Lecture 12: Comparative Anatomy Musculature Basics I

Question 1

Dumb question to start off on, but what sort of movements involve muscular contractions?

Answer 1

• Motor actions (movement of the skeletal system)
• Contractions of the heart and vessels
• Actions in the intestines
• Many other specific movements of and within the body

Question 2

What are the 3 main ways that muscle tissue can be classified?

Answer 2

• Muscle tissue can be classified on the basis of a number of its characteristics:
• Appearance:
• Striated
• Smooth
• Arrangement of nucleus (nuclei):
• Multinucleated (Syncytium)
• Mononucleated
• Function:
• Voluntary
• Involuntary

-The most common classification system is based on microscopic appearance and with what organs it is associated.

Question 3

Describe skeletal muscle

Answer 3

• Also referred to as striated or voluntary
• Subunits (muscle fibers) of skeletal muscle have a striated or banded appearance when viewed under magnification.
• Skeletal muscle is generally under the voluntary control of its owner.
• Skeletal muscle is typically attached directly or indirectly to the skeletal system.
• Skeletal muscle fibers develop embryologically from many mononucleated cells (myoblasts) that fuse into long fibers which become peripheral and multinucleated
• See Slide 7

Question 4

Describe smooth muscle

Answer 4

• Also referred to as visceral or involuntary
• Smooth muscle is not striated.
• Smooth muscle is generally involuntary –you cannot control its contractions.
• Smooth muscle consists of groups of spindle-shaped mononucleated cells with centrally located nuclei.
• Smooth muscle is most commonly associated with viscera –especially the gut tube and other hollow structures.

Question 5

Describe cardiac muscle

Answer 5

• Found only in the heart
• Cardiac muscle shares characteristics with both skeletal and smooth muscle.
• It is striated.
• It is involuntary.
• It consists of chains of individual cells that are both mononucleated and striated.
• A major characteristic of cardiac muscle are specialized intercellular junctions called intercalated discs.
• See Slide 10

Question 6

Describe the lesser known fourth type of tissue known as branchiomeric muscle

Answer 6

• Associated with pharyngeal arches:
• Somewhat of a transition between smooth and striated muscle tissue
• Innervated by cranial nerves

Question 7

Describe the basis for naming muscles

Answer 7

• Shape:
• Fusiform or spindle-shaped
• Pinnate:
• • Unipennate
• • Bipennate
• • Multipennate
• • See Slide 11 for Description for each pennate type
• Action (extensor, supinator)
• Location (pectoralis, latissimus dorsi)
• Number of heads (biceps, triceps, quadriceps)
• Fiber direction (oblique, rectus)
• Relative size (major, minor, magnus)
• Origin-insertion (sternocleidomastoid)

Question 8

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Answer 8

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Question 9

Describe the attachment structure for muscle anatomy

Answer 9

• Origin (proximal attachment):
• Usually proximal
• May be fixed with regard to movement
• Insertion (distal attachment):
• Usually distal
• Usually more movable

Question 10

Describe the tendon structure of muscle anatomy

Answer 10

• Attachments between muscle fibers and bone.
• Dense collagenous connective tissue.
• Surrounded by peritendineum.
• Bundles of collagen fibers.
• Poorly vascularized.

Question 11

Describe aponeurosis

Answer 11

Aponeuroses: Flat, fan-shaped tendons typically giving rise to other tendons

Question 12

Describe the hierarchical structure of skeletal muscle (again)

Answer 12

From smallest to largest units:

• Myofilament:
• Types: Myosin (thick filaments) and Actin (thin filaments)
• Organized into: Sarcomeres
• Myofibril: = chain of sarcomeres
• Myofiber: = bundle of myofibrils
• Often referred to as a muscle cell
• Each fiber formed from many fused myoblasts
• Fascicle: = bundle of myofibers
• Muscle: Composed of varying numbers of fascicles
• See Slide 14

Question 13

What are the 3 connective tissue supports surrounding skeletal muscle?

Answer 13

• Endomysium:
• Surrounds each muscle fiber
• Lies outside sarcolemma (cell membrane)
• Perimysium:
• Surrounds each fascicle
• Epimysium:
• Surrounds each muscle
• Becomes continuous with tendons
• Attached to periosteum

Question 14

What are the two main components of myofilaments

Answer 14

• Actin

* Myosin

Question 15

Describe the actin protein polymer

Answer 15

• Thin filamentous protein polymer (F-actin)
• Each filament is made up of two helically wound polymers of G actin.
• Associated molecules: Tropomyosin, & Troponin

Question 16

Describe myosin bundles

Answer 16

• Bundles of long molecules:
• Tail + ATPase head
• Head attached to tail via swivel mechanism
• Heads attach to binding sites on actin filaments.
• Attach-swivel-release cycles allow myosin and actin to slide along one another in opposite directions:
• Produces contraction = sliding filament theory of contraction.

Question 17

Describe sarcomeric arrangement

Answer 17

• Myosin and actin filaments are organized into cylindrical units that are aligned end-to-end to form the myofibrils.
• Each cylindrical unit is called a sarcomere.
• A myofibril is, therefore, a chain of sarcomeres.
• Sarcomere structure:
• • Z-lines: Separate adjacent sarcomeres in a fibril. Composed of Z-actin
• • H-band: In middle of each A-band. Composed entirely of myosin. Band width changes during contraction
• • I-band: On either side of A band. Split by Z lines
• • A-band: Between two I bands in middle of sarcomere. Composed of both myosin and actin. Represents length of myosin chains. Does not change width during contraction
• • I-bands: Located on either side of Z-line. Make up ends of each sarcomere. Composed entirely of actin
• • A-bands: Located in middle of sarcomere. Composed of both actin and myosin
• See slides 20,22

Question 18

Describe the contraction of a muscle fiber

Answer 18

• All-or-none:
• A muscle fiber will either contract completely or not at all.
• Action-potential Sequence:
• An action potential arrives at the sarcolemma from a motor neuron.
• Synaptic plate is the intervention point between the axon and the sarcolemma.
• Action potential is conducted from sarcolemma into the interior of the myofiber via T-tubules.
• Action potential carried by T-tubules causes the release of calcium ions from the sarcoplasmic reticulum cisternae.
• Calcium ions initiate mechanism by which actin and myosin filaments slide over one another resulting in a contraction.

Question 19

Describe myofiber growth

Answer 19

• After birth, the number of myofibers cannot be increased.
• The number of myofibrils can be increased; therefore:
• • The mass of a myofiberand a muscle may be increased.
• Lost muscle tissue will be replaced by scar tissue (fibrous connective tissue).

Question 20

What are motor units?

Answer 20

• A single nerve cell (neuron) may innervate from a few to several hundred myofibers.
• A neuron and the myofibers it innervates constitute a motor unit.
• When a neuron fires, all the myofibers in the motor unit contract.
• All-or-none really refers to a motor unit.
• See slide 25

Question 21

Briefly describe the two types of myofibers

Answer 21

• Myofiber type is determined by innervating neuron.
• Therefore, all the myofibers in a single motor unit will be of the Same type.
• Fiber type classification is based mainly on endurance (resistance to fatigue) and speed of contraction.
• Types:
• • Dark, slow fibers (red fibers)
• -Light, fast fibers (white fibers)

Question 22

Describe the Dark, Slow Fiber

Answer 22

• Fatigue resistant.
• Contract slowly (slow twitch).
• Rely on oxidative phosphorylation.
• Have a large number of mitochondria.
• Have a high concentration of myoglobin.
• Have a low concentration of ATPase.

Question 23

Describe the light, fast type of muscle fiber

Answer 23

• Fatigue easily.
• Contract rapidly (fast twitch).
• Rely on glycolysis.
• Have a small number of mitochondria.
• Have a low concentration of myoglobin.
• Have a high concentration of ATPase.

Question 24

What are the characteristics of myofiber types?

Answer 24

• Muscles usually have a mix of fibers.
• Some muscles are almost entirely of one fiber type or another:
• • Muscles predominantly composed of dark fibers: Soleus
• -Muscles predominantly composed of light fibers: Gastrocnemius

Question 25

Compare phasic fibers to tonic fibers

Answer 25

• Phasic fibers:
• Found in all vertebrate groups.
• Not multiply innervated
• Do propagate action potential
• Tonic fibers:
• Found in non-mammalian vertebrates.
• Involved in slow, sustained postural activities.
• Single nerve cell innervates many fibers:
• • (= motor unit) but each fiber is innervated by more than one nerve cell.
• Contract slowly.
• Do not propagate an action potential.

Question 26

Describe the muscle contraction types

Answer 26

• A contraction is a response to a stimulus.
• Types of contraction:
• • Isometric: Length of muscle does not change.
• • Isotonic: Length of muscle does change:
• • Muscle gets shorter = Concentric.
• • Muscle gets longer = Eccentric.

Question 27

What are examples of muscle names based on each type?

Answer 27

• Shape: Deltoid, Trapezius
• Origin-insertion: Coracobrachialis, Sternocleidomastoid
• Function: Pronator teres, Extensor digitorum
• Relative size: Adductor magnus, Adductor brevis
• Fiber arrangement: Rectus abdominis, Rectus cervicus
• Location: Pectoralis, Latissimus dorsi

Question 28

Describe the fiber arrangement types in musculature

Answer 28

• Straight: Example: rectus abdominis
• Fusiform: Example: biceps brachii
• Unipennate: Example: palmar interosseous muscles
• Bipennate: Example: dorsal interosseous muscles
• Multipennate: Example: deltoid muscle

Question 29

Describe the types of muscle action

Answer 29

• The result of a muscle contraction depends on many variables, including:
• What the muscle is attached to
• Which end of the muscle is fixed
• The force of the contraction and the force of the resistance
• The simultaneous actions of other muscles associated with the same structure.
• For example, when the muscle contracts, it pulls toward the middle, exerting equal force on attachments at both ends in an attempt to pull them toward each other.
• Which bone moves depends on the relative stability of the bones at that particular time.
• Stability of a bone at a given time is determined by contractions of muscles acting as stabilizers.

Question 30

What are terms related to muscle function

Answer 30

• Agonist: Muscle doing the desired action.
• Antagonist: Muscle that opposes the agonist.
• Synergist: Muscle that eliminates unwanted action by the agonist.
• Fixator: Muscle that stabilizes base of attachment of agonist.
• Unijoint: Muscle that crosses only one joint.
• Multijoint: Muscle that crosses more than one joint.
• Insufficiency: Inability of a multijoint muscle to contract maximally over all joints crossed simultaneously:
• • Active insufficiency: Refers to the agonist.
• • Passive insufficiency: Refers to the antagonist.

Question 31

Describe agonists, antagonists, and stabilizers

Answer 31

• Agonist (mover) muscle is a mover when its contraction contributes to the desired movement of a joint.
• Agonist muscles are classified as prime movers and assistant movers.
• The prime mover is a muscle whose primary function is to cause the particular movement, and one which makes a strong contribution to that movement.
• An assistant mover has the ability to assist in the movement but is only of secondary importance to the movement.
• Antagonists oppose the movement of an agonist.
• Stabilizer (fixator) muscles will stabilize the segment (bone) on which another segment (bone) moves.
• When a muscle acts as a stabilizer, it usually contracts isometrically.

Question 32

Describe the terms neutralizer, and multi-joint muscles

Answer 32

• Neutralizer (synergistic) muscles nullify one or more actions of another muscle.
• A pure neutralizer will cause the opposite motion of the prime mover without assisting in the movement.
• The triceps is a pure synergist to the biceps elbow flexion, so that the biceps can perform supination of the forearm.
• The pronator teres is a helping synergist for elbow flexion performed by the biceps while it nullifies the supination component of the biceps.
• A multi-joint muscle is one that extends across more than one joint and potentially can contribute to movement at each joint that it crosses.
• Multi-joint muscles do not allow complete range of motion in all joints at one time.

Question 33

Give muscle examples of flexors, extensors, and abductors

Answer 33

• Muscles that pass anterior to the axis of a joint are flexors:
• Examples: deltoid, biceps.
• Muscles that pass posterior to the axis of a joint are extensors:
• Examples: deltoid, triceps.
• Muscles that pass lateral to a joint are abductors:
• Example: deltoid.
• Muscle flexors can shorten about one-half of total length.

Question 34

Describe active muscle insufficiency

Answer 34

• Active insufficiency (involves active components):
• The diminished ability of a muscle to produce or maintain active tension.
• The muscle is elongated to a point where there is no overlap between myofilaments.
• The muscle is excessively shortened when all cross-bridges have been formed.
• One-joint muscles are arranged so this won’t occur.

Question 35

Describe Passive Muscle Insufficiency

Answer 35

• Passive insufficiency (involves passive elements):
• Occurs when the inactive antagonist muscle is of insufficient length to allow a force to complete the full range of motion available.
• Mostly applies to multi-joint muscles.