- occurs when the muscle spindle group la afferent fibers project to all of the a-motoneurons that innervate the homonymous muscle.

3. Recurrent inhibition (Renshaw cells)

- Renshaw cells are inhibitory cells in the ventral horn of the spinal cord.

- They receive input from collateral axons of motoneurons and, when stimulated, negatively feedback (inhibit) on the motoneuron.

E. Brain stem control of posture

1. Motor centers and pathways

-Pyramidal tracts (corticospinal and corticobulbar) pass through the medullary pyramids.

- All others are extrapyramidal tracts and originate primarily in the following structures of the brain stem:

a. Rubrospinal tract

- originates in the red nucleus and projects to interneurons in the lateral spinal cord.

- Stimulation of the red nucleus produces stimulation of flexors and inhibition of extensors.

b. Pontine reticulospinal tract

- originates in the nuclei in the pons and projects to the ventromedial spinal cord.

- Stimulation has a general stimulatory effect on both extensors and flexors, with the predominant effect on extensors.

c. Medullary reticulospinal tract

- originates in the medullary reticular formation and projects to spinal cord interneurons in the intermediate gray area.

- Stimulation has a general inhibitory effect on both extensors and flexors, with the predominant effect on extensors.

d. Lateral vestibulospinal tract

- originates in Deiters' nucleus and projects to ipsilateral motoneurons and interneurons.

- Stimulation causes a powerful stimulation of extensors and inhibition of flexors.

e. Tectospinal tract

- originates in the superior colliculus and projects to the cervical spinal cord.

- is involved in the control of neck muscles.

2. Effects of transections of the spinal cord a. Paraplegia

- is the loss of voluntary movements below the level of the lesion.

- results from interruption of the descending pathways from the motor centers in the brain stem and higher centers.

b. Loss of conscious sensation below the level of the lesion c. Initial loss of reflexes—spinal shock

- Immediately after transection, there is loss of the excitatory influence from a- and 7-motoneurons. Limbs become flaccid, and reflexes are absent. With time, partial recovery and return of reflexes (or even hyperreflexia) will occur.

(1) If the lesion is at C7, there will be loss of sympathetic tone to the heart. As a result, heart rate and arterial pressure will decrease.

(2) If the lesion is at C3, breathing will stop because the respiratory muscles have been disconnected from control centers in the brain stem.

(3) If the lesion is at CI (e.g., as a result of hanging), death occurs. 3. Effects of transections above the spinal cord

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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