Neuroscience/Objectives/Lecture 43

From PhysioWiki

Control of posture

Introduce the concept of reflex and involuntary movements.

Posture and equilibrium are maintained by reflexes and involuntary movements. Reflex responses include the knee-jerk and neck reflexes. These do not require planning and are carried out by reflex circuits of the spinal cord. Other involuntary movements include locomotion, whose initiation and termination requires cortical activity, but whose sustained activity is automatic. Such rhythmic motor patterns are maintained by pattern generators in the spinal cord.

Brainstem reflexes include the righting reflexes (used to align the body with respect to gravity) and postural and locomotive reflexes. These reflexes are triggered by 1) muscle proprioceptors, 2) vestibular receptors, and 3) visual input.

Discuss the role of the brainstem in the control of movement (control of posture).

The brainstem plays a key role in the control of involuntary movements initiated by the cerebral cortex and organized by the cerebellum and basal ganglia.

Describe examples of brainstem reflexes.

• Postural reflexes
  • Righting reflexes
    • Vestibular reflexes*
    • Neck reflexes*
    • Body righting reflexes
    • Grasp reflexes
  • Postural and locomotive reflexes
    • Positive supporting reflexes
    • Vestibular placing reflexes

Vestibular and neck reflexes are evoked by changes in the position of the head and neck, respectively. Triggering these reflexes allows the extension of the limbs in preparation for landing during a fall. The vestibular reflexes also stabilize the eyes while the head is in motion.

Positive supporting reflexes originate from pressure receptors in the digits that cause limb extension. Mediated by the vestibulospinal and reticulospinal tracts, the vestibular placing reflexes allow fanning of the toes and limb extension to meet the ground during movement.

List the major nuclei in the brainstem forming the descending motor pathways.

• Dorsolateral pathways
  • Red nucleus (magnocellular portion: rubrospinal tract)
• Ventromedial pathways
  • Superior colliculus (deep layer: tectospinal tract)
  • Vestibular nuclei (lateral and medial nuclei: vestibulospinal tract)
  • Reticular formation (reticulospinal tract)
• Aminergic pathways
  • Locus coeruleus (coeruleospinal tract)
  • Raphe nuclei (raphespinal tract)

Illustrate the origin, course, and termination of the descending pathways: ventromedial pathway (reticulospinal, tectospinal, and vestibulospinal), dorsolateral pathway (rubrospinal tract), and aminergic pathway.

Discuss the function of the ventromedial and dorsolateral pathways in the control of posture.

See above.

Describe the role of monoaminergic pathways in the brainstem control of locomotion.

See above.

Illustrate the anatomical basis for decorticate and decerebrate rigidity.

Decerebrate rigidity

Important innervation to the extensors necessary for upright posture is provided by the medial (pontine) reticulospinal and lateral vestibulospinal tracts. The medial reticulospinal tract elicits significant extension of limb muscles when a stimulus is applied to the pontine reticular formation. But under normal circumstances, this pontine activity is tonically inhibited by descending cortical projections. However, if a lesion occurs between the colliculi such that these cortical afferents are lost, the descending inhibitory input from the cortex to the pontine reticular formation is likewise lost. The medial reticulospinal tract is then free to exert its excitatory effect on the axial and limb extensors, producing the characteristic body position seen in decerebrate rigidity.

Decorticate rigidity

Decorticate rigidity results from a brainstem lesion superior to the red nucleus with consequent loss of cortical input to the brainstem and spinal cord. The corticospinal tract is lost, as is inhibitory input to the red nucleus, reticular formation, and vestibular nuclei. Bulbospinal tracts—including the rubrospinal, reticulospinal, and vestibulospinal tracts—are preserved. As in decerebrate rigidity, the reticulospinal and vestibulospinal tracts act to extend the limbs. However, because the red nucleus is preserved here, the rubrospinal tract is free to excite the flexors of the upper limbs; in fact, the rubrospinal tract's flexor influence overcomes the vestibulospinal and reticulospinal tracts' extensor influence, resulting in flexion of the upper limbs (only the arms are flexed because the majority of rubrospinal input is to the upper limbs). The result is the upper limb flexion and lower limb extension characteristic of decorticate rigidity.