Brain Anatomy and Hand Model Study Guide

The Hand Model Representation of Brain Anatomy

• Overall Brain Scale:

  • The human brain is approximately equal in size to two human hands placed together.

• Hand Model Setup:

  • For each individual hand, move the thumb all the way up until it sits beneath the pinky nail.

  • Place both hands together side-by-side to construct the physical brain model.

• Anatomical Orientations (Front vs. Back):

  • Front of the Brain:

  • Represented by the pinky fingers.

  • Physical gesture and verbal prompt: Touch the pinkies and state, "front of the brain".

  • Back of the Brain:

  • Represented by the thumb knuckles.

  • Physical gesture and verbal prompt: Touch one thumb knuckle and state, "back of the brain".

• Hemispheric Division:

  • The brain is divided into two distinct hemispheres: left and right.

  • Left Hemisphere:

  • Represented by the left hand.

  • Physical gesture and verbal prompt: Touch the left hand and state, "left hemisphere".

  • Right Hemisphere:

  • Represented by the right hand.

  • Physical gesture and verbal prompt: Touch the right hand and state, "right hemisphere".

Cortical Layering and Subcortical Structures

• Cortex vs. Subcortical Layering:

  • Cortex:

  • The outermost layer of the brain.

  • Structurally comprised of a thin sheet of nerve cells.

  • Mechanically analogous to a crumpled piece of paper folded tightly to fit within a small space.

  • Subcortical Structures:

  • A collection of deep structures residing beneath the outer cortex.

  • Structural Relationship:

  • The cortex wraps completely around the underlying subcortical structures, mirroring how the fingers wrap around the thumbs in the hand model.

  • Physical gesture and verbal prompt: Wrap the fingers around the thumbs and state, "the cortex wraps around the subcortical structures".

  • Physical gesture and verbal prompt: Move the thumbs and state, "the brain has subcortical structures underneath the cortex".

• Continuity with the Spinal Cord:

  • The subcortical structures extend directly down into and are continuous with the spinal cord.

  • In the hand model, this is demonstrated by how the thumbs extend continuously into the arms.

  • This structural continuity helps anchor and remember the location of the back of the brain.

  • Physical gesture and verbal prompt: Run a finger from the thumb down the arm and state, "the subcortical structures are continuous with the spinal cord".

Cortical Topography: Gyri, Sulci, and the Corpus Callosum

• Gyri and Sulci Surface Features:

  • Gyrus:

  • A region where the brain bulges outward due to the tight crumpling of the cortical sheet.

  • Represented in the hand model by the outward bulge of individual fingers.

  • Physical gesture and verbal prompt: Run a finger down the ring finger and state, "a gyrus is a place where the brain bulges out".

  • Sulcus:

  • A crevice or groove formed between adjacent bulges in the brain.

  • Represented in the hand model by the space where pressed fingers meet.

  • Physical gesture and verbal prompt: Run a finger down the crevice where the ring and middle fingers meet and state, "the sulcus is a crevice in the brain".

• Interhemispheric Connectivity (Corpus Callosum):

  • Corpus Callosum:

  • A large, thick band of nerve cells running horizontally across both hemispheres.

  • Serves as the primary physical bridge connecting the left and right hemispheres.

  • Hand Model Representation:

  • Represented by the alignment and junction where the fingernails meet across the two hands.

  • Physical gesture and verbal prompt: Press the fingernails of both hands directly together and state, "the corpus callosum connects the left and right hemispheres of the brain".

Lobes of the Cerebral Cortex

• Four Cortical Lobes Per Hemisphere:

  • Each hemisphere of the cortex is structurally organized into four principal lobes.

• Central Sulcus Boundary:

  • The central sulcus is the key dividing fissure on top of the brain separating the frontal lobe from the parietal lobe.

  • Physical gesture and verbal prompt: Spread the fingers apart to identify the central sulcus position between the ring and middle fingers, press the fingers back together, run a finger down where the ring and middle fingers join, and state, "Central sulcus".

• Frontal Lobe:

  • Anatomical Position: Situated at the front of the brain.

  • Hand Model Representation: Mapped to the pinky and ring fingers.

  • Physical gesture and verbal prompt: Touch the pinky and ring fingers and state, "frontal lobe".

• Parietal Lobe:

  • Anatomical Position: Situated on top of the brain, directly behind the frontal lobe.

  • Hand Model Representation: Mapped to the middle and index fingers.

  • Physical gesture and verbal prompt: Touch the middle and index fingers and state, "parietal lobe".

• Temporal Lobes:

  • Anatomical Position: Located on the lateral sides of the brain.

  • Hand Model Representation: Mapped to the knuckles of the index and middle fingers.

  • Physical gesture and verbal prompt: Touch the index and middle knuckles and state, "temporal lobes".

• Occipital Lobes:

  • Anatomical Position: Situated at the back of the brain.

  • Hand Model Representation: Located directly over the thumb knuckles.

  • Physical gesture and verbal prompt: Touch the area directly over the thumb knuckles and state, "occipital lobes".

Primary Motor Area (Primary Motor Cortex)

• Location and Structure:

  • Situated on the gyrus directly in front of the central sulcus.

  • Hand Model Representation: Mapped to the ring finger.

  • Distribution: Present as a single functional strip in each hemisphere.

  • Physical gesture and verbal prompt: Run a finger down one ring finger and state, "primary motor area".

  • Spatial Orientation: Runs in continuous bands starting at the corpus callosum and extending laterally down the sides of the brain.

• Functional Mapping (Somatotopic Organization):

  • Primary Function: Directly executes and controls voluntary body movements.

  • Topographical Map Layout:

  • Medial region (adjacent to Corpus Callosum): Controls the toes and feet.

  • Intermediate lateral region: Controls the legs, arms, and face in sequence.

  • Inferior lateral region (bottom of strip): Controls the mouth.

  • Hand Model Sensory/Motor Prompts:

  • Touch above the ring fingernail: State "Pete" (Feet).

  • Touch the top of the ring finger: State "arm".

  • Touch the side of the ring finger: State "face".

  • Complete sequence prompt: "Pete arm face".

• Direct Physiological Demonstration (Electrode Stimulation):

  • If a surgeon opens the skull and applies electrical stimulation via an electrode to specific functional regions of the primary motor cortex, corresponding muscle actions are involuntarily triggered:

  • Electrode in the foot region $\rightarrow$ The foot moves.

  • Electrode in the arm region $\rightarrow$ The arm moves.

Primary Somatosensory Area (Primary Somatosensory Cortex)

• Location and Structure:

  • Situated on the gyrus directly behind the central sulcus.

  • Hand Model Representation: Mapped to the middle finger.

  • Distribution: Present as a single functional strip in each hemisphere.

  • Physical gesture and verbal prompt: Run a finger down one middle finger and state, "primary somatosensory area".

  • Spatial Orientation: Runs in continuous bands starting from the corpus callosum down the sides of the brain.

• Functional Mapping (Somatotopic Organization):

  • Primary Function: Processes and enables tactile perception (the sensation of touch).

  • Topographical Map Layout:

  • Medial region (adjacent to Corpus Callosum): Receives tactile sensations from the toes and feet.

  • Intermediate lateral region: Receives tactile sensations from the legs, arms, and face.

  • Inferior lateral region (bottom of strip): Receives tactile sensations from the mouth.

  • Hand Model Sensory/Motor Prompts:

  • Touch above the middle fingernail: State "Pete" (Feet).

  • Touch the top of the middle finger: State "arm".

  • Touch the side of the middle finger: State "face".

  • Complete sequence prompt: "Pete Pete arm face".

• Direct Physiological Demonstration (Electrode Stimulation):

  • If a surgeon opens the skull and applies electrical stimulation via an electrode to specific regions of the primary somatosensory cortex, subjectively perceived tactile sensations are triggered without physical contact on the skin:

  • Electrode in the foot region $\rightarrow$ Patient experiences a subjective feeling that something is touching their foot.

  • Electrode in the face region $\rightarrow$ Patient experiences a subjective feeling that something is touching their face.