Brain Structure and Function: A Comprehensive Overview

Posted on Nov 7, 2024 in Biology

Brain Structure and Function

Anatomical Overview

Divisions of the Brain

The brain is divided by anatomical planes: sagittal (hemispheres), coronal (lobes), and horizontal (top and bottom).

Key components include:

  • Gray matter: Intracellular communication
  • White matter: Long-distance transmission of messages via myelin
  • Cerebrospinal fluid (CSF): Surrounds, protects, nourishes, and communicates within the CNS; produced in the lateral ventricles
  • Meninges: Protective layers covering the brain, including the dura mater (outermost), arachnoid mater, and pia mater (innermost)

Central Nervous System (CNS)

The CNS consists of:

  1. Forebrain
  2. Midbrain
  3. Hindbrain
  4. Spinal cord (coordinates simple reflexes)

The spinal cord transmits sensory information to the brain via afferent fibers and receives motor information from the brain via efferent fibers. The Bell-Magendie Law states that dorsal roots are sensory and ventral roots are motor.

Hindbrain (Rhombencephalon)

The hindbrain includes the myelencephalon and metencephalon. The medulla oblongata controls basic autonomic functions (reflexes), the reticular activating system (RAS), and contains the decussation of the pyramids. The cerebellum is responsible for balance, motor skills, and muscle tone regulation. The pons controls eye movements, facial expressions, and the superior olive (sound localization).

Midbrain (Mesencephalon)

The midbrain contains the RAS, substantia nigra, superior colliculus (visual stimuli orientation), and inferior colliculus (auditory stimuli orientation). These structures allow us to orient to specific stimuli.

Forebrain (Prosencephalon)

The forebrain, the largest part of the brain, is divided into the diencephalon and telencephalon.

  • Diencephalon: Contains the thalamus (relay station for sensory information) and hypothalamus (homeostasis control).
  • Telencephalon: Includes the cerebral hemispheres, basal ganglia, limbic system, and cerebral cortex.

Basal Ganglia

The basal ganglia (caudate nucleus, putamen, and globus pallidus) act as a relay station between the thalamus and neocortex, playing a role in motor control, eye movements, cognitive function, and emotional processing.

Cerebral Cortex (Neocortex)

The neocortex, a thin layer covering the cerebral hemispheres, is characterized by gyri (ridges) and sulci (grooves), maximizing surface area. Key sulci include the central sulcus (Rolando) and lateral sulcus (Sylvius).

Functional Areas

Sensory Processing Zones

  • Input: Somatosensory cortex (postcentral gyrus), visual cortex (occipital lobe), auditory cortex (superior temporal gyrus, including Heschl’s gyrus), and olfactory cortex (orbitofrontal cortex).
  • Output: Primary motor cortex (precentral gyrus) for voluntary motor actions.

Limbic System

The limbic system (amygdala, hypothalamus, cingulate cortex, anterior thalamus, mammillary bodies, hippocampus) regulates emotions, memory, and cognitive control.

Cortical Association Areas

  • Frontal Lobe: Dorsolateral, ventrolateral, medial, and orbitofrontal divisions; responsible for planning, decision-making, empathy, interpersonal behavior, and executive functions.
  • Parietal Lobe: Integrates sensory information, memory, internal states, external motor movements, and spatial awareness.
  • Temporal Lobe: Involved in memory (medial structures), emotion (temporal poles), auditory processing (superior temporal gyrus), and visual object recognition (inferior temporal gyrus).

Cortical Maps

The cortex is organized into layers (lamination), columns (columnarization), and dendritic structures, with varying neuronal density.

Cortical Areas (Brodmann’s Areas)

The cortex is divided into 47 areas based on cytoarchitecture.

Limbic/Paralimbic Cortex

  • Cortex-like: Septal nuclei and amygdala complex; lacks lamination, with disorganized dendrites.
  • Allocortex: Hippocampal complex and pyriform cortex (primary olfactory); 1-2 layers with organized dendrites.
  • Intermedia: Located between cortex-like and allocortex; 5 layers, with granular layers II and IV.

Association Cortex

  • Unimodal: Processes a single sensory modality.
  • Heteromodal: Processes multiple sensory modalities.

Sensory/Motor Cortex

  • Sensory: Granular and laminated.
  • Motor: Contains giant pyramidal cells (Betz cells).

Information Processing Flow

  1. Stimulus
  2. Sensory fibers
  3. Primary sensory cortex
  4. Unimodal association cortex
  5. Heteromodal association cortex
  6. Paralimbic area
  7. Limbic areas
  8. Hypothalamus

Brain Connectivity

White Matter Tracts

  • Corpus callosum: Connects the two hemispheres.
  • Uncinate fasciculus: Connects frontal and temporal lobes.
  • Superior longitudinal fasciculus: Connects frontal, parietal, and occipital lobes.
  • Inferior longitudinal fasciculus: Connects temporal and occipital lobes.

Blood Supply

Arterial System

  • Anterior circulation: Internal carotid artery supplies the frontal brain areas.
  • Posterior circulation: Vertebral artery supplies the posterior brain areas, including the posterior cerebral artery.
  • Circle of Willis: Connects the anterior and posterior circulation, providing redundancy in blood supply.

Four main arteries supply blood to the brain.