Reflex Integration of Motor Pathways Postural Reflexes

Because postural reflexes are difficult to study without special techniques y in normally functioning animals and humans, and because the pathways involved are intermingled with many other neural systems in the brain stem, postural reflexes abnormalities are detected primarily in

Figure 15-17 (Figure Not Available) The reticulospinal tracts. Note the multiple inputs including cerebral cortex and vestibular riu(From Snell RS: Clinical Neuroanatomy for Medical Students. Boston, Little, Brown & Co., Inc., 1987.)

Figure 15-18 The vestibulospinal tract. The inputs from the lateral vestibulospinal tract to the alpha and gamma motor neurons excite antigravity axial muscles. The medial vestibulospinal tract terminates in the cervical and upper thoracic segments to mediate reflex head movements in response to vestibular stimifFrom Kandel ER, Schwartz JH, Jessell TM: Principles of Neural Science. Norwalk, CT, Appleton & Lange, 1991.)

animals with large lesions of the brain stem. Experimentally, this was demonstrated most vividly when Sherrington transected the brain stem below the red nucleus in a cat. By interrupting the descending tracts from the cerebral cortex, the postural reflexes mediated by the brain stem systems were greatly exaggerated. This produced increased tone in the antigravity muscles, allowing the cat to be stood on all four legs. This is called decerebrate rigidity. Decerebrate rigidity with tonic extension of all four limbs is also seen in humans with lesions of the midbrain and upper pons. y In humans with brain stem lesions, the rigid extension of the limbs is not persistently maintained but may be paroxysmal, often in response to external stimuli, and is then referred to as decerebrate posturing (Fig. 15-20 (Figure Not Available) ).

Decorticate rigidity is a posture in which the arms are flexed and adducted and the legs are extended due to lesions above the midbrain in the cerebral hemispheres. The posture is usually bilateral and is essentially the same as a bilateral spastic hemiplegia.

TONIC NECK REFLEXES

Reflex changes in muscle tone and posture, especially in the extremities when the relationship of the head to the body is changed, are known as neck reflexes. Neck reflexes are not usually seen in normally functioning humans and animals. Stimulation of the labyrinths (labyrinthine reflexes) produces effects on muscle tone and posture that are exactly the opposite of the neck reflexes, thereby holding these in balance in the normal individual. However, in decerebrate animals, human infants, and children with cerebral palsy, the neck reflexes may be unmasked and can be demonstrated by turning the head from side to side. For example, when the head is rotated to the right, an increase in extensor tone occurs in the ipsilateral extremities and an increase in flexor tone is seen in the contralateral extremities, producing the so-called fencer's posture.

LOCOMOTOR REFLEXES

Pattern generators for locomotion reside in these brain stem systems. Pattern generators for locomotion also exist at a spinal level, although in humans the brain stem generators are more important. Locomotion is examined in more detail in Chapter 18 .

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