Reticular and Vestibular Nuclei
•Reticular nuclei are divided into two groups:
The pontine reticular nuclei- located in the pons end extending to the encephalon
The medullary reticular nuclei- near the middle of the medulla
(1) and (2) funcion antagonistically- pontine excite the antigravity muscles and the medullary inhibit them.
The Pontine Reticular Nuclei
•Transmit excitatory signals downward into the cord through the pontine reticular tract.
•Fibers terminate on the anterior motor neurons that excite the muscle of the spinal column and the extensor muscles of the limb (muscles that support the body against gravity).
The Medullary Reticular Nuclei
•Transmit inhibitory signals to the same antigravity anterior motor neurons by the way of the medullary reticulospinal tract.
•Receive input from:
The corticospinal tract
The rubrospinal tract
Other motor pathways
Vestibular Nuclei
•Function in association with the pontine reticular nuclei to excite the antigravity muscles.
•Lateral vestibular nuclei transmit signals by way of lateral and medial vestibulospinal tracts in the anterior column of the spinal cord.
The Vestibular Apparatus
•Is the organ that detects sensations of equilibrium.
•It is composed of bony tubes and chambers in the petrous portion of the temporal bone (the bony labyrinth) and of membranous tubes and chambers (membranous labyrinth).
•The membranous labyrinth is the functional part of the apparatus.
The Membranous Labyrinth
•Is composed of cochlea, three semicircular ducts, and two large chambers (utricle and saccule).
•The cochlea is the sensory area for hearing and has nothing to do with equilibrium.
•The semicircular ducts (the utricle and the sacule) are parts of the equilibrium mechanism.
The Maculae
•Are small sensory area located on the inside of each utricle and sacule.
•Are covered by a gelatinous layer in which small calcium carbonate crystals are imbedded.
•In the macula there are thousand of hair cells which project cilia up into the gelatinous layer.
•The bases and sides of the hair cells synapse with sensory endings of the vestibular nerve.
Directional Sensitivity of the Hair Cells
•Each hair cell has 50 to 70 cilia (stereocilia) plus one large cilium (the kinocilium).
•Minute filamentous attachments connect the tip of the stereocilium to the next stereocilium and finally to the kinocilium.
•When the stereocilia and the kinocilium bend (in the direction of kinocilium), the filamentous attachment tug one after another pulling stereocilia in the outward direction from the cell body. This opens channels in each cilium through which large quantities of positive ions pour into the cell from the surrounding fluids causing depolarization.
•Bending the cilia in the opposite direction (away from the kinocilium) closes the ion channels causing hyperpolarization.
•Appropriate signals are transmitted to the brain to control equilibrium.
•A different pattern of excitation occurs in the nerve fibers from the macula for each position of the head.
The Semicircular Ducts
- Anterior
- Posterior
- Lateral
•Arranged at right angles (the planes in space)
•Ampulla = enlargement in the semicircular ducts
•The ducts are filled with a viscous fluid (endolymph). Flow of this fluid from one of the ducts in the ampulla excites the sensory organ of the ampulla.
•Crista ampullaris = small crest in the ampulla
•Cupula = gelatinous mass on top of the crista
•Rotation of the head causes the fluid to flow from the duct into the ampulla, bending cupula to one side
•Into the cupula are projections cilia from hair cells. Bending the cupula causes depolarization ( hyperpolarization if head turned the other way). From the hair cells, signals are send through the vestibular nerve to the head.
•Vestibular- cerebellar- reticular motor systems excite by reflex the appropriate muscles to maintain equilibrium.