Transcript Unit One: Introduction to Physiology: The Cell and

Chapter 55: Cortical and Brain Stem Control
Of Motor Function
Guyton and Hall, Textbook of Medical Physiology, 12 edition
Motor Cortex and Corticospinal Tract
Fig. 55.1 Motor and somatosensory functional areas of the cerebral cortex
Motor Cortex (cont.)
• Primary Motor Cortex
Fig. 55.2 Degree of representation
of the different muscles of
the body in the motor
cortex
Motor Cortex (cont.)
• Primary Motor Cortex
Fig. 55.3 Representation of the different muscles of the body in the motor cortex and location
of other cortical areas responsible for specific types of motor movements
Motor Cortex (cont.)
• Transmission of Signals from the Motor Cortex to
the Muscles
a. Corticospinal (Pyramidal) Tract
Fig. 55.4
Motor Cortex (cont.)
• Red Nucleus As An Alternate Pathway
Fig. 55.5
Motor Cortex (cont.)
• Function of the Corticorubrospinal System
a. Accessory route for transmission of discrete signals
from the motor cortex to the spinal cord
•
Extrapyrimidal System- all portions of the brain and
brain stem that contribute to motor control but are
not part of the direct scorticospinal-pyramidal system
a. Include the basal ganglia, reticular formation, and
the vestibular nuclei
Motor Cortex (cont.)
• Excitation of the Spinal Cord Motor Control Areas by
the Primary Motor Cortex and the Red Nucleus
a. Vertical columnar arrangement of the neurons in the
motor cortex
b. Each column functions as a unit, usually stimulating
a group of synergistic muscles (sometimes a single muscle)
c. Each column operates as an integrative operating system
d. Each column can also function as an amplifying system
e. Dynamic neurons-excited at a high rate for a short period
of time at the beginning of a contraction
f. Static neurons-fire at a slower rate but maintain the force
of contraction
Motor Cortex (cont.)
• Somatosensory Feedback
a. Vertical columnar arrangement of the neurons in the
motor cortex
b. Each column functions as a unit, usually stimulating
a group of synergistic muscles (sometimes a single muscle)
c. Each column operates as an integrative operating system
d. Each column can also function as an amplifying system
e. Dynamic neurons-excited at a high rate for a short period
of time at the beginning of a contraction
f. Static neurons-fire at a slower rate but maintain the force
of contraction
Motor Cortex (cont.)
• Stimulation of Spinal Motor Neurons
Fig. 55.6 Convergence of different motor control pathways on the
anterior motor neurons
Role of the Brain Stem in Controlling Motor Function
• Brain Stem Provides Special Control Functions
a.
b.
c.
d.
Control of respiration
Control of the cardiovascular system
Partial control of GI function
Control of many stereotyped movements of the
body
e. Control of equilibrium
f. Control of eye movements
Role of the Brain Stem in Controlling Motor Function
• Support of the Body Against Gravity
a. Reticular Nuclei
1) Pontine reticular nuclei- transmit excitatory
signals; receive strong signals from vestibular
nuclei and from nuclei of the cerebellum
2) Medullary reticular nuclei- transmit
inhibitory signals to the same neurons as the
pontine nuclei
Role of the Brain Stem in Controlling Motor Function
• Support of the Body Against Gravity
b. Vestibular Nuclei- transmit strong excitatory
signals to control antigravity muscles
Role of the Brain Stem in Controlling Motor Function
Fig. 55.7 Locations of the reticular and vestibular
nuclei in the brain stem
Fig. 55.8 Vestibulospinal and reticulospinal descending
tracts to excite or inhibit anterior motor neurons
Vestibular Sensations and Maintenance of Equilibrium
Fig. 55.9 Membranous labyrinth
and organization of the
crista ampullaris and the
macula
Vestibular Sensations and Maintenance of Equilibrium
• Vestibular Apparatus- sensory organ for detecting
sensations of equilibrium
a. Encased in the bony labyrinth
b. Within are the membranous labyrinth which is the
functional part of the vestibular apparatus
•
Membranous Labyrinth- composed of
a.
b.
c.
d.
Cochlea
Three semicircular canals
Utricle
Saccule
Vestibular Sensations and Maintenance of Equilibrium
• Maculae
a. Sensory organs of the utricle and saccule for detecting
orientation of the head with respect to gravity
b. Covered with a gelatinous layer that contains calcium
carbonate crystals (statoconia)
c. Hair cells project into the gel layer; synapse with
sensory endings of the vestibular nerve
Vestibular Sensations and Maintenance of Equilibrium
• Maculae
d. Calcified statoconia have a specific gravity 2-3X the
specific gravity of the surrounding fluid and tissues;
e. Weight of the statoconia bends the hair cells (cilia)
in the direction of gravitational pull
Vestibular Sensations and Maintenance of Equilibrium
• Directional Sensitivity of the Hair Cells—Kinocilium
Fig. 55.10 Hair cell of the equilibrium
apparatus and its synapses
with the vestibular nerve
Vestibular Sensations and Maintenance of Equilibrium
• Directional Sensitivity of the Hair Cells—Kinocilium
a. Each hair cell has 50-70 small cilia (stereocilia) and
one large kinocilium
b. The kinocilium is located to one side and the cilia get
progressively shorter toward the other side of the cell
c. When the stereocilia and the kinocilium bend in the
direction of the kinocilium, this opens fluid channels
capable of conducting large numbers of positive ions
Vestibular Sensations and Maintenance of Equilibrium
• Directional Sensitivity of the Hair Cells—Kinocilium
d. Positive ions pour into the cell causing receptor membrane
depolarization (hyperpolarization occurs the bending is
in the opposite direction)
e. At rest: impulses are conducted continuously at a rate of
100 per second
f. When cilia are bending, impulses increase; as the
orientation of the head in space changes and the weight
of the statoconia bends the cilia, appropriate signals are
transmitted to the brain to control equilibrium
Vestibular Sensations and Maintenance of Equilibrium
• Semicircular Ducts- arranged at right angles so that
they represent all three planes in space
a. Anterior
b. Posterior
c. Lateral (horizontal)
Vestibular Sensations and Maintenance of Equilibrium
Fig. 55.11 Movement of the cupula and its embedded hairs at the onset of rotation
Vestibular Sensations and Maintenance of Equilibrium
• Semicircular Ducts (cont.)
a. Each has an enlargement at one end called the ampulla
b. Ducts and ampulla are filled with endolymph
c. Each ampulla has a crest called the crista ampullaris
d. On top of the crista is the cupula
Vestibular Sensations and Maintenance of Equilibrium
• Semicircular Ducts- Rotation
a. When a person’s head starts to rotate in any direction, the
inertia of the fluid causes the fluid to remain stationary
while the duct rotates with the head
b. This causes the fluid to flow from the duct through the
the ampulla, bending the cupula to one side
c. Rotation of the head in the opposite direction causes the
cupula to bend to the opposite side
Vestibular Sensations and Maintenance of Equilibrium
• Semicircular Ducts- Rotation
d. Hundreds of cilia project into the cupula from hair cells
on the crest
e. Kinocilia are oriented in the same direction, bending
the cupula in that direction causing depolarization of
the hair cells
f. Stimulates the vestibular nerve
Vestibular Sensations and Maintenance of Equilibrium
• Function of the Utricle and Saccule in Static Equilibrium
a. Function to maintain equilibrium when the head is in
a near vertical position
b. Do not operate for the detection of linear velocity
•
Dectection of Head Rotation by the Semicircular Ducts
Vestibular Sensations and Maintenance of Equilibrium
Fig. 55.12 Response of a hair cell when a semicircular canal is stimulated first by
the onset of head rotation and then by stopping rotation
Other Factors Concerned With Equilibrium
• Neck Proprioceptors
• Visual Information
• Proprioception and Exteroceptive Information
from Other Parts of the Body