An understanding of neuroanatomy is important to success as a neurologist and equally as important for success on neurology shelf, in-service, and board examinations. If you are more interested in reviewing the various syndromes seen with focal cortical lesions as opposed to basic neuroanatomy please check out the “Anatomic Syndromes” chapter.

Authors: James Eaton, MD, and Brian Hanrahan MD

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Cortex

    • The majority of the cortex has 6 cortical layers

          • The six layers from superficial to deep:
            1. Molecular
            2. External granular
            3. External pyramidal
            4. Internal granular
            5. Internal pyramidal (ganglion)
            6. Multiform.

      • The cortex is organized into 5 lobes: frontal, parietal, temporal, occipital, and limbic.
        • Frontal lobe: Bound by the central sulcus and lateral fissure.
        • Parietal lobe: Bound by the central sulcus, lateral fissure, and parieto-occipital fissure.
        • Temporal lobe: Bound by the Sylvian fissure and preoccipital notch.
        • Occipital lobe: Bound by the parieto-occipital sulcus and preoccipital notch.
        • Limbic lobe: Consists of the parahippocampal, cingulate, and subcallosal gyri. The limbic lobe interacts with various other structures that are part of the limbic system.
      Diagram of the lobes of the cortex, exam review
      Lobes of the brain

Frontal Lobe

  • There are four major gyri: precentral, superior frontal, middle frontal, and inferior frontal.
    • The precentral gyrus is the primary motor strip.
    • The motor homunculus of the precentral gyrus:
By mailto:[email protected] – File:Homunculus-ja.pngTranslated from Image:Homunculus-de.svg by Was a bee., CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49377875
  • The superior frontal contains the supplementary motor area (SMA).
  • The middle frontal contains the frontal eye fields which are necessary for voluntary saccadic eye movements.
  • The inferior frontal is organized into the pars orbitalis, pars triangularis, and pars opercularis.
    • Pars opercularis and triangularis are associated with Broca’s area.
    • Pars orbitalis has a role in thought, cognition, and planning behavior.

 

Diagram of The Frontal Lobe and Associated Gyri/sulci
The Frontal Lobe and Associated Gyri/sulci

Parietal Lobe

  • There are five principal parts: post-central gyrus, superior parietal lobule, inferior parietal lobule, the precuneus, and the posterior portion of the paracentral lobule.
    • The postcentral gyrus contains the primary sensory cortex.
    • The sensory homunculus of the postcentral gyrus:
By OpenStax College – Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148008
  • The superior parietal lobule contains the somatosensory association area.
  • The inferior parietal lobule has two components: angular and supramarginal gyri
    • This is the sensory association cortex and has a role in perception, vision, reading, and speech.
    • A lesion to this region can lead to Gerstmann’s syndrome.
  • The precuneus is an area of cortex just anterior to the occipital lobe on the medial surface. It has a broad spectrum of functions including visuospatial processing, memory, and first-person perspective.
    • It is an early region of atrophy in Alzheimer’s dementia
  • The posterior portion of the paracentral lobule is regarded as a tertiary somatosensory cortex involved in stereognosis.
    • Stereognosis: the perception, understanding, recognition, and identification of an object by touch.
      • Tested by having the patient feel an object and identify it, such as a paperclip or set of keys.
      • It is often accompanied by other deficits like agraphesthesia.

Occipital lobe

  • The occipital lobe is divided into the cuneus and lingual gyri by the calcarine fissure.
  • The primary visual cortex, also called the striate cortex, is within the banks of the calcarine fissure.
  • The secondary visual cortex is a visual association area necessary for visual processing and depth. It lies dorsal and ventral to the primary visual cortex.
  • The tertiary visual cortex is where color, motion, and depth are processed. It is located anterior to the secondary visual cortex.

The Limbic system

  • Responsible for emotion, behavior, and long-term memory formation.
  • The limbic system is comprised of multiple structures including the limbic lobe (parahippocampal, cingulate, and subcallosal gyri), amygdala, hippocampus, mammillary bodies, and anterior thalamus.

Papez Circuit

  • The Papez circuit is a collection of structures of the limbic system that connect the limbic lobe and the hypothalamus.

Cingulate gyrus

  • Lies immediately above the corpus callosum.
  • Receives input from the thalamus and surrounding cortex.
  • Projects fibers via the cingulum to the parahippocampal gyrus.
  • Unilateral damage to the cingulate gyrus can lead to apathy or cognitive dysfunction. Bilateral damage can cause akinesis and mutism.

Parahippocampal gyrus

    • Represents the cortex that surrounds the hippocampus.

  • Axons from the parahippocampal gyrus project to the hippocampus.

Hippocampus

    • Located in the inferomedial temporal lobe.

  • Associated syndromes:
    • Gliosis and neuronal loss of the hippocampal CA1 pyramidal cell layer can lead to mesial temporal sclerosis (MTS) and focal epilepsy.
        • Presents with focal seizures with impaired awareness.
        • Area CA1 is particularly sensitive to ischemic damage

    • Lesions to bilateral hippocampi can lead to profound anterograde amnesia.
  •  
Neuropathology slide showing CA1 Atrophy in Mesial Temporal Sclerosis
Normal hippocampus of a rat (left) compared to a hippocampus in a rat with neuronal degeneration secondary to transient global ischemia (right). Note the cell loss within the CA1 region.

Fornix

  • The major outflow white fiber tract from the hippocampus synapse at the mammillary bodies.
  • Located below the splenium of the corpus callosum.

Mammillary bodies

  • The mammillary bodies are round, paired structures located on the inferior surface of the hypothalamus.
  • The mammillothalamic tract connects the mammillary nucleus to the anterior nucleus of the thalamus.
  • Mammillary body dysfunction occurs in the setting of Wernicke’s encephalopathy.

Anterior nucleus of the thalamus

  • The anterior thalamic nucleus projects to the cingulate cortex through the thalamocingulate fibers, completing the Papez’s circuit.

Temporal Lobe

  • The lateral surface of the temporal lobe has three gyri: superior, middle, and inferior. They are separated by the superior, middle, and inferior sulci.
By Sebastian023, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20988757
  • The superior temporal gyrus is the region associated with language comprehension.
    • When injured in the dominant hemisphere it leads to a Wernicke’s aphasia.
    • The middle and inferior temporal gyri have a role in formed vision and processing.
    • The fusiform gyrus (also called the occipitotemporal gyrus) is necessary for facial recognition.
      • Lesions here can cause prosopagnosia/visual agnosia and Capgras delusion.
        • Prosopagnosia: The failure to visually identify objects and faces.
        • Capgras delusion is a variant of prosopagnosia and psychosis where the patient has the belief that close friends or family are replaced by an imposter.

Major White Matter Tracts

  • There is an incredible number of connections organized throughout the brain, with several distinct axonal bundles that have well-defined functions.

U-fibers:

  • Also called arcuate fibers, these link one gyrus to another.
  • Most leukodystrophies (errors in the initial creation of myelin) will characteristically “spare the U-fibers” as opposed to a demyelinating process (damage to existing myelin) that will not.

Long association fibers

  • Connect to different ipsilateral regions in the brain:

Arcuate fasciculus

  • Links Wernicke’s and Broca’s areas.
  • If damaged patients will develop conduction aphasia.

Commissural fibers

  • Connect contralateral cerebral hemispheres:

Corpus callosum

  • Organized into four segments; the rostrum, genu, body, and splenium.
    Sagittal view of the corpus callosum on MRI
    Sagittal view of the corpus callosum on MRI
  • Associated syndromes:
    • Alexia without agraphia: Occurs secondary to infarction of the splenium of the corpus callosum and left occipital lobe.
    • Agenesis of the corpus callosum: A midline patterning defect that can be seen with other developmental abnormalities.

Anterior commissure

  • Connects the olfactory bulbs, amygdala, and basal forebrain.

Posterior commissure

  • Connects language processing centers from both hemispheres.

Hippocampal commissure

  • Aides in memory formation.

Projection fibers

  • Link the brain and spinal cord.
  • They can be either afferent or efferent.
  • The internal capsule is the most important.
    • It is divided into three regions:
      • Anterior limb: Has multiple different radiations but includes many thalamocortical fibers.
      • Genu: Contains the corticobulbar tract (cortex to the brainstem).
      • Posterior limb: Contains corticospinal (axons from the primary motor cortex), somatosensory, and corticopontine fibers.

References

  1. Bartsch T, Butler C. Transient amnesic syndromes. Nat Rev Neurol. 2013;9(2):86-97.
  2. Campbell WC. Dejong’s The Neurologic Exam. 7th ed. Philadelphia, PA: Lippincott; 2013.
  3. Crossman AR, Neary D. Cerebral Hemisphere, and Cerebral Cortex. In: Neuroanatomy: An Illustrated Colour Text. 5th ed. Philadelphia, PA: Elsevier; 2015:131-145.
  4. Mega MS, Cummings JL. “Frontal-Subcortical Circuits and Neuropsychiatric Disorders.” The Journal of Neuropsychiatry and Clinical Neurosciences, vol. 6, no. 4, 1994, pp. 358–370., doi:10.1176/jnp.6.4.358.
  5. Mancall EL. Diencephalon. In: Gray’s Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience. Philadelphia, PA: Saunders; 2011:261-278.
  6. Waxman SG. The Limbic System in Clinical Neuroanatomy, 28e New York, NY: McGraw-Hill.
  7. Willis MA, Haines DE. The Limbic System. In: Fundamental Neuroscience for Basic and Clinical Applications. 5th ed. Philadelphia, PA: Elsevier; 457-467.

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