about animal anatomy and physiology; muscular system

1. 5. The Muscular System

2.  Muscle tissue specialized for contraction and is responsible for body
movements and changes in size and shape of internal organs.
 The prefixes myo and sacro refers to muscle, the terms such as
myofibril and myofilaments reference structure with in the muscle.
 The plasma membrane of muscle cell is called the sarcolemma.
 The cytoplasm is termed as sarcoplasm.
 Endoplasmic reticulum is termed as sarcoplasmic reticulum(SR).
 A single skeletal muscle is called a muscle fiber.

3. Properties of muscle
Excitability: Sometimes called irritability. Muscle cell maintain a
membrane potential and are able to respond to a stimulus such as a
neurotransmitter by developing an electrical impulse.
Contractility: When stimulated, the electrical impulse spreading
across a muscle cell can cause the cell to contract.
Extensibility: In addition to contraction, muscle cell can lengthen in
response to stretch.
Elasticity: Once stretched, muscle fibers can recoil to their original
resting length due to the elastic elements with in the muscle.

4. Functions of muscle
Production of movement: The action of skeletal muscle is responsible
for moving joints and thus allowing locomotion.
Maintaining posture: through the action of signals generated from
sensors located in joint, tendons, and muscles, minute adjustments
are anatomically made to maintain the position.
Generation of body heat: skeletal muscles are an important organ in
heat production, such as through the process of shivering.
Stabilizing joints: In addition to moving joints, muscle also stabilizes
the joints, thus minimizing dislocations.

5. Classifications
Smooth
 Individual cells are spindle shaped with centrally located
nuclei
 Regulated by the autonomic nervous system
 Located in visceral structures
 Aggregates of myofilaments are composed of actin and
myosin
 Filaments are not arranged in order

6. Smooth Muscle

7. Classifications
Cardiac
 Found only in the heart
 Regulated by the autonomic nervous system
 Characterized by alternating white and dark bands
 Composed of elongated, branching cells with
irregular contours at junctions with other cells

8. Classifications
 Boundary area where cells meet is called
intercalated disk
 This specialized structure facilitates transmission of
nerve impulses
 Each cell has a nucleus (sometimes 2) that is
centrally located

9. Cardiac Muscle

10. Classifications
Skeletal muscle
Long bundles of cells – striated
Multinucleated with nuclei at periphery of cells
Innervated by spinal or cranial nerves
Three types
red or dark (highest myoglobin and mitochondria content)
white or pale (lowest myoglobin and mitochondria content)
intermediate (characteristics between white and red fibers)

11. Classifications
Skeletal muscle comprises majority of body
muscle mass.
Skeletal muscle fiber can run the length of the
muscle with which it is a part.
Skeletal muscle fibers do not branch or
anastomose.

12. Classifications
Often described according to type of movement performed
Flexors – if they are located on the side of the limb towards which
the joint decreases the joint angle
Extensors – if they are located on the side of the limb towards which
the joint increases the joint angle
Adductors – if they pull a limb toward the median plane
Abductors – if they pull a limb away from the median plane
Sphincters – are arranged circularly to constrict body openings

13. Skeletal Muscle

14. Arrangement
 Function of muscles is to contract or shorten and thereby
move an object.
 Primary consideration for accomplishing this goal is
arrangement.
 Examples include:
 Sheets
 Sheets rolled into tubes
 Bundle
 Rings
 Cones
 Discrete cells or clusters of cells

15. Arrangement
 The effects of skeletal muscles (apart from sphincters) are
noted some distance from their location.
 This means the contraction must be transmitted.
 One end must be anchored and the other attached directly to a
tendon or moveable part.
 Accordingly, anatomic description of a skeletal muscle
sometimes refers to its origin and insertion.

16. Skeletal Muscle Harnessing
 Skeletal muscle accounts for approximately 40% of body
weight.
 Each skeletal muscle is considered as an organ and is made up
of:-
connective tissue
muscle fiber
blood vessels
 nerve fibers

17. Connective tissue
 Muscle cell has three connective tissue layers.
Epimysium: The entire cell muscle is surrounded by a dense, irregular
connective tissue layer called the epimysium containing a dense
concentration of collagen fibers.
 This layer separates the muscle from surrounding tissue.
Perimysium: In cross section, a muscle consists of multiple grouping
of muscle fibers called fascicles (bundles). Each fascicles
surrounded by the perimysium (peri – around) containing collagen
and elastic fibers.
 This layer contains blood vessels and nerves supplying the fascicles.

18. Skeletal Muscle Harnessing
 Endomysium: Within each fascicles are individual skeletal muscle
cells, called muscle fibers, each surrounded by the endomysium (endo
– within).
 Within this connective tissue layer are capillaries supply each muscle
fiber, nerve fibers controlling the muscle, and satellite cells.
 These later cells serve as stem cells that can help repair damaged
muscle.

19. Blood vessels and nerves
 The two innermost layers of connective tissue within the
muscle each contain blood vessels and nerves.
 Skeletal muscle is generally under voluntary nervous control,
and therefore requires stimulation for nerve fibers to initiate
contraction.

20. Arrangement

21. Skeletal muscles fiber
 Skeletal muscle tissue consists of large, multinucleated cells
commonly referred to as muscle fiber.
 Muscle fiber form the fusion of small, individual muscle cell called
myoblasts during development.
 Some remain unfused and become satellite cells.
 Skeletal muscle fibers are incapable of dividing, new muscle fibers
are produced from satellite cells located in the adult muscle.
 The nuclei located immediately under the plasma membrane, which
in skeletal muscle is called sarcolemma.
 The cytoplasm of skeletal muscle is called sarcoplasm.

22. Formation of skeletal muscle cell

25.  Muscle fiber are composed of functional subunits called myofibrils.
 Each muscle cell contains hundreds to thousands of myofibrils.
 The myofibril consists of bundles of myofilaments that are protein
filaments composed of primarily actin and myosin, the two
contractile protein in muscle.
 Actin forms – the bulk of the thin filaments
 Myosin forms – the bulk of the thick filaments
Myofibrils

26. Myofibrils contain three types of proteins
Contractile protein: generates the force during contraction. This
protein contain myosin and actin.
Regulatory protein: help to initiate and terminate the contraction
process and include tropomyosin and troponin found on thin
filaments.
Structural protein: help to maintain the alignment of the thin and
thick filaments and provide elasticity and extensibility. These
protein includes – titin, myomesium, and dystrophin.

27. Sarcoplasmic reticulum
 Similar to the endoplasmic reticulum in nonmuscle cells, the
sarcoplasmic reticulum (SR) forms a tubular network
surrounding each myofibril.
 The terminal cisternae have an active calcium pump that
pumps calcium from the sarcoplasm in to the SR.

28. Sarcomeres
 Sarcomeres is the functional unit of skeletal muscle.
 Sarcomeres contain the protein myofilaments actin and myosin
 In stained cross sections of skeletal muscle, alternating light and
dark bands are evident, which are called the I and A bands
respectively. These bands give skeletal muscle its striated
appearance.
 The dark bands alter the plane of the polarized light and are
therefore anisotropic (not having the same properties in all
direction)

29.  The light bands do not alter the plane of polarized light and are there
for isotropic(appear the same in all direction), thus the name A band
and I band, respectively.
 Sarcomere is the region between two Z discs. It consists of one-half
an I band, an A band, and one half an I band.
 In muscle at rest, a lighter region can be found in the center of the A
band called the H zone (from helle, meaning bright), which contains
only myosin.
 M – line, named for being in the middle of the sarcomere, transects
the H zone and composed of protein that stabilizes the position of
thick filaments.

31. Functional units of sarcomere cell

34. Skeletal muscle contraction
 Each skeletal muscle fiber is innervated by a motor neuron
innervating the muscle.
 The release of the neurotransmitter from these motor neurons
initiates excitation – coupling- contraction in which an
action potential is generated within the skeletal muscle fiber.
 The action potential causes the release of calcium from the SR,
which the causes muscle contraction.

35. Summary of skeletal muscle contraction

36. Summary of skeletal muscle contraction
Stimulation of α-motor neurons going to skeletal muscle
The release of acetylcholine at the neuromuscular junction
Production of an action potential in the muscle fiber
Release of calcium ions from the sarcoplasmic reticulum
Calcium diffuse to the thin filaments and binds to the troponin and
initiate contraction