Nervous System and Vestibular System

Anatomy of the Vestibular System and Sense of Balance

  • The nervous system affects other organs.
  • The vestibular system is linked with the sense of balance.

Multisensory Control of Balance

  • Sensory input:
    • Visual (eyes)
    • Proprioception (touch)
    • Vestibular (inner ear): equilibrium, spatial awareness, rotation, and linear movement.
  • These inputs feed into the cerebellum and cerebral cortex, which communicate with the brainstem.
  • The cerebellum and cerebral cortex feed into the vestibular ocular reflex.
  • The brainstem provides information for the vestibular ocular reflex.
  • Motor impulses control eye movement and postural adjustments.
  • These motor impulses contribute to the sense of balance.

Anatomy of the Ear

  • Outer ear:
    • Pinna
    • Earlobe
    • Auditory canal
    • Tympanic membrane
  • Middle ear:
    • Stapes, uncus, and malleus (small bones for auditory function)
    • Round window
  • Inner ear:
    • Cochlea (sense of hearing)
    • Semicircular canals, utricle, and saccule (vestibular system organs)

Vestibular Hair Cells

  • Responsible for transmitting electrical influences perceived as balance or sound.
  • Two types of hair cells in vestibular epithelia:
    • Type one: amphora-like shaped, engulfed by calyx with afferent ending
    • Type two: columnar, connected by button-type endings
  • Vestibular afferent neurons:
    • Calyx only afferents
    • Button only afferents

Semicircular Canals and Ortholithic Organs

  • Vestibule composition:
    • Outer bony labyrinth: perilymph (high in sodium, low in potassium ions)
    • Membranous labyrinth: semicircular ducts containing endolymph (high in potassium, low in sodium) - important for signal transduction

Semicircular Canals

  • Three canals: anterior, posterior, and lateral (90 degrees from each other)
  • Pick up information from angular acceleration in any axis.
  • Ampulla at each canal: contains gelatinous sensory epithelium (crista ampullaris) with hair cells and supporting cells.
  • Important for rotational (angular) equilibrium: looking left, moving head left to right.
  • Maintain dynamic equilibrium.

Hair Cells in the Ampulla

  • Kinocilium (one higher one) and multiple stereocilia (smaller ones) oriented in the same direction in the ampulla.
  • Each ampulla has hair cells pointing in different directions to cover movements.

Vestibular Hair Cell Function

  • If stereocilia beat towards the kinocilium:
    • Tip links shorten, channels open.
    • Potassium influx (positive charge).
    • Depolarization.
    • Calcium influx through channels.
    • Neurotransmitter release.
  • If stereocilia beat away from the kinocilium:
    • Tip links close, channels close.
    • No potassium influx.
    • No depolarization.
    • No neurotransmitter release.
    • No signal transmission.

Rotational Equilibrium

  • Important for cells in the semicircular canals.
  • Angular equilibrium depends on the movement of gelatinous moisture.
  • Movement pushes on hair cells, sending or not sending signals.
  • Important for movement to look through your head left and right.

Vestibular System Ortholithic Organs

  • Saccule and utricle.
    • Outer bony labyrinth: perilymph (high in sodium).
    • Inner labyrinth: endolymph (high in potassium).
  • Special sensory epithelium: macula (on the floor of the saccule, back of the utricle).
  • Calcium carbonate crystals in gelatinous matrix stimulate hair cells.
  • Utricle: horizontal movement.
  • Saccule: vertical movement.

Hair Cell Orientation in Saccule and Utricle

  • Hair cells in the saccule point away from the midline (striola).
  • Hair cells in the utricle point towards the striola.
  • The orientation is important for detecting movements and maintaining equilibrium.

Function of Utricle and Saccule

  • Kinocilium and stereocilia with ortholithic organs and ortholiths in the membranous layer.
  • The movement of the gel, caused by the weight of the otoliths, will either stimulate or not stimulate the hair cells.
  • Important for gravitational equilibrium.
  • Macula contains calcium carbonate granules.
  • When the body is still, the otoliths in the utricle and saccule rest on the otolithic membrane above the hair cells.
  • When the head bends forward or the body moves in a horizontal or vertical plane (like a plane taking off), the otolithic membrane will sag, bending the stereocilia or hair cells beneath.
  • If stereocilia move towards the kinocilium, nerve impulses increase.
  • If stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve.