Brain stem
1. Anatomy
- medullar oblongata
- pons
- midbrain
2. Brain stem centers evaluate the afferent information from a great many receptors in the body
- vestibular organ
- stretch receptors of the neck muscles
- joint receptors in the region of the cervical vertebra
3. Functional elements of the brain stem
- Cranial nerves and their nuclei
- Ascending lemniscal pathway
- Recticular nuclei : its nuclei and pathways
- Descending pathways
- Pathways to/from cerebellum
4. Motor Function
- control in the maintenance of posture
- maintenance of tone in gravity-opposing muscles (usually extensor muscles) that is required for standing upright
1. Brain stem structures are....
- responsible for automatic postural responses
- alter spinal cord function(controlled the interneuron of spinal reflex pathway)
2. Brain stem projections....
- Turn ON
- Turn Off various reflexes... depending on behavioral conditions
(Cerebral cortex has direct projections to the brain stem, perhaps conscious activity can alter reflex behavior as well)
3. Specialized structures of brain stem
- The red nucleus of the midbrain
- Vestibular nuclei
- The reticular formation
- Tecto-spinal tracts
- Mesencephalic/pontine locomotor regions
- Cranial nerve nuclei
Motor function of the brain stem reticular formation
1. Three function of the reticular system are of particular relevance of movement:
- Wakefulness
- Modification of sensory input
- Motor control
2. Two larger zones influence the reflex activity of the spinal cord
- 1. Facilitatory reticular formation
- lateral reticular extensor facilitator area
- 2. Inhibitory reticular formation
- medial reticular extensor inhibitary area
Influence a & g motor neurons to extensor muscles
1. Lateral reticular extensor facilitatory area
- from middle of the medulla to mesencephalic tegmentum
- receives strong tonic excitatory inputs from
- collaterals of ascending sensory pathways
- vestibular organ
- normal controlled or suppressed by descending influences from the cortex
- influence on spinal reflexs via medial reticulospinal tract
→ Extensor (axial & girdle muscles) facilitation
2. Medial reticular extensor inhibitory area
- caudal part of the medulla
- requires higher motor centers (cortex) to drive it into activity
- without this driving force unable to sustain its inhibitory influence on spinal reflexes via lateral reticulospinal tract
→ Extensor inhibition
In normal state....
Descending facilitation & inhibition on to segmental a & g motor neurons → Constant balance
Major brain stem - spinal (bulbospinal) tracts
| Medial brain stem pathways | Lateral brain stem pathways |
|---|---|
|
|
1. Medial pathways
- provide the basic postural control system
- three major tract
- vestibulospinal, recticulospinal & tectospinal tr - descend in the ipsilateral ventral columns of the spinal cord
- terminate prodominantly on interneurons and long propriospinal neurons in ventromedial part of the intermediate zone
- function :
- influence motor neurons that innervate axial and proximal muscles
- wide area of termination of individual axon is important in distributing control to a variety of functionally related motor nuclei
2. Lateral pathways
- more concerned with goal-directed limb movement such as reaching & manipulating
- main tract :
- rubrospinal tract originates in the magnocellular portion of the red nucleus
- terminate on interneurons in the dorsolateral part of the spinal gray matter
- function : influence motor neurons that control distal muscles in limbs
Body orienting reflex
- reflex control of posture & equilibrium in space
- postural reflexes - static
- righting reflexes - dynamic
- classification ; according to their peripheral afferent source
- Labyrynthine receptors
- Somatic proprioceptors of proximal portions of the limb & body axial parts (principally neck muscles)
- Visual receptors
1. Postural reflex
- motor abilities of the decerebrate animal
- appropriate correction of tonus distribution of muscles of the body occur when changed by manipulating the position of the head
- Tonic neck reflexes :
- afferent signals from the receptors in the neck muscles and joints
- Tonic labyrinthine reflexes
- afferent signals from labyrinths (about the position of the head in space)
2. Righting Reflex
an involuntary movement response to a stimulus; it serves to maintain the normal alignment of the head and body in space
- motor abilities of the midbrain animal
- serve to maintain normal body posture and balance without voluntary control
- midbrain animal lacks decerebrate rigidity & can right itself in normal posture from any abnormal starting position
- Chain of righting reflexes :
- labyrinthine righting reflex
- neck-muscle righting reflexes final posture