Muscle fascicle

Structure of a skeletal muscle. (Fascicle labeled at bottom right.)

Bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue.

- Muscle fascicle

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Perimysium

Structure of a skeletal muscle. (Perimysium labeled at top center.)

Perimysium is a sheath of connective tissue that groups muscle fibers into bundles (anywhere between 10 and 100 or more) or fascicles.

Skeletal muscle

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton.

A top-down view of skeletal muscle
3D rendering of a skeletal muscle fiber
Muscle types by fiber arrangement
Types of pennate muscle. A – unipennate; B – bipennate; 
C – multipennate
ATPase staining of a muscle cross section. Type II fibers are dark, due to the alkaline pH of the preparation. In this example, the size of the type II fibers is considerably less than the type I fibers due to denervation atrophy.
Structure of muscle fibre showing a sarcomere under electron microscope with schematic explanation.
Diagram of sarcoplasmic reticulum with terminal cisternae and T-tubules.
Human embryo showing somites labelled as primitive segments.
When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap.
Contraction in more detail
(a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
Exercise-induced signaling pathways in skeletal muscle that determine specialized characteristics of slow- and fast-twitch muscle fibers
Jogging is one form of aerobic exercise.
In muscular dystrophy, the affected tissues become disorganized and the concentration of dystrophin (green) is greatly reduced.
Prisoner of war exhibiting muscle loss as a result of malnutrition.

A skeletal muscle contains multiple fascicles – bundles of muscle fibers.

Asian elephant

Only living species of the genus Elephas and is distributed throughout the Indian subcontinent and Southeast Asia, from India in the west, Nepal in the north, Sumatra in the south, and to Borneo in the east.

Illustration of an elephant skeleton
The nail-like structures on the toes of an Asian elephant
Depigmented skin on the forehead and ears of an Asian elephant
A 5-month-old calf and its 17-month-old cousin in a sanctuary in Laos
Indian elephants in the Coimbatore Forests, Tamil Nadu
Prime elephant habitat cleared for jhum—a type of shifting cultivation practiced in Arunachal Pradesh
Elephants on the road in Khao Yai National Park, Thailand
Rhythmic swaying behaviour is not reported in free ranging wild elephants and may be symptomatic of psychological disorders.
Elephants are used for safari tourism in some Asian countries
At this elephant training camp, captive elephants are taught to handle logs.

The deeper muscles are best seen as numerous distinct fasciculi in a cross-section of the trunk.

Outer ear

External part of the ear, which consists of the auricle and the ear canal.

The muscles of the pinna
External and middle ear, opened from the front. Right side.
A diagram of the anatomy of the human ear:

The helicis minor is an oblique fasciculus, covering the crus helicis.

Pinniped

Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals in the clade Pinnipedia.

Restoration of Puijila
Fossil of Enaliarctos
Fossil skull cast of Piscophoca sp. from Phocidae
Reconstruction of Archaeodobenus akamatsui family Odobenidae
Male and female South American sea lions, showing sexual dimorphism
Light reflection on an elephant seal eye
Frontal view of brown fur seal head
Vibrissae of walrus
Weddell seal underwater
Northern elephant seal resting in water
Walrus on ice off Alaska. This species has a discontinuous distribution around the Arctic Circle.
Harbor seal hauled out on rock
Steller sea lion with white sturgeon
Leopard seal capturing emperor penguin
Orca hunting a Weddell seal
Walrus herd on ice floe
Northern fur seal breeding colony
Male northern elephant seals fighting for dominance and females
Harp seal mother nursing pup
Adult Antarctic fur seal with pups
Walrus males are known to use vocalizations to attract mates.
Sea lion balancing a ball
Inuit seal sculptures at the Linden Museum
Captive sea lion at Kobe Oji Zoo Kobe, Japan
Men killing northern fur seals on Saint Paul Island, Alaska, in the mid-1890s
Protests of Canada's seal hunts
Grey seal on beach occupied by humans near Niechorze, Poland. Pinnipeds and humans may compete for space and resources.

In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water.

Sternocleidomastoid muscle

One of the largest and most superficial cervical muscles.

Neck muscles, with the sternocleidomastoid shaded
The sternocleidomastoid muscle with nearby structures labeled, such as the triangles of the neck.

The sternal head is a round fasciculus, tendinous in front, fleshy behind, arising from the upper part of the front of the manubrium sterni.

Striated muscle tissue

Cardiac muscle (heart muscle)

Micrograph of HPS stained skeletal striated muscle (fibularis longus).

The perimysium organizes the muscle fibers, which are encased in collagen and endomysium, into fascicles.

Gluteus maximus

Main extensor muscle of the hip.

The gluteus maximus, with surrounding fascia. Right buttock, viewed from behind, skin covering removed.
Muscles of the gluteal and posterior femoral regions, showing origin and insertion of gluteus maximus muscle.
Image showing the outer surface of the ilium, showing the inferior gluteal line.
An Ancient Greek javelin thrower represented on a vase, c.520 B.C.
An Ancient Greek warrior in bronze. Riace Bronzes, c.450 B.C.
A sprinter passing the baton to his team mate, 2018
A sprinter warming up on the race track, 2018
Gluteus maximus is the most superficial muscle of the hips, here visible at top centre with skin removed from the entire right leg
The gluteus maximus as it appears on a skeleton without other muscles
All gluteal muscles, maximus in yellow
Structures surrounding right hip-joint (gluteus maximus visible at bottom)
Innervation and blood-supply of the gluteus maximus.
Gluteus maximus cut showing underlying structures.
Gluteus maximus cut showing underlying structures.
Structures visible under the gluteus maximus.
Innervation as seen from under the gluteus maximus.

The muscle is made up of muscle fascicles lying parallel with one another, and are collected together into larger bundles separated by fibrous septa.

Coccyx

Final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horses.

The coccyx
A coccyx with four vertebrae below the sacrum.
The coccyx sits below the sacrum and behind the pelvic cavity.

The extensor coccygis is a slender muscle fascicle, which is not always present.

Heart block

Disorder in the heart's rhythm due to a fault in the natural pacemaker.

ECG showing types of heart block
Conduction system of the heart
Sinus rhythm with acute inferior infarction complicated by Type I AV block manifest in the form of 5:4 Wenckebach periods; R-P/P-R reciprocity.
Sinus tachycardia with complete AV block and resulting junctional escape

Conduction is initiated by the sinoatrial node ("sinus node" or "SA node"), and then travels to the atrioventricular node ("AV node") which also contains a secondary "pacemaker" that acts as a backup for the SA nodes, then to the bundle of His and then via the bundle branches to the point of the apex of the fascicular branches.