Smooth muscle

Smooth muscle tissue, highlighting the inner circular layer (nuclei then rest of cells in pink), outer longitudinal layer (nuclei then rest of cells), then the serous membrane facing the lumen of the peritoneal cavity
The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.
A series of axon-like swellings, called varicosities from autonomic neurons, loosely form motor units through the smooth muscle.
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Involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations .

- Smooth muscle

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Types of muscle contractions

Muscle contraction

Activation of tension-generating sites within muscle cells.

Activation of tension-generating sites within muscle cells.

Types of muscle contractions
In vertebrate animals, there are three types of muscle tissues: 1) skeletal, 2) smooth, and 3) cardiac
Organization of skeletal muscle
Structure of neuromuscular junction.
Sliding filament theory: A sarcomere in relaxed (above) and contracted (below) positions
Cross-bridge cycle
Muscle length versus isometric force
Force–velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening), left of the axis eccentric contractions (the muscle is lengthened under load); power developed by the muscle in red. Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.
Swellings called varicosities belonging to an autonomic neuron innervate the smooth muscle cells.
Cardiac muscle
Key proteins involved in cardiac calcium cycling and excitation-contraction coupling
A simplified image showing earthworm movement via peristalsis
Asynchronous muscles power flight in most insect species. a: Wings b: Wing joint c: Dorsoventral muscles power the upstroke d: Dorsolongitudinal muscles (DLM) power the downstroke. The DLMs are oriented out of the page.
Electrodes touch a frog, and the legs twitch into the upward position

Unlike skeletal muscle, the contractions of smooth and cardiac muscles are myogenic (meaning that they are initiated by the smooth or heart muscle cells themselves instead of being stimulated by an outside event such as nerve stimulation), although they can be modulated by stimuli from the autonomic nervous system.

General structure of a skeletal muscle cell and neuromuscular junction: 1. Axon

2. Neuromuscular junction

3. Skeletal muscle fiber

4. Myofibril

Muscle cell

General structure of a skeletal muscle cell and neuromuscular junction: 1. Axon

2. Neuromuscular junction

3. Skeletal muscle fiber

4. Myofibril
Diagram of skeletal muscle fiber structure

A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells.

The iris in humans is the colored (typically brown, blue, or green) area, with the pupil (the circular black spot) in its center, and surrounded by the white sclera.

Iris (anatomy)

Thin, annular structure in the eye, responsible for controlling the diameter and size of the pupil, thus the amount of light reaching the retina.

Thin, annular structure in the eye, responsible for controlling the diameter and size of the pupil, thus the amount of light reaching the retina.

The iris in humans is the colored (typically brown, blue, or green) area, with the pupil (the circular black spot) in its center, and surrounded by the white sclera.
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Light micrograph of the iris near to the pupil. M. sph. sphincter muscle, L lens
Anterior chamber cross-section imaged by an SD-OCT.
Among human phenotypes, blue-green-gray eyes are a relatively rare eye color and the exact color is often perceived to vary according to its surroundings.
Blue eyes of a 17 year old.
Adult male with amber-colored eyes: This color is extremely rare and occurs when an unusually stronger presence of the yellow pigment (lipochrome) exists in the iris, with a rather small amount of pigment (melanin). This gives the eye an orange copper/gold hue.
Example of a blue iris featuring a yellow iris nevus (eye freckle)
Example of a green-brown (hazel) iris
Example of a brown iris
Example of heterochromia - one eye of the subject is brown, the other hazel.
Structures of the eye
Iris, front view
Fluorescein angiograpy of the iris reveals a radial layout of blood vessels.

The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles (including birds).

Cardiac muscle

3D rendering showing thick myocardium within the heart wall.
The swirling musculature of the heart ensures effective pumping of blood.
Cardiac muscle
Illustration of a cardiac muscle cell.
Intercalated discs are part of the cardiac muscle cell sarcolemma and they contain gap junctions and desmosomes.
Dog cardiac muscle (400X)

Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscle tissue, with the other two being skeletal muscle and smooth muscle.

1. Human urinary system: 2. Kidney, 3. Renal pelvis, 4. Ureter, 5. Urinary bladder, 6. Urethra. (Left side with frontal section)
7. Adrenal gland
Vessels: 8. Renal artery and vein, 9. Inferior vena cava, 10. Abdominal aorta, 11. Common iliac artery and vein
With transparency: 12. Liver, 13. Large intestine, 14. Pelvis

Urinary bladder

Hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination.

Hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination.

1. Human urinary system: 2. Kidney, 3. Renal pelvis, 4. Ureter, 5. Urinary bladder, 6. Urethra. (Left side with frontal section)
7. Adrenal gland
Vessels: 8. Renal artery and vein, 9. Inferior vena cava, 10. Abdominal aorta, 11. Common iliac artery and vein
With transparency: 12. Liver, 13. Large intestine, 14. Pelvis
Male and female urinary bladders in lateral cross-section
Bladder location and associated structures in the male
Calcifications on bladder wall caused by urinary schistosomiasis
Cross-section of the bladder showing a cancer within it. When a cancer occurs it is most likely to be a transitional cell carcinoma.
A diverticulum of the bladder
Urinary bladder (black butterfly-like shape) and hyperplastic prostate (BPH) visualized by medical ultrasound
Vertical section of bladder wall
Layers of the urinary bladder wall and cross-section of the detrusor muscle
Anatomy of the male bladder, showing transitional epithelium and part of the wall in a histological cut-out

The trigone is an area of smooth muscle that forms the floor of the bladder above the urethra.

Diagram of stomach, intestines and rectum in the average human

Gastrointestinal tract

Tract or passageway of the digestive system that leads from the mouth to the anus.

Tract or passageway of the digestive system that leads from the mouth to the anus.

Diagram of stomach, intestines and rectum in the average human
Illustration of the small intestine

It is considerably shorter in the living body because the intestines, which are tubes of smooth muscle tissue, maintain constant muscle tone in a halfway-tense state but can relax in spots to allow for local distention and peristalsis.

A top-down view of skeletal muscle

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.

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.

The other types of muscle are cardiac muscle which is also striated and smooth muscle which is non-striated; both of these types of muscle tissue are classified as involuntary, or, under the control of the autonomic nervous system.

Part of the myosin II structure. Atoms in the heavy chain are colored pink (on the left-hand side); atoms in the light chains are colored faded-orange and faded-yellow (also on the left-hand side).

Myosin

Myosins are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes.

Myosins are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes.

Part of the myosin II structure. Atoms in the heavy chain are colored pink (on the left-hand side); atoms in the light chains are colored faded-orange and faded-yellow (also on the left-hand side).
Myosin unrooted phylogenetic tree
Sliding filament model of muscle contraction.
Crystal structure of myosin V motor with essential light chain – nucleotide-free
State of myosin VI from PDB 2V26 before the power stroke
Phase 1
Phase 2
Phase 3
Phase 4

The term has been extended to include a group of similar ATPases found in the cells of both striated muscle tissue and smooth muscle tissue.

Image showing different structures around and relating to the human uterus

Uterus

For other uses of "Womb", see Womb (disambiguation).

For other uses of "Womb", see Womb (disambiguation).

Image showing different structures around and relating to the human uterus
Different regions of Uterus displayed & labelled using a 3D medical animation still shot
Diagram showing regions of the uterus
Uterus covered by the broad ligament
Schematic diagram of uterine arterial vasculature seen as a cross-section through the myometrium and endometrium
Vessels of the uterus and its appendages, rear view
Transvaginal ultrasonography showing a uterine fluid accumulation in a postmenopausal woman.
Vertical section of mucous membrane of human uterus
Schematic frontal view of female anatomy
Sectional plan of the gravid uterus in the third and fourth month
Fetus in utero, between fifth and sixth months.
Female pelvis and its contents, seen from above and in front
The arteries of the internal organs of generation of the female, seen from behind
Median sagittal section of female pelvis
(Description located on [[:File:Illu female pelvis.jpg|image page]])
Uterus

The myometrium of the uterus mostly consists of smooth muscle.

The choroid and iris. (Ciliary muscle is labeled near top.)

Ciliary muscle

The choroid and iris. (Ciliary muscle is labeled near top.)
Ciliary ganglion with parasympathetic fibers of ciliary nerves.
The arteries of the choroid and iris. The greater part of the sclera has been removed.
Iris, front view.

The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, uvea (vascular layer).