Brainstem Anatomy: Pyramidal Tract and Key Structures

• The brainstem serves as a conduit for ascending and descending tracts.
• It contains reflex centers vital for respiration, cardiovascular function, and consciousness.
• Cranial nerve nuclei III-XII are located within the brainstem.
• The medulla is identifiable by the olives and pyramids.
• The pons is identifiable by the basilar part of the pons.
• The midbrain is identifiable by the interpeduncular fossa.
• The pyramidal tract is found in the medulla.
• The inferior olivary nucleus is located within the olives.
• The hypoglossal nucleus is identifiable due to its checkerboard structure.

Pyramidal Tract/Corticospinal Tract

• Neurons in layer V of the cortex are pyramidal cells, due to their pyramidal-shaped cell bodies.
• Most fibers cross the midline at the junction between the medulla and spinal cord, forming the pyramidal decussation.
• In the spinal cord, these fibers synapse with interneurons, which then synapse with motor neurons in the ventral horn.
• Remaining fibers cross the midline at the spinal cord level.

Corticobulbar (Corticonuclear) Tract

• This tract originates from the cortex and terminates in the lower motor neurons (LMNs) within the cranial nerve nuclei.
• It innervates the skeletal muscles of the head and neck, including muscles of facial expression and mastication.
• Cranial Nerve VII innervates the muscles of facial expression.

Brown-Séquard Syndrome

• Results in specific neurological deficits based on the spinal tracts involved following damage to one side of the spinal cord.
• The spinal cord's organized sensory and motor pathways lead to characteristic clinical findings.

Key Tracts Affected in Brown-Séquard Syndrome

• Spinothalamic Tract:
  • Carries pain and temperature sensation from the contralateral side of the body.
  • Damage leads to loss of pain and temperature sensation on the opposite side below the lesion level, due to fibers crossing shortly after entering the spinal cord.
• Dorsal Column/Medial Lemniscus Pathway:
  • Carries fine touch, vibration, and proprioception from the ipsilateral side of the body.
  • Damage causes loss of these sensations on the same side below the lesion because fibers ascend on the same side and cross over in the brainstem.
• Corticospinal Tract:
  • Carries motor signals for voluntary movement from the brain to the ipsilateral side of the body.
  • Damage results in weakness or paralysis on the same side below the lesion.
  • Corticospinal fibers cross over in the medulla.

Functional Impact of Brown-Séquard Syndrome

• Ipsilateral Deficits:
  • Weakness or paralysis (motor function loss).
  • Loss of fine touch, vibration, and proprioception.
• Contralateral Deficits:
  • Loss of pain and temperature sensation.

Cerebral Aqueduct

• The cerebral aqueduct is a common site of blockage, such as by a tumor, due to its small size.
• Blockage can result in hydrocephalus, which is the accumulation of fluid in the brain.

Dopaminergic Neurons

• Located in the substantia nigra.
• In addition to motor function, they play a role in motivation and reward.
• In Parkinson's disease, the substantia nigra appears pale.

Reticular Formation

• Resembles a net, composed of nerve cells and fibers scattered throughout the brainstem center.
• Has direct and indirect connections with all levels of the central nervous system (CNS).

Major Nuclei within the Reticular Formation

• Raphe Nuclei:
  • Synthesize and secrete serotonin (5-HT).
  • Active in deep sleep.
• Cholinergic Reticular Nuclei:
  • Utilize acetylcholine.
  • Active in the awake state.
• Noradrenergic Neurons:
  • Located in the locus coeruleus.
  • Utilize noradrenaline.
  • Active when awake, regulating arousal and attention.

Functions of the Reticular Formation

• Control of skeletal muscles.
• Control of somatic and visceral sensations.
• Control of the autonomic nervous system (ANS).
• Control of the endocrine nervous system.
• Influences sleep, arousal, and consciousness.
• Pain modulation.
• Consciousness is the awareness of oneself and one's surroundings.