Chapter 13: The Brain, Cranial Nerves, and Sensory and Motor Pathways

Overview of Brain Anatomy and Development

The human brain develops from a hollow neural tube and exhibits significant structural and functional complexity. Brain volume averages around 1200 mL, with individual variation. Notably, male brains are typically about 10% larger than female brains due to body size differences, but there is no correlation between brain size and intelligence.

• Neural Tube: The embryonic structure from which the central nervous system (CNS) develops.

• Brain Volume: Average adult brain volume is approximately 1200 mL.

• Size vs. Intelligence: Brain size does not determine intellectual capacity.

Major Regions of the Brain

The brain is divided into four major regions, each with distinct structural and functional characteristics:

• Cerebrum: The largest part of the brain, responsible for higher cognitive functions.

• Cerebellum: Coordinates movement and balance.

• Diencephalon: Includes structures such as the thalamus and hypothalamus, involved in sensory relay and autonomic functions.

• Brainstem: Composed of the midbrain, pons, and medulla oblongata; controls basic life functions and connects the brain to the spinal cord.

Cerebrum

The cerebrum consists of left and right cerebral hemispheres, separated by deep fissures. The surface of the cerebrum is marked by gyri (folds) and sulci (grooves), which increase the surface area for neural processing.

• Cerebral Hemispheres: Two symmetrical halves of the cerebrum, each controlling opposite sides of the body.

• Fissures: Deep grooves that separate major regions of the brain.

• Gyri and Sulci: Gyri are raised folds; sulci are shallow grooves. Both serve to increase cortical surface area.

Cerebral Cortex

The cerebral cortex is the superficial layer of gray matter, ranging from 1.5 to 4.5 mm in thickness. It is the site of major cortical functions, including conscious thought, memory storage and processing, sensory processing, and control of skeletal muscles.

• Gray Matter: Contains neuron cell bodies and is responsible for processing information.

• Major Functions:

  • Conscious Thought: Awareness, reasoning, and decision-making.

  • Memory Storage and Processing: Encoding, storing, and retrieving information.

  • Sensory Processing: Interpretation of sensory input from the body.

  • Control of Skeletal Muscles: Initiation and regulation of voluntary movement.

Example: Surface Anatomy of the Cerebrum

The arrangement of gyri and sulci is unique to each individual and is used to identify specific functional areas, such as the primary motor cortex (precentral gyrus) and primary somatosensory cortex (postcentral gyrus).

Additional info: The cerebral cortex is organized into lobes named after the overlying cranial bones: frontal, parietal, temporal, and occipital lobes. The insula is a deep cortical area not visible from the surface.