Makindo Medical Notes

Structure of the Cerebrum

The cerebrum is the largest part of the human brain, responsible for higher brain functions such as thought, action, and sensory processing. Its complex and highly organized cellular structure comprises various types of neurons and glial cells. These cells work together to perform the sophisticated functions of the brain.

Neurons

Neurons are the primary functional units of the brain, responsible for transmitting information through electrical and chemical signals. The cerebral cortex, the outermost layer of the cerebrum, contains several types of neurons organized into six distinct layers (I to VI), each with specific cell types and functions.

• Pyramidal Cells
  • Shape: Pyramid-shaped cell bodies with a large apical dendrite extending toward the cortical surface and several basal dendrites projecting horizontally.
  • Function: Major excitatory neurons involved in cognitive functions, motor control, and communication between different brain regions. They use glutamate as a neurotransmitter.
  • Location: Predominantly found in layers III and V of the cerebral cortex.
• Stellate Cells
  • Shape: Star-shaped with multiple branching dendrites.
  • Function: Act as interneurons, processing sensory input and modulating local cortical activity. They can be excitatory or inhibitory.
  • Location: Found mainly in layer IV of the cerebral cortex, especially in primary sensory areas.
• Betz Cells
  • Shape: Large pyramidal neurons, among the largest neurons in the human nervous system.
  • Function: Specialized for motor control; they project directly to the spinal cord's motor neurons, controlling voluntary muscle movements.
  • Location: Found in layer V of the primary motor cortex (precentral gyrus).
• Interneurons
  • Function: Modulate and integrate signals between neurons within the cortex. They are primarily inhibitory, using gamma-aminobutyric acid (GABA) as a neurotransmitter.
  • Types: Include basket cells, chandelier cells, and double bouquet cells, each with unique morphology and connectivity.
  • Location: Distributed throughout all cortical layers.

Glial Cells

Glial cells, or neuroglia, provide support, protection, and nourishment to neurons. They outnumber neurons in the brain and perform various essential functions to maintain homeostasis and facilitate signal transmission.

• Astrocytes
  • Function: Provide structural support, regulate blood flow, maintain the blood-brain barrier, and modulate synaptic activity by regulating neurotransmitter levels.
  • Shape: Star-shaped with numerous branching processes extending in all directions.
  • Location: Throughout the brain and spinal cord.
• Oligodendrocytes
  • Function: Produce myelin sheaths that insulate axons in the central nervous system (CNS), enhancing the speed of electrical impulse conduction.
  • Shape: Smaller cells with fewer processes that wrap around axons.
  • Location: Mainly in the white matter, but also present in gray matter.
• Microglia
  • Function: Act as the brain's resident immune cells; they phagocytose debris, pathogens, and damaged neurons, and release cytokines during inflammatory responses.
  • Shape: Small cells with elongated nuclei and fine, highly branched processes.
  • Location: Distributed throughout the CNS.
• Ependymal Cells
  • Function: Line the ventricles of the brain and the central canal of the spinal cord; involved in producing and circulating cerebrospinal fluid (CSF) through the action of their cilia.
  • Shape: Ciliated, epithelial-like cells forming a single layer.
  • Location: Lining the ventricular system and central canal.

Layered Organization of the Cerebral Cortex

The cerebral cortex is organized into six horizontal layers, each characterized by specific types of neurons and connections:

1. Layer I – Molecular Layer: Contains few neurons; primarily composed of dendritic and axonal processes, facilitating intracortical communication.
2. Layer II – External Granular Layer: Contains small pyramidal and stellate neurons; receives inputs from other cortical areas.
3. Layer III – External Pyramidal Layer: Contains medium-sized pyramidal neurons; sends outputs to other cortical areas (association fibers) and the opposite hemisphere (commissural fibers).
4. Layer IV – Internal Granular Layer: Rich in stellate neurons; primary recipient of thalamic sensory input, especially prominent in sensory cortices.
5. Layer V – Internal Pyramidal Layer: Contains large pyramidal neurons (including Betz cells in the motor cortex); sends outputs to subcortical structures such as the brainstem and spinal cord.
6. Layer VI – Multiform Layer: Contains diverse neuron types; sends outputs to the thalamus and establishes feedback loops.

Functional Significance

• Sensory Processing: Neurons in the sensory cortices receive and interpret input from sensory organs. Layer IV is particularly important for receiving sensory information from the thalamus.
• Motor Control: Pyramidal neurons in the motor cortex (especially Betz cells) initiate voluntary movements by sending signals to the spinal cord's motor neurons.
• Cognitive Functions: Association areas composed of pyramidal and interneurons integrate information for complex processes like learning, memory, and decision-making.
• Support and Protection: Glial cells maintain the environment necessary for neuronal function, protect against pathogens, and facilitate repair processes.

Conclusion

The cerebrum's cellular structure is intricately organized to support its diverse and complex functions. Neurons, including pyramidal cells, stellate cells, Betz cells, and various interneurons, are systematically arranged in cortical layers, each contributing to specific neural pathways and processing capabilities. Glial cells—astrocytes, oligodendrocytes, microglia, and ependymal cells—play crucial supporting roles, from maintaining homeostasis and forming myelin to immune defense and CSF circulation. This sophisticated cellular architecture enables the cerebrum to perform higher-order processes essential for cognition, sensation, voluntary movement, and overall brain function.