Makindo Medical Notes

Related Subjects: |Abdominal Anatomy |Brainstem Anatomy |Thalamic Anatomy

The brainstem is a vital structure that connects the brain with the spinal cord and is also the bridge to the cerebellum. It regulates many basic life functions, including breathing, heart rate, and consciousness. It is composed of three main parts: the midbrain, pons, and medulla oblongata.

Structure of the Brainstem

• The brainstem is composed of three main parts:
  • Midbrain (Mesencephalon)
  • Pons
  • Medulla Oblongata

Midbrain (Mesencephalon)

• Location: Uppermost part of the brainstem, located between the diencephalon (thalamus and hypothalamus) and the pons.
• Structure: Contains the tectum (dorsal) and tegmentum (ventral).
  • Tectum: Contains the superior colliculi (involved in visual reflexes) and inferior colliculi (involved in auditory processing).
  • Tegmentum: Contains the red nucleus (involved in motor coordination) and the substantia nigra (important for dopamine production and motor control).
• Functions:
  • Visual and Auditory Reflexes: Controls reflex movements in response to visual and auditory stimuli.
  • Motor Control: Regulates movement through connections with the basal ganglia and motor cortex.
  • Pain Modulation: The periaqueductal gray (PAG) region modulates pain sensation.

Pons

• Location: Middle part of the brainstem, lying between the midbrain and the medulla oblongata.
• Structure:
  • Contains a large collection of nerve fibers that connect the cerebrum to the cerebellum and the spinal cord.
  • Includes cranial nerve nuclei for cranial nerves V (Trigeminal), VI (Abducens), VII (Facial), and VIII (Vestibulocochlear).
  • Contains the pontine nuclei, which relay signals from the cortex to the cerebellum.
• Connections to the Cerebellum:
  • Middle Cerebellar Peduncle: The largest connection between the pons and cerebellum, carrying fibers from the pontine nuclei to the cerebellum, critical for coordinating voluntary movements.
• Functions:
  • Relay Center: Acts as a bridge for signals traveling between the cerebrum, cerebellum, and spinal cord.
  • Respiratory Control: Works with the medulla to regulate breathing rate and depth.
  • Facial Sensation and Motor Control: Involved in facial expressions, chewing, and eye movement through the associated cranial nerve nuclei.
  • Balance and Hearing: Contributes to balance and hearing through the vestibulocochlear nerve.


Same but in Anatomical position

Medulla Oblongata

• Location: Lowest part of the brainstem, connecting directly to the spinal cord.
• Structure:
  • Contains vital centers for controlling autonomic functions like breathing, heart rate, and blood pressure.
  • Includes cranial nerve nuclei for cranial nerves IX (Glossopharyngeal), X (Vagus), XI (Accessory), and XII (Hypoglossal).
  • Contains pyramids, where corticospinal (motor) tracts decussate (cross over) from one side to the other.
• Connections to the Cerebellum:
  • Inferior Cerebellar Peduncle: Connects the medulla to the cerebellum, carrying proprioceptive and vestibular information to the cerebellum for coordination of balance and posture.
• Functions:
  • Autonomic Functions: Regulates heart rate, blood pressure, breathing, swallowing, and vomiting reflexes.
  • Corticospinal Tract Decussation: Motor signals cross over at the pyramids, explaining why each side of the brain controls the opposite side of the body.
  • Sensory Processing: Processes sensory information from the body, including touch and pressure, through the dorsal columns and spinothalamic tracts.

Ascending Pathways (Sensory)

• Dorsal Column-Medial Lemniscus Pathway: Conveys fine touch, vibration, and proprioception from the body.
  • Pathway: Sensory neurons ascend through the dorsal columns, synapse in the medulla at the nucleus gracilis and cuneatus, cross over (decussate) as the medial lemniscus, and then continue to the thalamus and somatosensory cortex.
• Spinothalamic Tract: Transmits pain, temperature, and crude touch.
  • Pathway: Sensory neurons enter the spinal cord, synapse, and cross over (decussate) within the spinal cord, then ascend through the brainstem to the thalamus and somatosensory cortex.
• Spinocerebellar Tracts: Convey proprioceptive information to the cerebellum.
  • Pathway: Proprioceptive information from muscles and joints ascends through the dorsal and ventral spinocerebellar tracts to the cerebellum via the inferior cerebellar peduncle.

Descending Pathways (Motor)

• Corticospinal Tract: Responsible for voluntary motor control of the body.
  • Pathway: Originates in the motor cortex, descends through the internal capsule, passes through the brainstem, and decussates at the pyramids of the medulla before continuing to the spinal cord to innervate muscles.
  • Divisions:
    • Lateral Corticospinal Tract: Controls fine motor movements of the limbs and decussates at the medullary pyramids.
    • Anterior Corticospinal Tract: Controls axial and proximal muscles and decussates at the spinal level.
• Corticobulbar Tract: Controls voluntary movements of the face and neck.
  • Pathway: Originates in the motor cortex, passes through the brainstem, and synapses with cranial nerve nuclei to control muscles of the face, tongue, and pharynx.
• Reticulospinal and Vestibulospinal Tracts: Maintain posture and balance.
  • Pathway: Originates in the brainstem (reticular formation and vestibular nuclei) and descends to the spinal cord to control muscle tone, balance, and reflexes.
• Rubrospinal Tract: Assists in the control of movement.
  • Pathway: Originates in the red nucleus of the midbrain, crosses over immediately, and descends to the spinal cord to facilitate upper limb movements.

Reticular Formation

• Location: Extends throughout the brainstem, from the medulla to the midbrain.
• Structure: A network of interconnected neurons that span the entire length of the brainstem.
• Functions:
  • Consciousness and Arousal: Plays a critical role in maintaining consciousness and alertness through the ascending reticular activating system (ARAS).
  • Regulation of Sleep-Wake Cycles: Controls transitions between sleep and wakefulness.
  • Autonomic Functions: Influences autonomic functions like heart rate, respiration, and gastrointestinal activity.

Common Diseases and Disorders of the Brainstem

• Stroke (Brainstem Stroke): A brainstem stroke can cause serious deficits such as loss of consciousness, difficulty breathing, paralysis, and cranial nerve palsies. Symptoms depend on which part of the brainstem is affected (e.g., Wallenberg syndrome with PICA infarct).
• Locked-In Syndrome: Caused by damage to the pons, usually due to stroke. Patients are conscious but cannot move or speak, except for vertical eye movements and blinking.
• Medullary Syndrome (Wallenberg Syndrome): A condition resulting from a stroke in the lateral medulla, leading to symptoms such as vertigo, dysphagia, hoarseness, and loss of pain and temperature sensation on one side of the face and the opposite side of the body.
• Decerebrate and Decorticate Posturing: These are abnormal postures that occur due to severe brainstem or cortical damage. Decerebrate posturing involves extension of the arms and legs, while decorticate posturing involves flexion of the arms and extension of the legs.
• Parkinson's Disease: A neurodegenerative disorder that affects the substantia nigra of the midbrain, leading to tremors, rigidity, bradykinesia (slowness of movement), and postural instability.
• Multiple Sclerosis (MS): A demyelinating disease that can affect the brainstem, leading to symptoms such as double vision, facial weakness, vertigo, and dysphagia (difficulty swallowing).
• Brainstem Tumors: Tumors in the brainstem can cause symptoms like cranial nerve palsies, ataxia, weakness, and changes in breathing or heart rate. These tumours may be primary (e.g., gliomas) or metastatic.
• Chiari Malformation: A structural defect where brain tissue extends into the spinal canal, often compressing the brainstem and causing symptoms such as headaches, dizziness, and coordination problems.
• Brainstem Death: Refers to the irreversible loss of all brainstem function, including the ability to breathe and maintain cardiovascular regulation. It is one of the key criteria for declaring brain death.

Summary

• The brainstem integrates sensory and motor pathways, with ascending pathways transmitting sensory information to the brain and descending pathways controlling voluntary and involuntary movements.
• It serves as a crucial relay center between the brain, cerebellum, and spinal cord, regulating vital autonomic functions like breathing, heart rate, and consciousness.
• Its connections to the cerebellum are critical for balance, coordination, and motor control.