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Action Potential, Muscle
When a neuromuscular junction is repeatedly stimulated by incoming neural action potentials, the end-plate potentials eventually exceed the threshold for an action potential to be initiated within the muscle fibre instead. The active ions in this new type of action potential are calcium ions instead of sodium, and the resulting calcium ion influx is the key enabling factor in excitation-contraction coupling.
Alpha Motor Neurons
These are large lower motor neurons, situated in the ventral spinal grey. Their axons form the bulk of the motor root of the spinal nerve.
Alpha-Gamma Control System
An important biological servosystem involving structures of the central and peripheral nervous systems. Helps to manage bodily posture and locomotion from the level of the spinal segment, thus (a) making for very rapid muscular response when problems are encountered, and (b) generally reducing neural traffic up and down the spinal cord. (See alpha motor neuron and pyramidal tract.)
These are the sensory receptors of the intrafusal muscle fibres. They are wound spirally around the belly of each such muscle fibre, and are thus admirably placed to detect the 'fatness' of that fibre at any moment in time. Their sensory information is conveyed back to the spinal segment by the sensory branch of the spinal nerve.
Type of muscle organisation where muscles groups oppose each other in moving a limb. When controlling such muscles it is necessary for one group to relax while the other is pulling. (See extensor muscle and flexor muscle.)
These are fibres derived from upper motor neurons whose lower motor neurons are situated in the cranial nerve nuclei of the medulla. (Compare corticospinal fibres.)
These are fibres derived from upper motor neurons whose lower motor neurons are situated in the spinal grey. (Compare corticobulbar fibres.) This is therefore the major class of fibres found in the pyramidal tract.
Electromyography is the technique of detecting the synchronised discharge of all the muscle fibres in a motor unit. This is made possible by the fact that this discharge creates an electrostatic field which can be picked up from some distance away by either an implanted or a skin surface electrode (the former being very sensitive, the latter less so but quicker and more convenient to apply). EMGs are much used in research and clinical medicine, as well as being available in kit form for personal amusement and biofeedback.
End-Plate Potential (EPP)
The potential in a motor end-plate following the arrival of a neural action potential. This is not yet a muscular action potential, rather it is the equivalent to the EPSP in neurotransmission. (It is 'graded' activity in other words, rather than all-or-none.) Typically, the EPP takes longer to decay (ca 10 msec) than do the traces of the acetylcholine which caused it (3 msec). It may therefore require the temporal summation of a succession of incoming impulses to produce a full muscular action potential.
Term used to describe the link between the action potential of muscle fibres, and the contraction of the myofibrils which then follows. This phenomenon results from sensitivity of the protein filaments within the sarcomere to a sudden influx of calcium ions.
In an antagonistic muscle pair, the one which straightens the limb in question rather than bending it. (Compare flexor muscle.)
Extrafusal Muscle Fibres
These are the main muscle fibres making up a skeletal muscle. They provide the main contractile power for bodily posture and locomotion.
This is the ancillary spinal motor tract. It arises in a variety of basal ganglia, midbrain, and hindbrain locations, descends more ventrally in the spinal white than does the pyramidal tract, and carries involuntary muscle instructions to the gamma motor neurons.
Fibre Bundle (Muscle)
The first level of muscle organisation above the muscle fibre. Consists of bundles of individual muscle fibres. These bundles are sometimes referred to as fasciculi, and consist of individual muscle fibres encased within a layer of connective tissue called the perimyseum.
In an antagonistic muscle pair, the one which bends the limb in question rather than straightening it. (Compare extensor muscle.)
Gamma Motor Neurons
These are small lower motor neurons, situated in the ventral spinal grey. Their axons join those of the alpha motor neurons to form the motor root of the spinal nerve.
Golgi Tendon Organs
These are the sensory receptors of the muscle tendons. They detect the amount of tension in the tendons, and therefore of the entire muscle. Their sensory information is conveyed back to the spinal segment by the sensory branch of the spinal nerve. Excitation of the Golgi tendon organs excites inhibitory interneurons within the spinal grey which inhibit the alpha motor neurons which caused the muscle contractions in the first place. In the absence of any other effect, therefore, muscle stretch inhibits itself, thus serving as a valuable safety mechanism against cramp, tetany, etc.
One of several neurons in a neuronal circuit, usually relatively small and with only a short axonal travel. Frequently inhibitory by nature, so that they act as negative feedback devices (as, for example, with Renshaw cells). Inhibitory interneurons in the spinal grey are involved in the alpha-gamma control system, where they are responsible for reducing the output from a given alpha motor neuron whenever the muscle being controlled by that neuron is determined to be contracting too vigorously.
Intrafusal Muscle Fibres
An individual muscle fibre which is smaller and thinner than normal. These are small muscle fibres contained in the muscle spindles, and wound about by the annulospiral endings. These fibres are excited by the gamma motor neurons, and respond via the annulospiral endings. Functionally, these are the control fibres, whereas the extrafusal muscle fibres are the strength fibres. See under muscle spindle for mode of operation. Also optionally known as spindle fibres.
Lower Motor Neuron
Synonym for alpha motor neuron. (Compare upper motor neuron.)
The motor axons of a spinal nerve branch into numerous telodendria as they approach the muscle they innervate. Where a single telodendrion touches the muscle fibre it flattens out to make a better contact. This enlargement is known as the motor end-plate, or neuromuscular junction. It is thus the nerve-muscle equivalent of a synaptic button.
One of the neural fibre bundles which emerge from the ventral horn of a given spinal segment and make up the motor element of that segment's spinal nerve. (Compare sensory root.)
The bundle of muscle fibres innervated by a single motor end-plate. Can be of widely differing size: in eye muscles there are seven muscle fibres per nerve fibre, but in leg muscles this rises to 1700 muscle fibres per nerve fibre.
Muscle Contraction, Overview
There is a six-stage sequence of events during the initiation of a striate muscle contraction. These are (a) arrival of the neural action potential at the neuromuscular junction, (b) release of the acetylcholine neurotransmitter, (c) creation of the partial depolarisation in the muscle fibre (the end-plate potential), (d) triggering of the muscular action potential, (e) triggering of the calcium ion action potential within the tubule membranes of the sarcoplasmic reticulum, and (f) contraction of the sarcomeres making up each myofibril.
Muscle cells. Striate muscle fibres are of two types. Red muscle fibres contain plentiful supplies of myoglobin, a variant of haemoglobin, because they have to satisfy the metabolic demands of large muscle movement. White muscle fibres contain less myoglobin and fatigue more easily. The proportion of each depends upon biological need (eye muscles are mainly white, for example), but can be affected by training. The main subcellular components of a muscle fibre are the myofibrils and the sarcoplasmic reticulum. (Not to be confused with nerve fibres!)
These are small bundles of intrafusal muscle fibres scattered in moderate numbers throughout the main bulk of a muscle. They make no real contribution to the power of that muscle's contraction, but serve instead as contraction sensors. The information they provide is one of the key factors in the alpha-gamma control system.
A collection of axons en route from their source neurons to their appointed destination. (Not to be confused with muscle fibres!)
Neuromuscular Junction (NMJ)
The junction between an efferent nerve axon and a skeletal muscle. Much like a synapse, but involving an alpha motor neuron and a motor unit rather than two neurons. Also known as the motor end-plate.
The connective tissue wrapped around a bundle of muscle fibres.
This is the direct corticospinal tract. It arises in the primary motor cortex, as a bundling together of axons from the upper motor neurons. By the time it reaches the medulla, it contains a million or so fibres, 90% of which decussate (cross over) and travel down the contralateral (or 'crossed') lateral corticospinal tract of the spinal cord. The pyramidal tract is conventionally believed to carry voluntary muscle instructions to the alpha motor neurons. (See also corticospinal fibres, and compare corticobulbar fibres.)
An inhibitory interneuron found in the spinal grey, and associated in two ways with an alpha motor neuron. Firstly, it receives an excitatory collateral from the alpha neuron's axon as it emerges from the motor root, so that is 'kept informed' of how vigorously that neuron is firing. Secondly, it sends its own inhibitory axon to synapse with that alpha neuron. The rate of discharge of the Renshaw cell is thus broadly proportional to the rate of discharge of the associated motor neuron, and the rate of discharge of the motor neuron is broadly inversely proportional to the rate of discharge of the Renshaw cell. Renshaw cells thus act as 'limiters', or 'governors', on the alpha motor neuron system, thus helping to prevent muscular damage from tetanus.
One of the six or so fibre bundles which emerge from the dorsal horn of a given spinal segment and make up the sensory element of that segment's spinal nerve. (Compare motor root.)
The mass of neuron cell bodies making up the central core of the spinal cord throughout most of its length. Shoes a distinctive butterfly shape if transected, the tips of the wings of which being known as the dorsal horns (dorsally) or the ventral horns (ventrally). The alpha motor neurons are located in the ventral horns. (Compare spinal white.)
A bilaterally symmetrical pair of peripheral nerves arising from each spinal segment, and responsible for the body's sensory, skeletomuscular and visceral operation.
A section of spinal cord corresponding more or less to a single vertebra, from which a single bilateral pair of spinal nerves originates.
The spinal cord's ascending and descending axon tracts. So called because the myelin sheathing of each component axon gives the tissue a white colouring upon dissection. In spinal cross-sections, the white matter can be seen to be channeled into the 'flutings' provided by the horns of the spinal grey. (Compare spinal grey.)
See intrafusal muscle fibres.
(1) State of constant muscle tension caused (a) by incoming excitations arriving so quickly that their individual twitches overlap and merge ('fused tetanus') (see end-plate potential), or (b) pathological state such as tetanus (2) or strychnine poisoning. Tetanus (2): Name of disease (commonly known as 'lock-jaw') characterised by severe and life-threatening tetanus (1) caused by action of toxins produced by the bacteria responsible.
Muscle spasm (uncontrolled overcontraction) caused by insufficient calcium ions available at the muscle end-plate.
Minute oscillations of a muscle which accompany its contraction. Their frequency is in the range 8 - 12 Hz, and their amplitude about 1% that of the main muscle contraction.
A sudden but unsustained contraction of one or many muscle fibres.
Upper Motor Neuron
A relatively large neuron in Layer V of the primary motor cortex (Brodmann's Area 4). The axons pass down through the corona radiata to the internal capsule, then down through the cerebral peduncles of the midbrain to form the pyramidal tract.
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