Term of the Day

Rami Communicantes: Short nerve fibers that connect an intercostal nerve to the sympathetic trunk. The white ramus communicans travels from the nerve to the sympathetic ganglion, and the gray ramus communicans travels from the ganglion to the intercostal nerve.

Term of the Day

Internal Anal Sphincter: An involuntary sphincter surrounding the superior 2/3 of the anal canal. It is a thickening of the circular smooth muscle layer of the large intestine and is innervated by the pelvic splanchnic nerves. The sphincter responds to the pressure of feces in the rectal ampulla by relaxing.

Concept of the Week : The Motor Unit

The cell bodies of motor neurons that innervate skeletal muscles are located in the ventral horn of the grey matter of the spinal cord. Axons of these nerves leave via the ventral root and travel to the appropriate skeletal muscle.

A single motor neuron can form synapses with many muscle fibers, due to a multiplicity of axon collaterals. An individual motor neuron and all the muscle fibers it innervates is called a motor unit, the functional unit of the neuromuscular system.

Motor units can vary in size. The smallest consist of a single motor neuron that innervates just a few muscle cells. Such an arrangement is typical in situations where very precise control of fine movements is necessary (e.g. finger or eye movements). A large motor unit may innervate hundreds of muscle cells. Such units are responsible for gross movements such as leg or trunk movements. The smaller the motor unit, the greater precision of fine motor skills.

Term of the Day

Hilton's Law: Joints (articulations) have an extensive nerve supply. These articular nerves are actually branches of the nerves that supply the overlying skin and the muscles that move the joint. Hilton's Law essentially says that the nerves that supply a joint also supply the muscles that move the joint and the skin covering their attachments.

Term of the Day

Hypersomnia Sleep Apnea (HSA) Syndrome: Periodic breathing and episodes of apnea (absence of air flow for at least 10 seconds during sleep) associated with stage I and II and REM sleep. Decreased arterial oxygen saturation leads to arousal, thus interrupting the sleep cycle and leading to sleep and REM deprivation. HSA syndrome is mainly the result of upper airway obstruction occurring during sleep. Patients may experience hundreds of episodes of apnea per night, which leads to extreme daytime sleepiness. Snorting and thrashing about during sleep are common, and some patients may experience sleepwalking or enuresis.

Term of the Day

Splanchnic Nerve: A preganglionic autonomic nerve of the abdominopelvic viscera. Preganglionic, sympathetic fibers that pass through the paravertebral (sympathetic chain) ganglia without synapsing form splanchnic nerves. Instead, they synapse in a prevertebral (visceral) ganglion such as the celiac ganglion, the superior mesenteric ganglion, or the inferior mesenteric ganglion (e.g. thoracic -- greater, lesser, least -- splanchnic nn, lumbar splanchnic nn, and sacral splanchnic nn). Pelvic splanchnic nerves are parasympathetic and formed by branches of the ventral rami of S2, S3, and S4. Their ganglia are at or near the target organ.

P.S. Have a very Happy New Year!