Chapter 14 Brainstem and Reticular Formation

The Brainstem and Reticular Formation

Objective

  • Describe the structures and functions of the brainstem and reticular formation.

Overview of the Brainstem

  • The brainstem is located between the spinal cord and the diencephalon.

  • Consists of three structures:

    1. Medulla oblongata

    2. Pons

    3. Midbrain

  • The reticular formation, a netlike region of interspersed gray and white matter, extends through the brainstem.

Medulla Oblongata

  • Continuous with the superior part of the spinal cord.

  • Forms the inferior part of the brainstem.

    • Begins at the foramen magnum and extends to the inferior border of the pons (approx. 3 cm or 1.2 in).

Structure

  • Contains white matter consisting of sensory (ascending) and motor (descending) tracts that extend between the spinal cord and other parts of the brain.

  • The pyramids (bulges on the anterior aspect of the medulla) are formed by large corticospinal tracts that control voluntary movements of limbs and trunk.

Decussation of Pyramids

  • Defined as the crossing of axons where 90% of axons in the left pyramid cross to the right side and vice versa.

  • This explains the phenomenon where each side of the brain controls voluntary movements on the opposite side of the body.

Nuclei of the Medulla

  • Contains several important nuclei that control vital functions:

    • Cardiovascular center

    • Regulates rate and force of heartbeat and the diameter of blood vessels.

    • Medullary rhythmicity center

    • Adjusts the basic rhythm of breathing.

    • Controls reflexes for:

    • Vomiting

      • Forcible expulsion of contents of the upper digestive canal through the mouth.

    • Swallowing (deglutition)

      • Promoted by the deglutition center.

    • Sneezing

      • Involves spasmodic contraction of breathing muscles that expel air through the nose and mouth.

    • Coughing

      • Involves deep inhalation followed by a forceful exhalation that sends air through the upper respiratory passages.

    • Hiccupping

      • Caused by spasmodic contractions of the diaphragm, producing a sharp sound.

Olive and Inferior Olivary Nucleus

  • Just lateral to each pyramid, an oval-shaped swelling called an olive contains the inferior olivary nucleus, which has inputs from the:

    • Cerebral cortex

    • Red nucleus of the midbrain

    • Spinal cord

  • Influences cerebellar neuron activity, helping to adjust muscle activity as one learns new motor skills.

Sensory Nuclei

  • Nuclei for sensations of touch, pressure, vibration, and conscious proprioception are located in the posterior part of the medulla.

    • Gracile nucleus (slender) and cuneate nucleus (wedge) receive sensory axons from the gracile fasciculus and the cuneate fasciculus, respectively.

    • Postsynaptic neurons relay sensory information to the thalamus via the medial lemniscus (ribbon) in a pathway known as the posterior column-medial lemniscus pathway.

Nuclei for Other Sensory Pathways

  • Gustatory nucleus for taste.

  • Cochlear nuclei for hearing.

  • Vestibular nuclei for balance and equilibrium.

Cranial Nerves Associated with the Medulla

  1. Vestibulocochlear (VIII)

    • Sensory input related to hearing.

  2. Glossopharyngeal (IX)

    • Related to taste, swallowing, and salivation.

  3. Vagus (X)

    • Motor impulses to pharynx, larynx, thoracic, and abdominal viscera.

  4. Accessory (XI)

    • Controls swallowing via vagus nerves.

  5. Hypoglossal (XII)

    • Controls tongue movements during speech and swallowing.

Clinical Connection: Injury to the Medulla

  • Injuries to the medulla from impacts such as falling can be fatal due to vital functions controlled here.

  • Nonfatal injuries may cause:

    • Cranial nerve dysfunction (same side as injury)

    • Paralysis/loss of sensation (opposite side)

    • Respiratory or heart rhythm irregularities.

  • Alcohol overdose can suppress the medullary rhythmicity center, potentially leading to death.

Pons

  • Located directly superior to the medulla and anterior to the cerebellum, about 2.5 cm (1 in) long.

  • Acts as a bridge connecting different parts of the brain via bundles of axons.

  • Contains two structural components:

    1. Ventral Region

    • Large synaptic relay station with the pontine nuclei, facilitating connections between the cortex of one cerebral hemisphere and the opposite hemisphere of the cerebellum.

    1. Dorsal Region

    • Contains ascending/descending tracts and cranial nerve nuclei.

Respiratory Control

  • Contains the pontine respiratory group, which collaborates with the medullary respiratory center to regulate breathing.

Cranial Nerves Associated with the Pons

  1. Trigeminal (V)

    • Sensory impulses from the head and face, governs chewing.

  2. Abducens (VI)

    • Motor impulses for eye movement.

  3. Facial (VII)

    • Taste sensation; motor impulses for salivation, tears, and facial expression.

  4. Vestibulocochlear (VIII)

    • Sensory and motor impulses related to balance and equilibrium.

Midbrain

  • Extends from the pons to the diencephalon, approximately 2.5 cm (1 in) long.

  • Contains the aqueduct of the midbrain connecting the third and fourth ventricles.

Structure

  • Composed of both nuclei and tracts.

  • Anterior part consists of the cerebral peduncles (little feet) housing axons from the motor areas of the cerebral cortex.

Tectum

  • The posterior part of the midbrain, contains four rounded elevations:

    • Superior colliculi: Reflex centers for visual activities.

    • Inferior colliculi: Part of the auditory pathway for hearing and startle reflex.

Nuclei in the Midbrain

  • Substantia nigra: Neurons releasing dopamine, influencing muscle activities, and associated with Parkinson's disease symptoms.

  • Red nuclei: Richly vascularized nuclei involved in muscular control.

Cranial Nerves Associated with the Midbrain

  1. Oculomotor (III)

    • Controls eyeball movements and smooth muscles regulating pupil constriction and lens shape.

  2. Trochlear (IV)

    • Provides motor impulses for eyeball movement.

Reticular Formation

  • Consists of clusters of neuronal cell bodies (gray matter) interspersed among myelinated axons (white matter) in a net-like arrangement.

  • Extends from superior part of the spinal cord through the brainstem and into the diencephalon.

Functions of the Reticular Formation

  • Ascending Portion (Reticular Activating System - RAS):

    • Comprises sensory axons projecting to the cerebral cortex, crucial for maintaining consciousness and arousal.

    • Activated by visual/auditory stimuli, mental activities, and sensory info about body position.

    • Filters excessive sensory information to prevent overload.

  • Status Changes:

    • Inactivation leads to sleep (partial consciousness).

    • Damage can lead to coma (unconsciousness with no arousal).

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