The Muscular System

The three distinct types of muscle tissue in the human body:

• Skeletal muscle

• Cardiac muscle

• Smooth muscle

Skeletal muscles

Attach to the skeleton to provide strength and mobility to the body; the contractions of large skeletal muscles also function to generate heat

Cardiac muscles

In the heart provide the force necessary to pump blood throughout the body

Smooth muscles

Provide steady contraction within organs (and blood vessels) throughout the body

Muscle movements

Can be either voluntary or involuntary

Muscles

Both produce and resist movement

Muscle contractions

Resist movement when maintains posture (skeletal muscles) and maintaining the diameter of blood vessels (smooth muscles)

-contraction of skeletal muscles generate heat - shivering

Shivering

In which skeletal muscles alternatively and rapidly contract and relax, is used to generate heat in cold environments

Muscles are excitable by

Chemical and electrical signals

Excitation of muscles

Causes muscle contraction

Most skeletal muscles

Interact with the skeleton and are organized into groups

Skeletal muscles attach to bones

Via tendons

Skeletal muscle origin

Attaches to a relatively stationary bone

-proximal

The muscle insertion

Attaches to a bone across a joint

-distal

When skeletal muscles contract

Insertion is pulled towards the origin

Synergistic

Muscle groups that work together

Antagonistic

Muscle groups that oppose each other

Whole muscles

Are groups of muscle cells that share a common origin, insertion, and function

Fascicles

Are bundles of muscles cells sheathed in connective tissue (fascia)

Fasciae come together at

The ends of muscles to form the tendons

Muscles cells (fibres)

Are long, tube-shaped cells of variable length, are multinucleated, and are packed with myofibrils (organelles primarily made of actin and myosin)

muscle make-up order

whole muscle → muscle bundle (fascicle)→ single muscle cell (fiber)→ myofibril → sarcomeres → actin, myosin

contractile units

within myofibrils are segments (which extend between Z lines) called sarcomeres

sacromeres contain

thick filaments of myosin and thin filaments of actin

myosin

thick filaments

actin

thin filaments

parallel arrangement of the filaments

gives rise to striated appearance of skeletal muscle

muscle contractions occur

when the two filaments slide past each other and draw the two Z lines together

skeletal muscle cells are innervated by

nerves (motor neurons); there is one neuron per muscle cell, but multiple muscle cells per neuron

stimulation by a nerve (motor neurons)

increases the concentration of Ca2+ ions in the vicinity of the contractile proteins (myosin) in the sarcomeres

presence of Ca2+

permits the contraction of the fibre

absence of Ca2+

prevents contraction

when never stimulation stops

contraction stops

tetanus

caused by a bacterial infection, generally by Clostridium tetani

C. tetani produces

tetanospasmin, a neurotoxin that prevent inhibitory neurotransmitters that are necessary for muscle relaxation
- those inhibitory neurons typically function to limit the activity in CNS

during an infection with C. tetani

a lack of activity in inhibitory motor neurons can cause muscle spasms and spastic paralysis

how can infection by C. tetani be prevented

by immunization; treatment of an infection may include immunoglobulin therapy

motor neurons that innervate skeletal muscles secrete

the neurotransmitter acetylcholine into a neuromuscular junction

acetylcholine binds to

chemically-gated ion channels, which open (allowing Na+ to enter) to generate an electrical impulse in the muscle fibre

the electrical impulse is carried

into the interior of the cell via T tubules

T tubules

invaginations of the cell membrane

electrical impulse triggers

the release of Ca2+ ions from the sarcoplasmic reticulum (which stores Ca2+) into the scarcoplasm

sarcoplasm

specialized muscle fibre sarcoplasm

Ca2+ comes into

proximity of the myofibrils

Ca2+ initiates

the sliding filament mechanism responsible for the contraction (shortening) of the myofibril sarcomeres

contractions occur

when cross-bridges are formed between myosin and actin filaments (myosin heads bind to the actin)

in relaxed muscles

the myosin heads do not contact the actin filaments

after binding, the myosin heads

bend, which pulls the filaments toward the centre of the sarcomere causing shortening

Ca2+ ions must be present for

cross-bridges to form between the myosin and actin filaments

in the absence of Ca2+, a protein complex of

troponin-tropomyosin covers the myosin binding sites on the actin filaments

when Ca2+ binds to troponin

the protein complex shifts to expose the myosin binding sites on the actin filaments

primary source of energy in muscle cells

is ATP, but only a little ATP is stored in muscle cells

ATP

is required for both muscle contraction and muscle relaxation

ATP is used when

myosin heads bind to actin filaments, when those bound myosin heads detach, and when Ca2+ is transported back into the sarcoplasmic reticulum

ATP is replenished by

transferring a phosphate group from creatine phosphate to ADP, by breaking down stored glycogen into glucose for use in anaerobic metabolism, and by the aerobic metabolism of glucose and fatty acids circulating in the blood

cardiac and smooth muscles contract

involuntarily and can do so spontaneously, in the absence of stimulation by a nerve

cardiac muscle cells

are joined together at their ends by intercalcated discs, which contain gap junctions that permit electrical impulses to flow between connected cells

smooth muscle cells

also contain gap junctions, and most can contract spontaneously, except for those in the trachea and iris

cardiac and smooth muscle cells are innervated by

the autonomic nervous system, which can modify the degree of contraction

cardiac muscle cells also

contains actin and myosin filaments arranged in sarcomeres, which gives it the appearance of a striated muscle- similarly to skeletal muscles

cardiac pacemaker cells

contract with the fastest rhythm of all the cardiac muscle cells, which ultimately causes the other cardiac muscle cells to contract at the same pace

smooth muscle cells contain

actin and myosin filaments in bundles anchored to the cell membrane, which are arranged in a criss-cross pattern; gives the smooth appearance

interstitial cells of Cajal

serve as pacemaker cells in the smooth muscle layer (muscularis) of the intestine

skeletal muscle has

the fastest contraction speed, but those contractions are the least sustainable

cardiac muscle has

both a moderate contraction speed and contraction sustainability

smooth muscle cells have

the slowest contraction speed, but the highest contraction sustainability, generally, these muscles are constantly partially contraction (as in blood vessels)

slowest contraction speed

in smooth muscle cells ensures that ATP usage does not outpace ATP generation

muscles are largely derived from

the paraxial mesoderm

paraxial mesoderm

is divided into both head mesoderm and somitic mesoderm

head mesoderm

in the anterior part of the embryo, which will form the muscles of the face/skull

somitic mesoderm

in the posterior of the endoderm which will form the somites

somites

are transitory blocks of mesoderm divided into two regions:
-ventral somites
-dorsal somites

ventral somites

become sclerotome

dorsal somites

become dermomyotome

sclerotome

contributes to the axial skeleton and the tendons (via syndetome)

dermomyotome

gives rise to the musculature of the limbs, the back, and the rib cage (via mytome) and the dermis (via dermatome)- body’s muscles

somite pairs

periodically form along the dorsal embryo, as boundaries get created in the paraxial mesoderm

rate of somitogenesis

is different in every species, but progresses by the same mechanism, according to the clock-wavefront model

humans have

between 38-45 pairs of somites

individual somite identities (i.e. some somites will form cervical vertebrae with no ribs, while others will form thoracic vertebrae with ribs)

is established by the collinearity of Hox gene expression

the syndetome

is the dorsal-most region of the sclerotome
-is specified by neighboring myotome

responsible for tendon formation

syndetome

dorsomedial lip forms

the primaxial muscle ( the deep muscles of the back and intercostal muscles_

the ventrolip forms

abaxial muscle (the muscles of the ventral body wall and the limbs)