Basal electrical rhythm
Encyclopedia
The Basal or Basic electrical rhythm (BER) or electrical control activity (ECA) determines the frequency of the contractions in the gastrointestinal (GI) tract.

Physiology

Smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

 within the GI tract causes the involuntary peristaltic
Peristalsis
Peristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar...

 motion that moves consumed food down the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

 and towards the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

. The smooth muscle throughout most of the GI tract is divided into two layers: an outer longitudinal layer and an inner circular layer. Both layers of muscle are located within the muscularis externa
Muscularis externa
The muscular coat is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa membrane...

. The stomach has a third layer: an innermost oblique layer.

The physical contractions of the smooth muscle cells can be caused by action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

s in efferent motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...

s of the enteric nervous system
Enteric nervous system
The enteric nervous system is a subdivision of the autonomic nervous system that directly controls the gastrointestinal system in vertebrates.It is derived from neural crest.-Function:...

, or by receptor mediated calcium influx. The inner circular layer is innervated by both excitatory and inhibitory motor neurons, while the outer longitudinal layer is innervated by mainly excitatory neurons. These action potentials cause the smooth muscle cells to contract or relax, depending on the particular stimulation the cells receive. Longitudinal muscle fibers depend on calcium influx into the cell for excitation-contraction coupling
Excitation-contraction coupling
Excitation-contraction coupling is a term coined in 1952 to describe the physiological process of converting an electrical stimulus to a mechanical response . This process is fundamental to muscle physiology, whereby the electrical stimulus is usually an action potential and the mechanical...

, while circular muscle fibers rely on intracellular calcium release. Contraction of the smooth muscle can occur when the BER reaches its plateau (an absolute value less than -45mV) while a simultaneous stimulatory action potential occurs. A contraction will not occur unless an action potential occurs. Generally, BER waves stimulate action potentials and action potentials stimulate contractions.

The Interstitial cells of Cajal are specialized pacemaker cells located in the wall of the stomach, small intestine, and large intestine. These cells are connected to the smooth muscle via gap junction
Gap junction
A gap junction or nexus is a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells....

s and the myenteric plexus. The cell membranes of the pacemaker cells undergo a rhythmic depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...

 and repolarization from -65mV
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

 to -45mV. This rhythm of depolarization-repolarization of the cell membrane creates a slow wave known as a BER, and it is transmitted to the smooth muscle cells. The frequency of these depolarizations in a region of the GI tract determines the possible frequency of contractions. In order for a contraction to occur, a hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

 or neurocrine
Chemical synapse
Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie...

 signal must induce the smooth muscle cell to have an action potential. The basal electrical rhythm allows the smooth muscle cell to depolarize and contract rhythmically when exposed to hormonal signals. This action potential is transmitted to other smooth muscle cells via gap junctions, creating a peristaltic wave.

Frequency

The number of action potentials during the plateau of a particular BER slow wave can vary. Multiple action potentials during the same slow wave peak create a stronger muscle contraction.

The frequency of the BER, and thus the contractions, changes throughout the GI tract. The frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 in the stomach is 3 per minute, while the duodenum
Duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...

 is 11 to 12 per minute and the ileum
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...

is 9 per minute. The colon can have a BER frequency between 2 and 13 per minute. The electrical activity is oscillatory, so that the BER has peaks and valleys when graphed over time.
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