Shepherd's purse (Capsella bursa-pastoris) is an annual or biennialherb of the family Cruciferae with entire or pinnately lobed basal leaves arranged in a rosette. The stem leaves are small, lanceolate, clasping and sagittate at the base. The flowers are small, white and inconspicuous and arranged in terminal racemes. The fruit is a triangular-obcordate silicula resembling an old-fashioned purse, from whence the plant derives it's name. In some countries the leaves are eaten as a vegetable, and it was traditionally used as a medicine to check haemorrhaging in childbirth and to stop bleeding from open wounds. The active constituents of the plant include the amineschlorine and acetylcholine, bursine, diosmin, organic acids and tannins. Research Shepherd's Purse
Acetylcholine (Ach) is a chemical that serves as a neurotransmitter, communicating nerve impulses between the cells of the nervous system. It is largely associated with the transmission of impulses across the synapse between nerve and muscle cells, causing the muscles to contract. ACh is produced in the synaptic knob and stored in vesicles until a nerve impulse triggers its discharge across the synapse. When the ACh reaches the membrane of the receiving cell it binds with a specific site and brings about depolarisation - a reversal of the electric charge on either side of the membrane causing a fresh impulse in nerve cells or a contraction in muscle cells. Its action is short-lived because it is quickly destroyed by the enzyme cholinesterase. Anticholinergic drugs have a number of uses in medicine to block the action of ACh, thereby disrupting the passage of nerve impulses and relaxing certain muscles, for example in premedication before surgery. Research Acetylcholine
Skeletal muscles contract rapidly in response to messages from the central nervous system. Each group of several fibres receives a nerve supply that allows voluntary contraction of the muscle. Muscles can move some body parts in several directions and others in only two directions. The direction the body part is moved depends largely on the shapes of the bones at the joints. The stimulus for the muscle contraction begins in the cerebral cortex and passes down the spinal cord and the nerveroot to the junction between the nerve fiber and the muscle surface. This gap, called the end plate, acts as a kind of amplifier, increasing the effect of the tiny current coming down the nerve fiber to stimulate the much larger muscle fiber. On the arrival of the nerve impulse, a chemical called acetylcholine is released from the motor nerve ending and passes across the gap to stimulate the membrane of the muscle fiber. This stimulation is in the form of an electric current which passes along the surface of the muscle, causing it to contract. It takes one millisecond (1/ 1000th of a second) for the current to pass along the surface of the muscular fiber. Cardiac muscle differs slightly from skeletal muscle because it has a built-in mechanism to maintain the necessary rhythmical contraction independently of any nervous connections. Smooth muscles react much more slowly to stimulation than skeletal muscles. The nerves, when present, alter the activity of the muscle rather than initiating it. This action is somewhat similar to cardiac muscle. The contractions take place rhythmically without direct control from the central nervous system. The impulses for contraction come from within the muscle itself. Research Control of Muscles
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Shepherd's Purse
