July 5, 2012

Health Minute with CASI – The Amazing Brain – Part 3

This month we continue a discussion of the Central Nervous System which we began in our May issue. The Central Nervous System includes the brain and spinal cord.

Corticospinal or Pyramidal

The area has been named “pyramidal” since it crosses through the medulla. Nerve fibers originate in the motor cortex and travel to the brain stem, crossing to the opposite side (pyramidal decussation) then down the lateral column of the spinal cord. Ten percent of the corticospinal fibers do not cross, and these desend in the anterior column of the spinal cord. The corticospinal tract is a newer, “higher” motor system that humans have that permit very skilled and purposeful movements. The motor cortex is arranged in a specific pattern called somatotopic organization. We also have another body map, this one of a person, or homunculus, hanging “upside down.” Body Parts are not equally represented. It is more like an electrical map. That is body parts whose movements are more important to humans occupy more space on the brain. (e.g., the hand)

Extrapyramidial Tracts

Include all the motor nerve fibers which originate in the motor cortex, basal ganglia, brain stem, and the spinal cord are outside the pyramidal tract. This is an older, lower more primitive motor system. The motor fibers maintain muscle tone and control body movements.

Cerebellar System

This motor system coordinates movements, maintains equilibrium and helps with posture. The cerebellum receives the information about the position of muscles and joints, the body’s equilibrium; from there the motor messages are sent to the cortex to the muscles. Information is integrated, and the cerebellum uses feedback to exert control back on the cortex, or down to lower neurons of the spinal cord. This all occurs on a subconscious level.

Peripheral Nervous System

A bundle of nerve fibers “outside” the central nervous system. The peripheral nerves carry input to the CNS via their sensory afferent fibers and deliver output from the CNS via the efferent fibers.

Reflex Arc:

Our reflexes are the basic defense mechanisms of the nervous system. They are involuntary and operating below the level of conscious control and permitting a quick reaction to painful or damaging situations. Reflexes also help the body maintain balance and muscle tone. There are four types: deep tendon reflexes, patellar or knee jerk, superficial, e.g., corneal and abdominal reflexes, visceral e.g., papillary response to light. Pathological e.g., Babinski’s or extensor plantar reflex.

In the most simple reflex, this stretches the muscle spindles in the muscle, which the sensory afferent nerve. Sensory afferent fibers then carry the message from the receptor, trough the dorsal root into the spinal cord. The synapse with the motor neuron in the anterior horn, Motor efferent fibers the leave the ventral root and travel to the muscle. Deep tendon reflex has five components. (1) Intact sensory nerve (afferent): (2) a function synapse in the cord; (3) an intact motor nerve fiber (efferent); (4) neuromuscular junction; and (5) a competent muscle.

Join me next month for part 4 of the amazing brain.