Brain Anatomy & Organization: Structure and Function

Brain Anatomy & Organization

Surface Features of the Brain

The surface of the brain is marked by distinct features that increase its surface area and separate functional regions.

• Gyri: Ridges or folds of the cerebral cortex that increase surface area, allowing for more neurons.

• Sulci: Shallow grooves between gyri.

• Fissures: Deeper grooves that separate major brain regions (e.g., the longitudinal fissure divides the left and right hemispheres).

Cerebrum (Cerebral Cortex)

The cerebrum is the largest part of the brain, divided into two hemispheres and four main lobes. The outer layer (gray matter) contains neuron cell bodies, while the inner layer (white matter) contains myelinated axons.

Frontal Lobe

• Functions: Decision-making, judgment, planning, calculation, personality, and cognition.

• Primary Motor Cortex (M1): Controls voluntary skeletal movement; exhibits contralateral control (left M1 controls right side of body).

• Broca's Area: Located in the dominant (usually left) hemisphere; responsible for speech production.

• Clinical Note: Damage to Broca's area causes Broca's (expressive) aphasia—patients understand speech but cannot produce it fluently.

Temporal Lobe

• Primary Auditory Cortex (A1): Detects and integrates sounds.

• Wernicke's Area: Located in the dominant hemisphere; controls language comprehension.

• Clinical Note: Damage to Wernicke's area causes Wernicke's (receptive) aphasia—patients produce fluent but nonsensical speech and cannot understand language.

Occipital Lobe

• Primary Visual Cortex (V1): Responsible for detection of light and visual processing.

• Receives input from the contralateral visual field.

Parietal Lobe

• Primary Somatosensory Cortex (S1): Detects and interprets general sensations (touch, pressure, temperature, pain, proprioception).

• Processes sensory information from the contralateral side of the body.

Insula (Insular Cortex)

• Located deep to the temporal lobe, hidden within the lateral sulcus.

• Functions: Autonomic regulation, appetite/taste processing, visceral sensation, emotional processing, and interoception (awareness of internal body states).

Brainstem

The brainstem connects the cerebrum to the spinal cord and controls automatic, life-sustaining functions. It contains the reticular formation, a network of neurons that regulates alertness, arousal, sleep-wake cycles, and sensory filtering (reticular activating system).

Midbrain (Mesencephalon)

• Corpora Quadrigemina: Four rounded bumps on the dorsal surface:

  • 2 Superior Colliculi: Visual reflex pathway (e.g., tracking moving objects, pupillary light reflex).

  • 2 Inferior Colliculi: Auditory reflex pathway (e.g., reflexive turning toward a sound).

• Cerebral Peduncles: Paired anterior bundles of white matter carrying motor tracts from cortex to spinal cord.

• Substantia Nigra: Dopamine-producing nucleus; degeneration leads to Parkinson's disease.

Pons

• Acts as a bridge connecting the cerebellum to the cerebrum and medulla.

• Contains pontine respiratory centers (apneustic and pneumotaxic centers) that fine-tune breathing rhythm.

• Houses nuclei for cranial nerves V, VI, VII, and VIII.

Medulla Oblongata

• The most vital part of the brainstem; connects the brain to the spinal cord.

• Contains autonomic reflex centers controlling:

  • Breathing (inspiratory/expiratory centers)

  • Heart rate and blood pressure (cardiac and vasomotor centers)

  • Swallowing, vomiting, hiccupping, sneezing, coughing, urination, defecation

• Clinical Note: Damage to the medulla is life-threatening due to its control of breathing and heart function.

Cerebellum

• Located posterior and inferior to the cerebrum, dorsal to the pons and medulla.

• Surface folds are called folia (distinct from gyri of the cerebrum).

• Divided into two hemispheres connected by the vermis.

Functions:

• Balance and equilibrium

• Proprioception (awareness of body position in space)

• Coordination and smoothing of voluntary movement

• Regulation of postural muscles and muscle tone

• Note: The cerebellum does not initiate movement; it fine-tunes movement initiated by the motor cortex. Damage leads to ataxia (uncoordinated, staggering gait).

Limbic System ("Emotional Brain")

The limbic system is a group of primitive brain structures forming a ring around the diencephalon, responsible for emotion, memory, and basic survival drives.

Structure	Function
Amygdala	Fear, aggression, emotional responses
Hippocampus	Formation of new long-term memories
Cingulate Gyrus	Emotional processing, pain response
Hypothalamus	Hunger, thirst, sexual drive, temperature
Olfactory Bulbs	Smell (explains why smells trigger strong memories/emotions)

Functions and Urges:

• Basic emotions (fear, anger, pleasure, lust)

• Hunger and thirst

• Urge to urinate and defecate

• Sexual drive and reproductive behavior

• Survival instincts

• Emotional memory and smell processing

Basal Ganglia (Basal Nuclei)

The basal ganglia are deep gray matter nuclei within the cerebral white matter that regulate movement in conjunction with the substantia nigra and subthalamus.

• Key Structures: Caudate nucleus, Putamen, Globus pallidus

• Combinations: Caudate + Putamen = Striatum; Putamen + Globus pallidus = Lentiform nucleus

Functions:

• Initiation and regulation of voluntary movement

• Suppression of unwanted movements

• Motor learning and habit formation

• Regulation of muscle tone

• Clinical Note: Dysfunction leads to movement disorders such as Parkinson's disease (too little dopamine → rigid, slow movement) or Huntington's disease (degeneration → uncontrolled jerky movements).

Diencephalon

The diencephalon is located between the brainstem and cerebrum and forms the central core of the brain. It contains four main parts:

Thalamus

• The major relay station for sensory and motor information.

• Almost all sensory information (except smell) passes through the thalamus before reaching the cerebral cortex.

• Sorts, filters, and directs information to the appropriate cortical area.

Hypothalamus

• Located below the thalamus; controls the pituitary gland.

• Functions:

  • Regulates the autonomic nervous system (sympathetic and parasympathetic)

  • Part of the limbic system

  • Produces and releases hormones (ADH, oxytocin; releasing/inhibiting hormones for the anterior pituitary)

  • Salt-water (osmotic) balance via ADH

  • Temperature regulation (the body's thermostat)

  • Hunger, thirst, sleep, sexual drive, and circadian rhythms

Epithalamus

• Most dorsal/posterior part of the diencephalon.

• Contains the pineal gland, which secretes melatonin to regulate the circadian rhythm and sleep-wake cycle.

• Note: Serotonin is the precursor to melatonin; it is converted to melatonin within the pineal gland. The body's main serotonin production occurs in the raphe nuclei of the brainstem and in the gut, not the pineal gland.

Subthalamus

• Located inferior to the thalamus.

• Contains the subthalamic nucleus, which is part of the basal ganglia circuit that modulates motor activity.

Structures Associated with the Diencephalon

• Corpus Callosum: The largest white matter commissure in the brain; connects the left and right cerebral hemispheres, enabling communication. Divided (anterior to posterior) into: rostrum, genu, body (trunk), and splenium.

• Septum Pellucidum: Thin, translucent membrane of white matter that separates the two lateral ventricles; attached superiorly to the corpus callosum and inferiorly to the fornix.

• Fornix: C-shaped bundle of white matter fibers; main output pathway of the hippocampus and part of the limbic system; connects the hippocampus to the hypothalamus and mammillary bodies.