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