Cerebrum: Lateral View

  • Primary cortices (areas), which initiate motor output or are primary cortical sensory reception centers.
  • Association cortices (areas), which process and integrate cortical information.

We divide the lateral surface of the brain into 4 key anatomic lobes:

Frontal lobe (anteriorly)
Occipital lobe (posteriorly)
Temporal lobe (inferiorly)
Parietal lobe (which borders each lobe)

Key Sulci

  • The Sylvian fissure (aka the lateral sulcus) distinguishes the cerebral lobes.
  • The central sulcus distinguishes the frontal and parietal lobes.

Key Gyri/Areas

  • The primary motor cortex lies within the precentral gyrus (given its location in front of the central sulcus)
    • It comprises primary motor neurons in pathways that control skeletal muscle (under volitional control) (NOT smooth muscle (under autonomic control), thus it’s often called the somatic motor area (somatic implies conscious control).
    • Each hemisphere innervates the opposite side of the body.
    • See: Stroke: Precentral Gyrus
  • The primary sensory cortex lies within the postcentral gyrus(given its location behind the central sulcus).
    • It receives sensory input from body regions that we consciously perceive, thus it’s often called the somatic sensory area.
    • Each hemisphere receives sensory input from the opposite side of the body.
  • The primary visual cortex (V1) lies at the occipital pole and is far more prominent on the medial aspect of the brain.
    It mostly lies along the calcarine sulcus, which is on the medial surface of the brain.
    • Visual stimuli first reach the cortex within the primary visual cortex.
    • Each cerebral hemisphere receives visual input from the opposite visual field.
    • Each eye captures both halves of the visual world.
    • See: PCA Stroke
  • The primary auditory cortex lies within the transverse temporal gyri (Heschl’s gyri).
    • Auditory stimuli first reach the cortex within the primary auditory cortex.
    • Each hemisphere receives auditory input from BOTH sides of the world.
  • The somatosensory association cortex interprets and integrates sensory information and is an important sensory planning region.
  • The premotor area assembles complex motor programs.
  • Clinically we assume the frontal eye fields control volitional eye movements (in reality eye movements come from disparate motor areas).
  • The prefrontal cortex in the anterior frontal lobe governs three discrete cognitive domains:
    • Laterally: Logistical (ie, task-sequencing) (use the L as a mneomnic)
    • Inferiorly: Impulse control (use the I)
    • Medially: Motivation (use the M)
  • Broca’s area is the language output area.
    • It lies in the inferior frontal gyrus.
    • Broca’s aphasia is a non-fluent language disorder, meaning language output is severely impaired but comprehension is mostly preserved (it is hesitant and effortful). Broca’s aphasia localizes to Broca’s area but also to many other (mostly motor) brain regions.
  • Wernicke’s area is the language reception area.
    • It lies within the superior temporal gyrus (posteriorly)
    • Wernicke’s aphasia is a fluent aphasia, meaning that there’s preserved speech output but poor comprehension (it is melodious but meaningless). Wernicke’s aphasia localizes to Wernicke’s area and neighboring temporo-parietal regions plus the insula.
  • The occipital lobe comprises cortical visual processing; it’s so dedicated to vision that clinicians can easily miss large posterior cerebral strokes, they fail to check the patient’s visual fields.

We subdivide visual processing into:

  • The dorsal stream (“where”) visual pathway, which lies along the superior occipital lobe and parietal lobe, and provides visuo-spatial localization processing.
  • The ventral stream (“what”) visual pathway, which lies along the inferior occipital lobe and temporal lobe, and provides object recognition processing.
  • The insula lies beneath the cerebral folds (the opercula).
    • It participates in many, varied (mostly subconscious) functions: pain modulation, appetite, visceral sensation, anxiety and emotion, socialization, and auditory processing.
    • See: Insular Stroke
  • The motor homunculus is a representation of the topographical distribution of neurons that (through relay neurons) command volitional (striated) muscle.

Sensory homunculus is similar but NOT the same as the motor homunculus; it contains sensory input from regions NOT under volitional command (like the genitals and abdomen).

  • The insula begins just above the sylvian fissure and wraps around the convexity, as follows:
    -tongue
    -face
    -thumb
    -hand

The aforementioned are enlarged compared to what follows because they require significant more muscular innervation to produce fine movements than the rest of the body.

-upper limb
-trunk
-lower limb
-foot (which ends just above what we’ll see is the cingulate gyrus (on the medial aspect of the brain)).

Cerebrum: Key Surface Anatomy

  • We present the 5 cerebral lobes a diamond formation, which should be a helpful mnemonic:
    Frontal up front.
    Parietal on top.
    Occipital in the back.
    Temporal at the bottom.
    Limbic in the center.

The Sylvian fissure (aka the lateral sulcus)

  • Distinguishes the cerebral lobes.

The central sulcus

  • Distinguishes the frontal and parietal lobes.

The parieto-occipital sulcus

  • Distinguishes the parietal and occipital lobes.

Brainstem anatomy

From superior to inferior:

Midbrain
Pons.
Medulla oblongata.

Cerebellum & Diencephalon

  • The cerebellum rests on the back of the brainstem.
  • The diencephalon lies within the center of the brain and contains the thalamus.
  • The corpus callosum is a prominent white matter pathway.

The precentral gyrus (the primary motor cortex)

  • Lies anterior to the central sulcus.

The postcentral gyrus (the primary sensory cortex)

  • Lies behind the central sulcus.

2 reliable indicators:

-The characteristic omega-shaped knob of the precentral gyrus
-The precentral gyrus is thicker than the postcentral gyrus

Optic pathway.

  • Optic nerves (anteriorly)
  • Optic chiasm (when they cross) and transition into the:
  • Optic tracts.

The pituitary gland

  • Lies beneath the optic chiasm.

The mammillary bodies

  • Are small but easily identifiable.