia axon and the alpha motor neurons on which it synapses?
The monosynaptic myotatic reflex arc
- Monosynaptic because only synapse separates the primary sensory input from the motor neuron output. This pathway serves as an antigravity feedback loop.
How does stretching of muscle spindles cause action potentials at the end of Ia axons?
By the stretching of mechanosensitive ion channels (leading to the opening and causing depolarization).
How does the monosynaptic myotatic reflex arc serve as an antigravity feedback loop?
When a muscle is lengthened the muscle spindle induces Ia axons to fire, which excite alpha motor neurons, causing the muscle to contract. This allows the muscle to re-shorten back to its former length, even when it has a weight bearing it down.
What are muscle fibres inside muscle spindles called? Outside?
Inside: intrafusal fibres
outside: extrafusal fibres (form the bulk of the muscle)
What type of motor neurons innvervates extrafusal muscle fibres?
Alpha motor neurons
