Brain Areas: Cerebral Hemispheres, Lobes, and Primary Cortices

The surface of cerebral hemispheres exhibits convolutions: ridges (gyri) and depressions (sulci).
Gyrus: A single ridge on the surface of the cerebral hemisphere.
Sulcus: A single depression or groove on the surface of the cerebral hemisphere.
Central Sulcus: A prominent sulcus that divides the frontal lobe from the parietal lobe.

Precentral Gyrus: The ridge located anterior to the central sulcus; part of the frontal lobe.
Primary Motor Area: Located within the precentral gyrus; responsible for the generation and control of skeletal muscle, thus controlling voluntary movement.

Postcentral Gyrus: Located within the parietal lobe.
Primary Somatosensory Area: Located within the postcentral gyrus; responsible for receiving and interpreting sensory information from the periphery, especially the skin.

The central sulcus separates motor and sensory areas.
- Precentral gyrus (frontal lobe): Primary motor area (motor control).
- Postcentral gyrus (parietal lobe): Primary somatosensory area (sensory processing).
This distinction is crucial for understanding brain function and organization.

Within both the primary motor area and primary somatosensory area, limbs and body parts are spatially mapped.
Specific areas are dedicated to specific body parts:
- Primary Motor Area: Separate areas for moving hands, face, legs, etc.
- Primary Somatosensory Area: Separate areas for receiving sensory information from the skin of the hand, face, legs, etc.

A homunculus is a caricature that reflects the size of brain areas dedicated to different body parts within the primary motor and somatosensory cortices.
Body parts with larger representations in the brain appear larger in the homunculus.
Features of the Homunculus:
- Massive hands and face: Indicate a large amount of brain area is dedicated to their movement and sensation.
- Large tongue: Reflects the brain area needed for speech.
- Small limbs, thorax, and abdomen: Indicate less brain area is dedicated to these parts compared to the hands and face.

Hands: A large area of the primary motor cortex is dedicated to hand movement due to the need for fine motor control (e.g., writing, playing musical instruments).
- Many muscles in the hand require precise control.
- Necessary for complex tasks requiring dexterity.
Face: A significant area is dedicated to facial movement for speaking and expressing emotions.
Sensory Input: Similarly, a large area of the primary somatosensory cortex is dedicated to sensory information from the hands and face.
- Allows for fine tactile discrimination and detailed sensory perception.

Motor control and sensory processing are contralateral (opposite side).
- The right motor cortex controls the left side of the body, and vice versa.
- The right somatosensory cortex receives input from the left side of the body, and vice versa.
Axons from the primary motor area cross over to supply the opposite side of the body.

Definition: A stroke, or cerebrovascular accident, occurs when blood flow to the brain is impaired or stopped (occluded).
Causes:
- Blockage of blood flow by a blood clot or foreign body.
- Rupture of an artery or vein.
Effects: Brain is highly sensitive to blood flow changes; neurons can die within minutes of oxygen deprivation.
- Neuronal death occurs if blood flow is disrupted for more than 4-5 minutes.
- Brain has limited ability to repair or regenerate neurons.
Symptoms of Right Frontal Lobe Occlusion:
- Damage to the right primary motor area, which controls movement on the left side of the body.
- Weakness or paralysis on the left side of the body (contralateral effect).
- Possible damage to the right primary somatosensory area, affecting sensory perception on the left side of the body.