You people are all heartless bastards! [Archive]

beef curtains

04-02-2003, 05:34 AM

The body is a highly complex interacting machine. Peripheral nerves penetrate all portions of the body, receiving motor information from and relaying sensory information to the central nervous system in the spinal chord and brain. There are two major branches of the peripheral nervous system- the somatic and autonomic. Somatic regulation affects skeletal muscle, leading to voluntary movements. Autonomic regulation affects smooth muscle, cardiac muscle and glands. These involuntary reflexes are regulated by competing sympathetic and parasympathetic output that average to a moderate value. Sympathetic stimulation causes an increase in cardiac output, heart rate, vasoconstriction, and a decrease in digestive activity. Parasympathetic stimulation causes a decrease in cardiac output, heart rate, vasodilatation and an increase in peristalsis. These two branches affect most organs, as in the case of the heart. Without any autonomic input, the intrinsic heart rate at 100 is much higher than 70, which occurs under normal conditions of autonomic regulation.
Sympathetic and parasympathetic nerves are fundamentally different. Sympathetic systems have short pre-ganglionic axons, and long post ganglionic axons. A nicotinic receptor on the post-ganglionic cell body releases Acetylcholine, which causes a release of Neuroepinephrine. This system works on cardiac and smooth muscle. In contrast, parasympathetic nerves have long pre ganglionic axons, and short post-ganglionic axons. The release of Acetylcholine to the nicotinic receptor causes the release of Acetylcholine to a muscarinic receptor. This system only affects the pacemaker cells of cardiac muscle, which in turn regulates heart rate.
Frank-Starling’s relationship states that changes in initial myocardial fiber length cause different forces during systole and diastole. An increase in fiber length increases force during both periods of contraction. After a certain length, the force decreases. This principle helps to explain differences in myocardial fiber length.

Gap junctions are the main method of communication between cells. Structurally, they are areas of proximal plasma membranes between exciteable cells that form a hexagonal connexon. Currents excitable cells synapse and propagate the action potential, transmitting Ca++ or depolarizing energy. Both myocardial and intestinal smooth muscle cells contain gap junctions.
Cardiac muscle is structurally similar to skeletal muscle. It is striated, however, cardiac muscle is also branched which leads to more connections. The cardiac tissue is adapted for continual activity, containing Type 1 fibers. Cardiac muscle contracts in a similar fashion to skeletal muscle.