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The Muscular System
ANS 215Anatomy & Physiology
of Domesticated Animals
Primary Functions Movement
Blood Ingesta/excreta Gametes/conceptus
Support Generation of body heat
Calssifications Smooth
No striations 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
Smooth Muscle
Classifications Cardiac
Found only in the heart Regulated by the autonomic nervous
system Shows striations characterized by
alternating white and dark bands Composed of elongated, branching cells
with irregular contours at junctions with other cells
Classifications Cardiac
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
Cardiac Muscle
Classifications Skeletal
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)
Classifications Skeletal
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
Classifications Skeletal
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
Skeletal Muscle
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 Bundles Rings Cones Discrete cells or clusters of cells
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
Arrangement
Skeletal Muscle Harnessing Harness for skeletal muscle fibers is
composed of connective tissue elements Epimysium Perimysium Endomysium
Continuous from the individual muscle fibers to the connective tissue structure to which it attaches and on which it exerts its contraction
Skeletal Muscle Harnessing Connective tissue elements of
skeletal muscle are: Muscle fibers comprise a muscle bundle
which are attached by their cell covering (sarcolema) to connective tissue division, endomysium
The endomysium is continuous with the connective tissue that envelopes muscle bundles, the perimysium
Skeletal Muscle Harnessing The perimysium is continuous with the
connective tissue that envelops the muscle (collection of bundles), the epimysium
Then epimysium is continuous with the tendon or aponeurosis, which can travel some distance from its attachment
Microstructure of Skeletal Muscles Depending on diameter of the
muscle fiber, there might be several hundred to several thousand myofibrils within one muscle
Each myofibril has striations or banding
Myofibrils are further broken into repetitive units (sarcomeres)
Microstructure of Skeletal Muscles Sarcomeres contain the protein
myofilaments actin and myosin Give rise to striations by their arrangement
Sarcomeres of a myofibril are in alignment with the sarcomeres of all the other myofibrils of the muscle fiber
The Z line is located at each end of a sarcomere and is common to both sarcomeres that it separates
Microstructure of Skeletal Muscles Actin filaments project from the Z
line into the sarcomeres that it separates
Actin of two sarcomeres common to the same Z line comprise an I band
Microstructure of Skeletal Muscles The myosin filaments are centrally located
within a sarcomere, coupled with the overlap of actin filaments provide for the dark banding (A band)
There is a regular 2:1 ratio of actin to myosin
Sarcotubular System Skeletal muscle fibers contain a
network of tubules known as the sarcotubular system
Tubules are located within the muscle fiber, but outside the myofibrils
Sarcotubular System Two tubule sets
Arranged transversely (right angles) to myofibrils
Open to the outside of fibers • contain extracellular fluid
Found at boundary of A and I bands• each sarcomere has two
Neuromuscular Junction Each skeletal muscle fiber is innervated by a
motor neuron Axon terminal end bulb forms synapse with
muscle fiber Release of Acetylcholine (Ach) causes
depolarization One motor neuron innervates many muscle fibers
Motor unit Motor unit ration of 1:150 means one nerve fiber
supplies 150 muscle fibers Smaller ration increases precision
Scanning Electron Micrograph of
Neuromuscular Junction
Skeletal Muscle Contraction Depolarization of muscle fibers
Acetylcholine released into synapse • Ca++ facilitates release • Parturient paresis occurs if not enough Ca++
Opens ligand-coated Na+ channels Causes a propagated action potential
• sarcolema • T-tubule • sarcoplasmic reticulum
Cholinesterase destroys Ach
Skeletal Muscle Contraction Depolarization of muscle fibers
Depolarization of sarcoplasmic reticulum causes Ca++ release
Ca++ diffuses to myofibrils Ca++ initiates contraction Ca++ pumped back into sarcoplasmic
reticulum • active transport • Ca++ ATPase Pump