S1) Skeletal Muscle Morphology, Structure & Mechanics Flashcards
What are the 3 types of muscle cells?
- Skeletal
- Cardiac
- Smooth
State the structure and function of skeletal muscle
- Structure: striated muscle due to the ordered arrangement of the myofibrillar apparatus producing a characteristic banding pattern
- Function: permit movement through voluntary contractions
Describe the role of skeletal muscle in heat generation
- Approximately 80% of chemical energy is lost as heat as a by-product of muscle activity
- This inefficiency is used by the physiological process of shivering to raise core body temperature by the involuntary activation of skeletal muscle
Describe the numerical distribution of skeletal muscles in the body
- Approximately 640 skeletal muscles
- Most of which are present as bilateral pairs
The most common arrangement of skeletal muscle is for the fibres to run parallel to the force-generating axis.
Identify the three types of parallel muscles
Describe the structure of strap muscles
Strap muscles are shaped like a strap with the fibres running longitudinally, parallel to the direction of contraction e.g. sartorius
Describe the structure of fusiform muscles
Fusiform muscles are cylindrical and wider in the centre while tapering off at the ends e.g. biceps brachii
Describe the structure of fan-shaped muscles
Fan shaped muscles have fibres that converge at one end e.g. pectoralis major
What are pennate muscles?
Pennate muscles are muscles which 1/more aponeuroses running through the muscle body from the tendon and fascicles which attach to these aponeuroses obliquely to the direction of movement
Identify and describe the different kinds of pennate muscles
- Unipennate where all the fascicles are on the same side as the tendon
- Bipennate where fascicles are on both sides of a central tendon
- Multipennate where a central tendon branches
What is a pennation angle?
The pennation angle is the angle between a fascicle’s orientation and the tendon axis
What are circular muscles?
Circular muscles are muscles where fibres form concentric rings around a sphincter or opening by attaching to skin, ligaments and fascia of other muscles rather than to bone
E.g. orbicularis oculi around the eye, orbicularis oris around the mouth
What are agonist and antagonist muscles?
- An agonist is the prime muscle(s) responsible for a particular movement
- An antagonist is the muscle(s) which opposes this movement
What are synergists?
Synergists are muscles which act to assist the prime mover as they cannot act alone to perform the movement e.g. brachioradialis and pronator teres in elbow flexion
What are neutralisers?
Neutralisers are muscles which prevent the unwanted actions that an agonist can perform
E.g. rotator cuff muscles stabilise the glenohumeral joint whilst biceps, (whose long head acts to cause shoulder flexion) cause elbow flexion
What are fixators?
Fixators are muscles which act to hold a body part (proximal joint) immobile whilst another body part (distal joint) is moving
E.g. elbow flexion – fixators are the muscles that stabilise the scapula and those that stabilise the shoulder joint
Distinguish between neutraliser and fixator muscles
- Neutralisers prevent the unwanted actions of a muscle
- Fixators stabilise a joint
What is muscle contraction?
Muscle contraction refers to the active cycling of cross-bridges between the actin thin filaments and the myosin thick filaments within the sarcomeres
What is isotonic contraction?
Isotonic contraction is where the tension within the muscle remains constant and the length changes
Identify and describe the 2 types of isotonic contraction
- Concentric contraction: muscle shortens e.g. biceps during elbow flexion
- Eccentric contraction: muscle lengthens e.g. quadriceps during walking
What is isometric contraction?
Isometric contractions is where the length of the muscle stays contant but tension is generated e.g. holding a weight in a fixed position, or trying to push a brick wall
What is passive stretch?
Passive stretch is the lengthening of the muscle muscle in a passive state i.e. not being stimulated to contract
E.g. pull felt in the hamstrings whilst touching the toes
The protein titin is now known to be important in the contraction of striated muscle tissues.
What is its role in passive stretch?
- Titin connects the Z line to the M line in the sarcomere and contributes to force transmission at the Z line and resting tension in the I band region
- Titin limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle
Briefly, describe the role of motor neurons in the skeletal muscle
Each skeletal muscle is supplied by a number of motor neurons which stimulate the muscle fibres to contract
What type of motor neurons innervate the skeletal muscle fibres?
α-motor neurons
Where are the cell bodies of alpha motor neurones found?
- Ventral horn of the spinal cord for muscles of the limbs and trunk
- Motor nuclei of the brainstem for the muscles of the head and face
What is a neuromuscular junction?
A neuromuscular junction is a specialised synapse between an a motor neurone and a skeletal muscle fibre
In 6 steps, outline the events which occur at a neuromuscular junction
⇒ Action potential arrives at the synapse
⇒ Voltage-gated Ca2+ channels open
⇒ Increased [Ca2+] causes release of ACh from vesicles into synaptic cleft
⇒ ACh activates nicotinic ACh receptors in the sarcolemma
⇒ Influx of Na+ depolarises sarcolemma (local)
⇒ Voltage sensitive Na+ channels are activated to produce general depolarisation
What is a motor unit?
- A motor unit is an α-motor neuron and the group of individual muscle fibres that it innervates
- Any single muscle fibre is innervated by only one α-motor neuron, but each α-motor neuron can innervate a number of different muscle fibres
The number of muscle fibres in a motor unit varies between different muscles.
Illustrate this
- Muscles that perform precise fine movements have less muscle fibres each unit motor unit
- Powerful muscles with crude movements have far more muscle fibres in each motor unit
What is muscle tone?
Muscle tone is the continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state
All muscle has some degree of baseline tone.
What contributes to this?
- Elasticity of the muscle tissue
- Low levels of motor neuron activity
Why is hypotonia?
Hypotonia is a lack of skeletal muscle tone and results from damage to the motor cortex, cerebellum or spinal cord
Skeletal muscle tone is controlled by motor control centres in the brainstem.
Describe this control
The locus coeruleus, containing noradrenergic cells, projects ascending axons to spinal motor neurons where it facilitates muscle tone
The contractile force produced by a muscle depends on two factors.
Identify and describe them
- Size principle: slow type I fibres are recruited first followed by those containing mostly fast IIa fibres and then those containing fast IIX fibres
- Rate code: the frequency at which the muscle fibres are stimulated by their α-motor neuron
Identify the sources of energy for muscle contraction
- ATP stores (very little)
- Stores of creatine phosphate (short term)
- ATP from glycolysis and oxidative metabolism (long-lasting)
State an advantage and disadvantage of glycolysis
- Advantage: function under anaerobic conditions so not reliant on the blood supplying oxygen at the rate required for maximal contraction
- Disadvantage: lactate is produced from pyruvate by LDH and accumulates
What are the effects of the accumulation of lactate?
Lactate accumulation is associated with acidification of the muscle cell environment leading to cramps and muscle fatigue
Describe contraction in Type I muscle fibres
- Slow muscle fibres express the type I myosin heavy chain
- They contract relatively slowly, produce low amounts of force and are extremely resistant to fatigue (high mitochondria content, oxidative metabolism)
Describe contraction in Type II muscle fibres
- Fast muscle fibres express the type II myosin heavy chain
- They contract relatively quickly and produce large amounts of force
Identify and describe the two different types of Type II muscle fibres
- The fast IIX muscle fibres rely on glycolytic metabolism, produce great amounts of force and fatigue rapidly
- The fast IIA muscle fibres are an intermediate form between Type I oxidative fibres and fast IIX glycolytic muscle fibres
What is malignant hyperthermia?
Malignant hyperthermia is a disease that causes a fast rise in body temperature and severe muscle contractions when triggered by some volatile anaesthetic agents
What is the most common cause for malignant hyperthermia?
Polymorphism in the ryanodine receptor (genetic cause)
In 4 steps, explain how malignant hyperthermia presents with an uncontrolled increase in oxidative metabolism
⇒ Ryanodine receptor is activated by anaesthetic agents
⇒ Massive increase in [Ca2+]i from intracellular stores
⇒ SERCA must function at a dramatically increased rate
⇒ Excessive heat production
How can malignant hyperthemia be treated?
Dantrolene – a muscle relaxant which antagonises the ryanodine receptor
