The cerebral cortex, basal ganglia, and meninges

Abstract

The brain is the most complex organ of the body and is responsible for cognition as well as the coordination of most of the body's vital processes. Together with the spinal cord, it forms the central nervous system. The brain is protected by the meninges and the neurocranium. Physiologically, the brain functions to control both conscious and unconscious processes by generating action potentials that travel through axons connected to the spinal cord and the peripheral nervous system.

For information on the blood supply of the brain, see cerebrovascular system.

Cerebral cortex

The cerebral cortex contains gyri that are separated by sulci. Deep sulci separate the cerebral cortex into different parts (frontal, temporal, parietal, and occipital lobes). The main sulci are:

Frontal lobe

Area Location Characteristics (motor functions and cognition) Effect of lesion

Primary motor cortex

(Brodmann area 4)

  • Primary motor control of the corticospinal tract
  • Motor homunculus
    • Body parts of higher sensitivity or fine motor skills are represented in larger areas than others
    • Arranged medially to laterally as follows: toes, ankle, knee, hip, trunk, shoulder, elbow, wrist, hand, little finger to thumb, eye, facial expression, mouth, chin, tongue, swallowing

Premotor cortex

(Brodmann area 6)

Frontal eye field

(Brodmann area 8)

  • Posterior part of the middle frontal gyrus
  • Controls voluntary saccadic eye movements
  • Stimulation causes contralateral conjugate deviation of the eyes
  • Transient ipsilateral conjugate deviation of the eyes

Prefrontal cortex

(prefrontal association area)

  • Connected to the mediodorsal nucleus of the thalamus
  • Contain Brodmann areas 8–14, 24, 25, 32, and 45–47

Broca area

(Brodmann area 44 and 45)

  • Posterior part of the inferior frontal gyrus (dominant hemisphere)
  • Speech area
  • Connected to Wernicke area via the arcuate fasciculus
  • Broca aphasia:
    • Nonfluent, telegraphic, and grammatically incorrect speech
    • Intact comprehension of simple language

Parietal lobe

Area Location Characteristics (sensory functions) Effect of lesion

Primary somatosensory cortex

  • Receives input from the ventral posterior nuclei of the thalamus
  • Contributes to the corticospinal tract
  • Sensory homunculus
    • Body parts of higher sensitivity or fine motor skills are represented in larger areas than others
    • Arranged medially to laterally as follows: genitals, toes, feet, leg, hip, trunk, neck, head, shoulder, arm, elbow, forearm, wrist, hand, little finger to thumb, eye, nose, face, lips, oral cavity, tongue, pharynx
  • Lesions cause contralateral loss of tactile discrimination (e.g., astereognosis)
Somatosensory association cortex
  • Located in the superior parietal lobule and the supramarginal gyrus
  • Integrates palpatory sensory information for pattern recognition
  • Lesions cause
    • Contralateral loss of tactile discrimination (e.g., astereognosis)
    • Stereognosis (inability to recognize shape)
    • Statognosis (inability to recognize the position of one's own body parts)
    • Apraxia (e.g., inability to manipulate objects, but ability to explain what object is used for is retained)
    • Facial apraxia (most common type)
    • Conduction apraxia (e.g., inability to repeat verbal commands)
      • Due to damage of the arcuate fasciculus
  • Unilateral injury to the somatosensory association cortex causes hemispatial neglect syndrome (the deficit is contralateral to the lesion)
Visual association cortex
  • Located in the angular gyrus
  • Integrates visual sensory information for pattern recognition
  • Lesions (if dominant hemisphere) cause Gerstmann syndrome
    • Inability to recognize right or left
    • Finger agnosia (cannot recognize one's own or others' fingers)
    • Agraphia (inability to write one's own thoughts)
    • Dyscalculia (inability to do calculations: addition, subtraction, division, etc.)

Temporal lobe

Area Location Characteristics (hearing) Effect of lesion

Primary auditory cortex

(Brodmann areas 41 and 42)

Wernicke's area
  • Posterior part of the superior temporal gyrus

Occipital lobe

Area Location Characteristics (visual functions) Effect of lesion
Primary visual cortex
  • Part of the striate cortex (Brodmann area 17)
  • Receives input from the lateral geniculate body via optic radiations (also known as visual radiations, geniculocalcarine tract)
  • Controls involuntary pursuit and tracking eye movements
  • Stimulation causes contralateral conjugate deviation of the eyes
Secondary visual cortex
  • Integrates visual properties such as color, position in space, and illusory contours

Internal capsule

The internal capsule is the largest collection of nerve fibers that travels to and from the cerebral cortex.

Structure Characteristics
Anterior limb
  • Contains the thalamocortical tract
Posterior limb
Genu
  • Contains the corticobulbar tract

References:[1]

Meninges

The meninges are 3 layers of connective tissue that cover and protect the brain and the spinal cord. They are divided into the dura mater, arachnoid mater, and pia mater (from outer to inner layer). In addition, the meninges, particularly the dura mater, fold to form septae which include the falx cerebri, falx cerebelli, and tentorium cerebelli.

Meningeal layer Origin Anatomy Function
Dura mater
  • Thick and strong outer layer
  • Adheres to bone (e.g., cranium)
  • Forms the epidural space
    • Potential space between bones of the cranium and the dura mater
    • Contains blood vessels and fat tissue
Arachnoid mater
Pia mater
  • Thin inner layer
  • Adheres to the brain and the spinal cord

Falces of the brain

Structure Location Characteristics Attachments Sinuses
Falx cerebri
  • Separates the cerebral hemispheres
  • Crista galli of the ethmoid bone anteriorly
  • Internal occipital protuberance posteriorly
  • Midline of the cranium superiorly
  • Superior surface of the tentorium cerebelli inferiorly
Falx cerebelli
  • Floor of the posterior cranial fossa
  • Separates the two cerebellar hemispheres
  • The posterior attachment contains the occipital sinus
Tentorium cerebelli
  • Separates the cranial cavity into infratentorial and supratentorial
  • Falx cerebri on the midline
  • Anterior clinoid processes of the sphenoid bone anteriorly
    • Forms the lateral part of the cavernous sinus
  • Petrous part of the temporal bone bilaterally (anterolateral margins)
  • Posteroinferior angles of the parietal bone and transverse sulci of the occipital bone (posterolateral margins)
  • Internal occipital protuberance posteriorly
  • The midline attachment with the falx cerebri contains the straight sinus
  • The posterolateral attachments contain the transverse sinus


References:[2][1][3][4][5]

Basal ganglia

The basal ganglia are a group of nuclei that lie beneath the cortex. They are distributed over the telencephalon, diencephalon, and mesencephalon (midbrain). They aid in the initiation of movement and control of skeletal muscles. The basal ganglia have two main pathways: the direct pathway and the indirect pathway.

Basal ganglia Structures Location
Striatum
  • Caudate nucleus
  • Putamen
Lentiform nucleus
  • Globus pallidus
  • Putamen
Subthalamic nucleus
Substantia nigra

Direct pathway

Substantia nigradopamine release → stimulates D1 receptor in striatumGABA release → inhibits globus pallidus internus (GPi) → no GABA release → disinhibition of thalamusmovement

Indirect pathway

Substantia nigradopamine release → stimulates D2 receptor in striatumGABA release → inhibits globus pallidus externus (GPe) → no GABA release → disinhibits subthalamic nucleusstimulation of GPiinhibits thalamusmovement

Nigrostriatal pathway

Microscopic anatomy

The nervous system is primarily composed of neurons and supporting (glial) cells. Neurons are specialized cells that are polarized and have the ability to pass signals from one cell to the other. For more information on the microscopic anatomy of the nerve tissue, see the learning card on nerve tissue.

Layers of the cerebral cortex

Most of the brain is composed of six layers. These areas are referred to as the neocortex. The hippocampus and the olfactory cortex are only composed of three layers and together are referred to as the allocortex. In addition, the cerebral cortex is divided into 47 Brodmann areas.

  • The neocortex is formed by the following layers (from external to internal)
    • Molecular layer
    • External granular layer
    • External pyramidal layer
    • Internal granular layer → termination area of thalamocortical projections (form lines of Gennari in the primary visual cortex)
    • Internal pyramidal layer → area where axons of the corticospinal and corticobulbar tracts form
    • Multiform layer → composed of polymorphic cells

Blood-brain barrier

  • Function: protects the central nervous system from microorganisms, cells, proteins, and drugs that can cause damage to the brain and other structures.
  • Structure: made up of four components
  • Transport mechanisms: Substances can be transported across the blood-brain barrier via
    • Ion channels (e.g., sodium, potassium)
    • Selective transport (amino acids, glucose, vitamin K, vitamin D)
    • Diffusion (lipid soluble substances: oxygen and carbon dioxide)

Blood-cerebrospinal fluid barrier (BCSFB):

  • Function: separates cerebrospinal fluid from the main circulation
    • Impermeable to cells, almost all proteins, and toxins (e.g., hydrophilic medications, bacteria)
    • Selectively permeable to substances, including glucose and electrolytes
  • Structure: It is formed by the choroid plexus.

References:[6][6]

Embryology

The CNS is derived from 3 primary vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These go on to develop into 5 subdivisions: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

Primary vesicles Brain vesicle Derived parts of the brain Derived fluid-filled structure

Forebrain

(prosencephalon)

Telencephalon
Diencephalon

Midbrain

(mesencephalon)

Mesencephalon

Hindbrain

(rhombencephalon)

Metencephalon
Myelencephalon
  • Medulla
  • Spinal cord
  • Central canal

For more information on the embryology of the nervous system see neurulation, and spinal cord tracts and reflexes.

References:[6]

  • 1. Murray E, Wise S, Graham K. The Evolution of Memory Systems. Oxford, UK: Oxford University Press; 2016.
  • 2. Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • 3. Bell DJ, Gaillard F. Falx Cerebri. https://radiopaedia.org/articles/falx-cerebri. Updated May 1, 2018. Accessed July 23, 2018.
  • 4. Murphy A, Maingard J. Falx Cerebelli. https://radiopaedia.org/articles/falx-cerebelli. Updated July 1, 2017. Accessed July 23, 2018.
  • 5. Murphy A, Maingard J. Tentorium Cerebelli. https://radiopaedia.org/articles/tentorium-cerebelli. Updated July 1, 2017. Accessed July 23, 2018.
  • 6. Kaplan. USMLE Step 1 Anatomy Lecture Notes 2016. Kaplan Publishing; 2015.
  • Ghetti G. Putaminal hemorrhages. Front Neurol Neurosci. 2012; 30: pp. 141–144. doi: 10.1159/000333617.
last updated 11/18/2018
{{uncollapseSections(['bqcHCW0', 'Xqc9CW0', 'WqcPxW0', 'dqcoxW0', 'cqcaxW0', '1qc2xW0'])}}