Week 3: Muscular System Flashcards
Skeletal Muscle Contraction:
Muscle cell/fiber (wrapped in endomysium)
bundled together
Fascicle (wrapped in perimysium)
bundled together
Skeletal Muscle (wrapped in epimysium)
Skeletal Muscle Cell Structure:
Key Components:
- Cell membrane of a skeletal muscle fiber is called the sarcolemma (sits directly underneath the endomysium and contains transverse (T) tubules
- Muscle cell cytoplasm is called the sarcoplasm, and contains hundreds of myofibrils (contractile organelles)
- Each myofibril contains bundles of protein myofilaments that interlock (myofilaments: Actin (thin) and Myosin (thick)
Myofilaments contain repeating contractile unites called sarcomere
Transvers Tubules and Muscle Contraction
- T-tubules are invaginations in the sarcolemma that tunnel in from the surface toward the centre of every muscle fiber
- They are open to the outside of the cell and therefore contain interstitial fluid
- Muscle action potentials travel along the sarcolemma and through the T-tubules quickly spreading throughout the muscle fiber
T-tubule arrangement ensures that the action potential excites all parts of the muscle cell at the same instant
Sarcomeres, Striations and contraction:
- Overlap of myofilaments (actin and myosin) gives the appearance of striped on a muscle fiber called striations
During muscle contraction, the actin slides over the myosin, shortening each sarcomere and thus the muscle cell
- Sarcomeres are separated by dense regions of protein, referred to as the Z-line/disc
- The total region where actin and myosin overlap is referred to as the A-band
- The region where there is only actin present is referred to as the I-band
- The region in the center of the sarcomere, where there is only myosin present is referred to as the H-band/zone
The region in the very center of the H-band, that contains proteins to hold all myosin filaments together is called the M-line
Skeletal Muscle Cell contraction:
- Myosin (thick) binds to Actin (thin)
- Actin slides inwards, covering the myosin
- This action causes the striated appearance of the muscle
Thick and thin filaments do not actually change in length, sliding over each other contracts the sarcomere
Types of muscle contraction:
- Isometric: No change in length, therefore no movement e.g. Wall sit, plank hold, bicep curl hold
- Isotonic: Changes in muscle length
- Isotonic-concentric: Muscle length shortens while generating force e.g. controlled weighted bicep curl to flex elbow joint
Isotonic-Eccentric: Muscle elongates (lengthens) while under tension due to an opposing force that is greater that the force generated by the muscle e.g. lowering of a weighted bicep curl in a controlled manner
Cooperative action of muscles:
- Although several muscles may be involved in an action, the principal muscle is called the prime over or agonist
- e.g. to raise a cup, biceps brachii is the prime mover/agonist and flexing the elbow joint
- A muscle that performs the opposite action to the prime mover is called the antagonist
- e.g. the triceps brachii is the antagonist as the muscle is relaxing
- A muscle whose contraction helps a prime mover/agonist in an action is called a synergist e.g. the brachialis is the acting as a synergist as it also works to flex the elbow joint
- A type of synergist that helps a prime mover/agonist by preventing or reducing movement at another joint is called a fixator
e.g. a fixator could be any muscles of the rotator cuff that are stabilizing the shoulder joint
Functional organization of the nervous system:
Nervous system: CNS and PNS
- CNS: Brain and Spinal cord
- PNS: Sensory Division, Motor Division
- Sensory: Somatic Sensory, Visceral Sensory
- Motor: Somatic (voluntary) motor division, Autonomic (involuntary) motor movement
Autonomic: Sympathetic, Parasympathetic
Motor Units and Muscular Contraction:
- Neurons responsible for innervating skeletal muscles for contraction are called somatic motor neuron
- Each motor neuron travels from the CNS to the skeletal muscle fibers
- Each motor neuron may innervate many muscle fibers e.g. 150 on average
Motor Unit: One somatic motor neuron and all of muscle fibers it innervates, large weight bearing muscles typically contain large motor units e.g. Quads
The neuromuscular junction:
- A skeletal muscle fiber will contract in response to an action potential moving along the sarcolemma and through the T-tubules
The action potential arise at the synapse between the somatic motor neuron and the skeletal muscle fiber (referred to as the neuromuscular junction and involved a neurotransmitter release from the pre-synaptic neuron and receptors on muscle fibers)
Strength of muscle contraction:
- The frequency of the stimulation of the muscle cells/fibers
- Repeated and rapid stimulation (via action potentials) = greater force of contraction compared to single stimulation
2. The degree of muscle fiber stretch - The greatest tension, thus force, occurs when there is optimal zone of overlap between actin and myosin fibers
3. The number of muscle fibers that are activated (determined by the number of motor units that are recruited) - More motor units = more muscle fibers = more force
4. The size of the activated muscle fibers
The larger the muscle fiber = greater the force
- Repeated and rapid stimulation (via action potentials) = greater force of contraction compared to single stimulation
Muscle tone:
- At rest skeletal muscle exhibits a small amount of tautness or tension due to weak, involuntary contractions of motor units referred to as muscle tone
- Muscle tone keeps skeletal muscles firm but does not result in a force strong enough to produce movement e.g. when awake, the muscles in the back of the neck are in normal tonic contraction thus the head is held upright
Deviations from normal muscle tone: Low muscle tone = hypotonia (muscle is flaccid/limp), high muscle tone = hypertonia (spasticity or rigidity)
- Muscle tone keeps skeletal muscles firm but does not result in a force strong enough to produce movement e.g. when awake, the muscles in the back of the neck are in normal tonic contraction thus the head is held upright
Components of skeletal muscle:
- Perimysium: Dense irregular connective tissue
- Endomysium: Reticular fibers, separates muscle fiber from one another
- Attaches skeletal muscle to other muscle fibers
- Tendon connects skeletal muscle periosteum of bone
- Calcaneus (heel) connects to Achilles with gastrocnemius muscle
Connective tissue can extend as a broad flat sheet called aponeurosis e.g. rectus sheet of the abdominal muscle
What is direct and indirect force?
- For skeletal muscle to exert force and produce movement it must be attached to two different muscles: Direct or indirect
- Indirect: Muscle ends short of the bone and is joined to bone by tendon e.g. biceps brachii, palmaris longus and flexor carpi radialis. Or when connective tissue is broad e.g. palmer aponeurosis
- In some locations groups of tendons from different muscles pass under a band of connective tissue (retinaculum) e.g. tendons of several muscles of forearm pass under on its way to the hand
- Direct: little separation between skeletal muscle and bone e.g. lateral head of triceps bra chi muscle
Majority of skeletal muscles are attached to a different bone at each end, at least one joint is spanned
Fascicle orientations:
- Parallel muscles cover long distance but shorten less than others, therefore they produce less force
- Triangular/convergence e.g. pectoralis major. Strong as they have many muscle fibers in the wider part
- Pennate muscles: feather like.
- Unipennate: All fascicles approach tendon on one side
- Bipennate: Fascicles approach tendon from both sides
- Multipennate muscles: Bunch of feathers with quills to a single point
Circular/sphincters: Circular motion, forms rings around openings to prevent passage of material through it
Skeletal Muscle striations, sarcomeres and myofilaments:
- 10 cm length
- Cells have many nuclei
- Unable to divide, number you have at birth is fixed
- Cell membrane = sarcolemma
Nuclei of cell is just below the sarcolemma
What are the components of skeletal muscle?
- Transverse tubules: ensure muscle action potentials excites all of muscle fiber at the same time
- Sarcoplasm: Cytoplasm of the cells. Contains lots of glycogen to synthesize ATP for energy to function. Contain myoglobin protein which releases oxygen for ATP production
Myofibrils: Striations, zebra, parallel to muscle fiber to allow muscle contraction. Each myofibril contains myofilaments (thin/actin, or thick/myosin) combined to create a sarcomere
- Sarcoplasm: Cytoplasm of the cells. Contains lots of glycogen to synthesize ATP for energy to function. Contain myoglobin protein which releases oxygen for ATP production
What are the bands of skeletal muscle?
- Separating sarcomeres are dense areas of protein called Z-discs. Can be seen as alternating dark and light striations running perpendicular to muscle fiber. Dark = A bands, I = lighter stripes
- Actin and Myosin overlap to interact and shorten efficiently
- Overlap = A bands, appear darker
- Only Actin = lighter, I bands
Actin and myosin are anchored to myofibrils, and them to sarcolemma of muscle fiber called Z discs
Naming Skeletal Muscles:
- Reflect name and function provides information about shape, size, orientation, location, action, attachments
- Muscle must cross a joint to act at that joint e.g. two bones, joint lying between the bones is the joint the muscle will act on
- Position of muscle relative to a joint will determine the plane and direction of movement it will produce at that joint
- Direction or orientation of fascicles in relation to the midline is included in the name of skeletal muscles
e.g. Rectus are parallel to the midline, transverse are horizontal to the midline and external are diagonal to the midline
Motor Units and Muscle Strength
- Somatic Motor Neurons: Responsible for innovating, stimulating muscles to contract
- Each has an axon (from CNS to skeletal muscle fibers)
- Will contract in response to an action potential moving along the sarcolemma and through t-tubules (arrives at neurological junction or synapse between motor neuron and skeletal muscle fiber)
- Axon of motor neuron enters skeletal muscle and branches into numerous axon terminals spreading through muscle belly
- Each terminal forms neuromuscular junction with a different muscle fiber
A motor neuron can form a motor unit with hundreds of individuals muscle fibers with as few as 4 individua muscle fibers
How do skeletal muscles travel?
- As neurons operate on an all or nothing principle, if an action potential travels down neuron, it will excite every muscle fiber that has form a neuromuscular junction with
- If a motor unity includes hundreds of muscle fibers, all muscle fibers will contract at once producing force
- If it only has a few muscle fibers, excitation will only result in a weak contraction
Large motor units such as those found in quads, are able to produce a large amount of force from a small number of nerve fibers
What does strength of contraction depend on?
- In contrast, small motor units e.g. in eyes are able to make subtle movements. Large weight baring muscles typically contain large motor units to generate a lot of force whereas small finely controlled muscles contain small motor units to produce weak yet finely controlled units
Strength of muscle contraction depends on how many of the myosin heads interact with actin when the muscle fibers are stimulated to contract, how many of these are activated and thus the strength of contraction is dependent on 4 major factors.
What 4 factors is strength of contraction dependant on?
- Number of muscle fibers activated: determined by number of motor units recruited in a muscle. More MU = more muscle fibers, which = more force
- The size of the activated muscle fibers: Larger muscle fibers will contain more actin and myosin and therefore more crossbridge therefore large muscle fibers produce more force. However smaller muscle fibers are typically recruited first followed by medium and then large muscle fibers. Skeletal muscles are able to change strength of muscle contraction dependent on their situation
- Frequency of stimulation of muscle fibers: Frequency of stimulation, when a skeletal muscle fiber is stimulated to contract multiple times in succession, the force of each contraction is added on top of existing force of contraction. Therefore contraction becomes stronger and stronger. If a muscle is stimulated repeatedly and rapidly, it will contract with greater force than if it is stimulated only once. When contraction reaches peak and cant get any stronger, it is referred to as tetanus
- Degree of muscle stretch: Maximal contraction will occur when the muscle is only slightly stretched and the actin and myosin overlap optimally. If muscle is stretched too little/much, the contraction will not be as strong as there is not any cross bridges (referred to as length tension relationship, when tension the muscle can produce is related to the length of the sarcomeres). If muscle is stretched too much, the actin and myosin will not overlap and contraction can not occur. If it is shortened too much, myofilaments overlap too much and myosin filaments run into the z-discs, preventing any further contraction
Types of Muscle contraction:
- Always pulling rather than pushing
- Muscle shortens, which facilitates movement
- Can be voluntary, however number of automatic movements are not (reflexive contraction) e.g. tendon reflex, myotatic reflex and is movement after a muscle stretch produces by tapping a tendon with a reflex hammer
- Can be at rest, relaxed and in use, however even when relaxed, muscles are always minimally contracted (muscle tone/small amount of tension required even when at rest to maintain firmness, assisting joint ability, maintenance of posture, enable body to respond to appropriate stimuli)
- Muscle tone = Tonic contraction
- Muscle tone is generally absent when unconscious (general anesthetic)
Also absent following nerve lesions
Skeletal muscles contract in 1 of 2 main types (phasic or active contraction)
- Phasic: Isometric contraction (muscle length remains the same, but no movement occurs, but muscle tension or force is increased above tonic levels to resist gravity, important for posture)
- Isotonic: Skeletal muscle changes in length and relationship in production of movement. Two types:
- Concentric: Movement occurs following the skeletal muscle shortening, e.g. pushing a wall. Occurs when the force of the skeletal muscle exceeds the force of gravity, using the arm as an example, the deltoid muscle will chorten and therefore contract to raise arm above head in the movement of abduction.
Eccentric: Contracting skeletal muscle lengthens, when the skeletal muscle relaxes in a controlled and gradual way whilst exerting decreasing force. E.g. deltoid muscle lengthens and therefore relaxes to enable the arm to lower in the movement of adduction. Force becomes less than gravity, and gravity will pull the limb down as muscle resistance decrease
Muscles that act as prime movers:
- Agonists, this type of muscle is responsible for producing specific body movement (contracts in concentric contraction) to achieve desired movement, for majority of movements, there is a single prime mover, e.g. lifting a cup, the prime mover is the biceps brachii.
- Antagonist: Directly opposes the prime mover through eccentric contractions (relaxing in a progressively coordinated fashion to enable a smooth movement) e.g. antagonist to prime mover is the triceps brachii
- Can be used to describe muscles (fixator muscle, and synergist muscle)
- Fixator: Stabilized the proximal parts of the leg or arm through isometric contraction during movement through the distal parts of the same limb. e.g. trapezius acts as a fixator for the elbow joint
- Synergist: complements the action of a prime mover by providing a less strong component of the same movement e.g. brachialis
Three types of Muscle Activity:
- Muscle Contraction
* Isometric contraction
* Isotonic contraction - Muscle Tension
* Active tension produced by the contraction of your muscles after receiving a signal from the nervous system
* Passive tension caused by the resistance to stretching that your muscles possess
* Thus joint stability depends on the active and passive tension of muscles - Muscle Tone
* Reflex activity maintains a level of muscle contraction at all times
* Keeps muscles firm and ready for contraction
If muscle loses innervation, it will become flaccid
Trapezius:
has multiple functions, including (but not limited to) elevation, retraction, upward rotation and depression of the scapula
Deltoid:
has multiple functions, including (but not limited to) abduction, flexion, extension, medial rotation and lateral rotation of the arm at the shoulder joint
Pectoralis major
has multiple functions, including (but not limited to) adduction, flexion, extension and medial rotation of the arm at the shoulder joint
Latissimus dorsi
acts to adduct, medially rotate and extend the arm at the shoulder joint.
Biceps brachii
has multiple functions, including (but not limited to) flexion of the arm at the shoulder joint, and flexion and supination of the forearm at the elbow joint.
Triceps brachii
has multiple functions, including (but not limited to) extension of the arm at the shoulder joint and extension of the forearm and the elbow joint.
Brachioradialis
has multiple functions, including (but not limited to) flexion, supination and pronation of the forearm at the elbow joint.
Flexors of the wrist and fingers
act to flex the hand and digits at the at the wrist joint and metacarpo-phalangeal joints, respectively.
Extensors of the wrist and fingers
act to extend the hand and digits at the at the wrist joint and metacarpo-phalangeal joints, respectively
A motor unit is defined as:
One somatic motor neuron and all of the muscle fibres it innervates
In normal circumstances, muscle cell contraction occurs when an action potentials from the neuromuscular junction move through which structure?
T-tubules
Examples of Muscle tone, contraction and tension
Muscle contraction: Active shortening or lengthening of muscles (e.g., bicep curls).
Muscle tone: Low-level, continuous muscle activity, even at rest (e.g., maintaining posture while sitting).
Muscle tension: The force generated by muscles, often in response to external demands (e.g., lifting weights, stress-induced tightness).
