Neuroanatomy: Brain Organization

Basic Neuroanatomy

• The central nervous system consists of the brain and spinal cord.
• The peripheral nervous system consists of cranial and spinal nerves.

Brain Organisation

• The brain is organized into the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).
• The forebrain includes the telencephalon and diencephalon.
• Telencephalon structures include the cerebral cortex, limbic system, and basal nuclei.
• The diencephalon consists of the thalamus and hypothalamus.
• The midbrain is the mesencephalon.
• The hindbrain includes the metencephalon and and myelencephalon.
• Metencephalon contains the pons and cerebellum.
• Myelencephalon contains medulla oblongata.

Anatomical Directions

• Anatomical directions in the brain include dorsal (superior), ventral (inferior), rostral (anterior), and caudal (posterior).
• In the forebrain, the longitudinal axis differs from that of the body, while medial and lateral remain consistent.

Anatomical Planes

• Coronal/Frontal Plane: Divides the brain into front and back sections.
• Transverse/Axial Plane: Divides the brain into upper and lower sections.
• Sagittal Plane: Divides the brain into left and right sections.

Telencephalon Anatomy

• The telencephalon includes the cerebral cortex, limbic system, and basal ganglia.
• Subcortical structures are neural structures located deep to the cerebral cortex.

Cerebral Cortex

• The cerebral cortex covers the right and left cerebral hemispheres.
• Gray matter in the cerebral cortex consists of neural tissue with neuron cell bodies and synapses, including the cerebral cortex and basal ganglia.
• White matter in the cerebral cortex consists of neural tissue with mostly myelinated axons.
• Landmarks of the cerebral cortex include sulci (crevices) and gyri (bumps).

Frontal Lobe

• The frontal lobe boundaries are rostral (frontal pole), caudal (central sulcus), and ventral (lateral sulcus).

• Structures within the frontal lobe:

  • Primary Motor Cortex/Precentral Gyrus: Involved in voluntary movement and production of contralateral half of the body movement.
  • Supplementary Motor Area: Involved in complex movement planning, sequencing, and bilateral coordination.
  • Premotor Area: Engaged in movement preparation and sensory/spatial guidance of movement.
  • Frontal Eye Field: Controls voluntary scanning movements of the eyes (horizontal movement).
  • Broca's Area: Involved in the motor function of speech, in Broca's aphasia, patients cannot physically form words and has effortful speech.
  • Prefrontal Cortex: Key for executive function, personality, intelligence, social behavior, and working memory.

• Functions of the frontal lobe include voluntary movement, executive functions, working memory, personality, and social behavior.

• The motor cortex is comprised of the primary motor cortex, supplementary motor area, and premotor area.

• The primary motor cortex/precentral gyrus location is precentral gyrus.

• Somatotopic arrangement in the frontal lobe shows medial for lower limbs and lateral for face motor control.

• Body part size in the frontal lobe is proportionate to the motor function of the respective body region with large hands and face and small limbs and trunk.

Parietal Lobe

• Rostral boundary is the central sulcus

• Caudal boundary is the parieto-occipital sulcus

• Ventral boundary is the lateral sulcus

• Structures:

  • Primary Somatosensory Cortex/Postcentral Gyrus: Processes contralateral sensory information from touch, pressure, pain, etc.
  • Somatosensory Association Area: Receives and integrates different sensory modalities for tactile object recognition and spatial awareness.

• Functions:

  • Process and interpret somatosensory input.
  • Proprioception

• Parietal lobe location in postcentral gyrus.

• Organization of somatotopic maps with medial lower limb and lateral face control.

• Body parts in the parietal lobe are sized in proportion to sensitivity and shows big hands and lips and small limbs and trunk.

Temporal Lobe

• Rostral boundary is the temporal pole.
• Dorsal boundary is the lateral sulcus.
• Structures found:
  • Primary Auditory Cortex: Receives low (anterior portion) and high (posterior portion) frequency sounds
  • Wernicke's Area: Interprets written and spoken language and if damaged, patients have fluent but nonsensical speech (Wernicke's aphasia).
• Functions:
  • Auditory input
  • Language comprehension

Occipital Lobe

• Rostral boundary is the parieto-occipital sulcus.
• Caudal boundary is the occipital pole.
• Structures:
  • Primary Visual Cortex: Processes visual information.
  • Secondary Visual Cortex.
• Functions:
  • Receives visual input
  • Visual recognition
• A lesion in the primary visual cortex leads to blindness in the corresponding visual field.

Insular Lobe

• Lateral boundary is the lateral sulcus.
• Structure: Primary gustatory cortex.
• Functions: -Taste processing -Risk-reward behavior -Autonomics -Pain pathways -Vestibular functions
  • Neuropsychiatric disorders e.g. OCD, PTSD, addiction, schizophrenia

Brodmann Areas

• Brodmann areas are regions of the cerebral cortex defined by cytoarchitecture.
• Primary motor cortex BA 4
• Primary somatosensory cortex BA 3, 1, 2
• Premotor and supplementary motor cortex BA 6
• Visual cortex BA 17
• Auditory cortex BA 41, 42
• Wernicke’s areas BA 22, 39, 40

Limbic System

• Structures that are responsible for emotion, survival behaviors, and memory. Split into cortical and subcortical structures.

• Cortical structures:

  • Cingulate gyrus: Processes sensory informatio; regulates emotions and involved in memory and attention.
  • Parahippocampal gyrus: Involved in memory formation and navigation.

• Subcortical structures:

  • Hippocampus: memory formation
  • Amygdala: Processes emotions (fear, anger): helps the brain respond to emotional events
  • Hypothalamus: Regulates physiological functions like hunger, thirst, sleep, and reproduction
  • Septum pellucidum: Thin tissue separating lateral ventricles
  • Fornix: Pathway from hippocampus and limbic arera.

• Mammillary bodies : Passes information for fornix and hippocampus to anterior thalamis nuclei and involved with memory recall

  • Thalamus: Brain information relay center.
  • Olfactory bulbs: Processes small.

Basal Nuclei

• Basal Ganglia/Nuclei: Help initiate and regulate voluntary movement.

Diencephalon Anatomy

• The diencephalon consists of the thalamus, hypothalamus, epithalamus, and subthalamus.

Thalamus

• Sensory/Motor Relay Station of the Brain
• Dorsomedial Nuclei (DM): Sensory Information & Emotional Feelings/States
• Anterior Nuclei (AN): Emotional Tone & Recent Memory (Limbic System)
• Ventrolateral & Ventral Anterior Nuclei (VL/VA): Influence Motor Activity
• Ventral Posterolateral/posteromedial Nuclei (VPL/VPM): Sensory Processing

Epithalamus

• Responsible for circadian rhythms and mood and behavior.
• Structures within include Habenular Nucleus and Pineal Gland.

Subthalamus

• It assists with motor activity control, integration, and accuracy.

Hypothalamus

• Autonomic system control: Controls parasympathetic and sympathetic system.
• Body temperature regulation: Monitors body temperature and adjusts as needed.
• Feeding and satiety centres: Induces hunger and fullness.
• Thirst centre to control water intake.
• Sexual arousal for reproduction.
• Limbic system structures for memory. Regulates sleep-wake cycles: regulates sleep wake cycles and arousal for circadian rhythm control.
• It controls activity of pituitary glands.

Hypothalamus Pituitary Gland

• Posterior Pituitary (Neurohypophysis):
  • Part of diencephalon which connects to the hypothalamus via the infundibulum.
  • Stores and secretes Hypothalamus Hormones (Oxytocin & ADH) in contrast to producing its own hormones.
• Anterior Pituitary (Adenohypophysis):
  • Not part of diencephalon regulated by hypothalamus. This is through hypophyseal portal system.

Brainstem Anatomy

• The brainstem includes the midbrain, pons, and medulla oblongata.

Midbrain

• Superior Colliculi: Visual reflexes.
• Inferior Colliculi: Auditory reflex centre.

Pons

• Basilar Groove

Medulla Oblongata

• Landmark identifiers include: Olives, Pyramids, Anterior Median Fissure, Cuneate Tubercles, and Gracile Tubercles.

Brainstem Cranial Nerves

• Cranial Nerves 2-12 originate from the brainstem.

Cerebellum Anatomy

• The cerebellum functions in muscle coordination, posture, & motor learning.
• The structure consists of two cerebellar hemispheres and a vermis.
• Primary and Horizontal Fissure of Cerebellum

Cerebullum Classification

• Gross anatomical classification:
  • Anterior lobe,
  • Posterior lobe, and
  • Flocculonodular lobe.
• Functional classification:
  • Spinocerebellum for muscle and limb co-ordination.
  • Cerebrocerebellum for planning and execution.
  • Vestibulocerebellum or balance and eye control.

Cortical Communication

• Communication can be done through three main types of fibers:
• Association fibers
  • Connect the cortex regions in the same hemisphere such as short fibers that connect adjacent gyri.
• Commissural fibers: Connect corresponding regions of 2 hemispheres examples are corpus callosum and fornix.
• Project fibers
  • Connect cerebral cortex to the brainstem examples are internal capsule.

Corpus Callosum

• Connects the two hemispheres of the brain.

White matter cerebellar peduncles for the cerebellum

• Superior Connects Cerebellum & Midbrain/Diencephalon
• Middle Connects Cerebellum & Pons
• Inferior Connects Cerebellum & Medulla/Spinal Cord