The human nervous system. Sky blue is PNS; yellow is CNS.
Nerves (yellow) in the arm
A top-down view of skeletal muscle
3D Medical Animation still shot of Lumbosacral Plexus
Cross-section of a nerve
3D rendering of a skeletal muscle fiber
Micrograph demonstrating perineural invasion of prostate cancer. H&E stain.
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 nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.

- Nerve

The PNS consists of the nerves and ganglia outside the brain and spinal cord.

- Peripheral nervous system

The somatic nervous system is under voluntary control, and transmits signals from the brain to end organs such as muscles.

- Peripheral nervous system

Efferent nerves conduct signals from the central nervous system along motor neurons to their target muscles and glands.

- Nerve

The efferent leg of the peripheral nervous system is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement.

- Skeletal muscle

Nerves move muscles in response to voluntary and autonomic (involuntary) signals from the brain.

- Skeletal muscle
The human nervous system. Sky blue is PNS; yellow is CNS.

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