Chapter 55: Cortical and Brain Stem Control Of Motor Function Guyton and Hall, Textbook of Medical Physiology, 12 edition

Unit Eleven: The Nervous System: C. Motor and

Integrative Neurophysiology

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


• 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


• Transmission of Signals from the Motor Cortex tothe Muscles

a. Corticospinal (Pyramidal) Tract

Fig. 55.4


• Red Nucleus As An Alternate Pathway

Fig. 55.5


• Function of the Corticorubrospinal System

a. Accessory route for transmission of discrete signalsfrom the motor cortex to the spinal cord

• Extrapyrimidal System- all portions of the brain andbrain stem that contribute to motor control but arenot part of the direct scorticospinal-pyramidal system

a. Include the basal ganglia, reticular formation, andthe vestibular nuclei


• Excitation of the Spinal Cord Motor Control Areas bythe Primary Motor Cortex and the Red Nucleus

a. Vertical columnar arrangement of the neurons in themotor cortex

b. Each column functions as a unit, usually stimulatinga group of synergistic muscles (sometimes a single muscle)

c. Each column operates as an integrative operating systemd. Each column can also function as an amplifying systeme. Dynamic neurons-excited at a high rate for a short period

of time at the beginning of a contractionf. Static neurons-fire at a slower rate but maintain the force

of contraction


• Somatosensory Feedback

a. Vertical columnar arrangement of the neurons in themotor cortex

b. Each column functions as a unit, usually stimulatinga group of synergistic muscles (sometimes a single muscle)

c. Each column operates as an integrative operating systemd. Each column can also function as an amplifying systeme. Dynamic neurons-excited at a high rate for a short period

of time at the beginning of a contractionf. Static neurons-fire at a slower rate but maintain the force

of contraction


• 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. Control of respirationb. Control of the cardiovascular systemc. Partial control of GI functiond. Control of many stereotyped movements

of the bodye. Control of equilibriumf. Control of eye movements


• 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


• Support of the Body Against Gravity

b. Vestibular Nuclei- transmit strong excitatory signals to control antigravity muscles


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 Apparatus- sensory organ for detectingsensations of equilibrium

a. Encased in the bony labyrinthb. Within are the membranous labyrinth which is the

functional part of the vestibular apparatus

• Membranous Labyrinth- composed of

a. Cochleab. Three semicircular canalsc. Utricled. Saccule


• Maculae

a. Sensory organs of the utricle and saccule for detectingorientation of the head with respect to gravity

b. Covered with a gelatinous layer that contains calciumcarbonate crystals (statoconia)

c. Hair cells project into the gel layer; synapse with sensory endings of the vestibular nerve


• Maculae

d. Calcified statoconia have a specific gravity 2-3X thespecific gravity of the surrounding fluid and tissues;

e. Weight of the statoconia bends the hair cells (cilia) in the direction of gravitational pull


• Directional Sensitivity of the Hair Cells—Kinocilium

Fig. 55.10 Hair cell of the equilibrium apparatus and its synapses with the vestibular nerve



a. Each hair cell has 50-70 small cilia (stereocilia) andone large kinocilium

b. The kinocilium is located to one side and the cilia getprogressively shorter toward the other side of the cell

c. When the stereocilia and the kinocilium bend in thedirection of the kinocilium, this opens fluid channelscapable of conducting large numbers of positive ions



d. Positive ions pour into the cell causing receptor membranedepolarization (hyperpolarization occurs the bending isin the opposite direction)

e. At rest: impulses are conducted continuously at a rate of100 per second

f. When cilia are bending, impulses increase; as the orientation of the head in space changes and the weightof the statoconia bends the cilia, appropriate signals aretransmitted to the brain to control equilibrium


• Semicircular Ducts- arranged at right angles so that they represent all three planes in space

a. Anterior

b. Posterior

c. Lateral (horizontal)


Fig. 55.11 Movement of the cupula and its embedded hairs at the onset of rotation


• 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


• Semicircular Ducts- Rotation

a. When a person’s head starts to rotate in any direction, theinertia 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 thethe ampulla, bending the cupula to one side

c. Rotation of the head in the opposite direction causes thecupula to bend to the opposite side


• Semicircular Ducts- Rotation

d. Hundreds of cilia project into the cupula from hair cellson the crest

e. Kinocilia are oriented in the same direction, bending the cupula in that direction causing depolarization ofthe hair cells

f. Stimulates the vestibular nerve


• Function of the Utricle and Saccule in Static Equilibrium

a. Function to maintain equilibrium when the head is ina near vertical position

b. Do not operate for the detection of linear velocity

• Dectection of Head Rotation by the Semicircular Ducts


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 Informationfrom Other Parts of the Body

Documents

Chapter 55: Cortical and Brain Stem Control Of Motor Function Guyton and Hall, Textbook of Medical Physiology, 12 edition