NEURO ANA - Structure of the Cerebral Cortex

Cerebral cortex

• Completely covers the cerebral hemisphere

• Composed of gray matter

• Contains approximately 10 billion neurons

Gray matter

What is the cerebral cortex composed of?

10 billion neurons (approx)

About how many neurons does the cerebral cortex contain?

• Gyri (ridges)

• Sulci/fissures (grooves)

What increases the surface area of the cerebral cortex?

Over the crest of a gyrus

Where is the cerebral cortex thickest?

In the depth of a sulcus

Where is the cerebral cortex thinnest?

1.5 to 4.5 mm

What is the thickness range of the cerebral cortex?

• Nerve cells

• Nerve fibers

• Neuroglia

• Blood vessels

The cerebral cortex consists a mixture of:

1. Pyramidal cells

2. Stellate cells

3. Fusiform cells

4. Horizontal cells of Cajal

5. Cells of Martinotti

Types of nerve cells present in the cerebral cortex:

Pyramidal cells

These are named from the shape of their cell bodies

10 to 50 µm

How long are pyramidal cells?

Betz cells

What are giant pyramidal cells also called?

120 µm

How long are Betz cells?

Motor precentral gyrus of the frontal lobe

Where are Betz cells found?

Faces the pial surface of the cortex

Which way do the apices of pyramidal cells face?

Thick apical dendrite

Extends upward toward the pia, giving off collateral branches

Basal dendrites

Extends laterally from the basal angles into the surrounding neuropil

Base of the cell body

From where does the axon of pyramidal cells arise?

• Projection fiber

• Association fiber

• Commissural fiber

The axon arises from the base of the cell body and more commonly enters the white matter as a:

Stelate cells

• Also known as granule cells

• small, polygonal in shape

• measures 8 µm in diameter

• have multiple branching dendrites

• short axon

Fusiform cells

• Long axis is vertical to the cortical surface

• Found mainly in the deepest cortical layers

• Dendrites: Arise from both poles of the cell body.

• Axon: arises from inferior part of the cell body

Superficial dendrite (of fusifom cells)

Ascends toward cortex surface, branches in superficial layers

Inferior dendrite (of fusifom cells)

Branches within the same layer

Axon (of fusifom cells)

Arises from inferior part of the cell body

Horizontal Cells of Cajal

• Small, fusiform, horizontally oriented cells found in the most superficial layers of the cortex.

• Dendrites: emerges from each end of cell

• Axon: runs parallel to the surface of the corteх

Cells of Martinotti

• Small, multipolar cells that are present throughout the levels of the cortex

• Dendrites: short

• Axon: directed toward the pial surface of the cortex, ends in a more superficial layer, commonly the most superficial layer. Gives origin to a few short collateral branches.

Radially and tangentially

How are nerve fibers in the cortex arranged?

Radial fibers

• Run at right angles to the cortical surface.

• Includes afferent entering projection, association, and commissural fibers (terminates within cortex)

• Axons of pyramidal, stellate, and fusiform cells, which leave the cortex to become projec tion, association, and commissural fibers of the white matter of the cerebral hemisphere

Tangential Fibers

• Run parallel to the cortical surface and are mostly collateral and terminal branches of afferent fibers.

• Include the axons of horizontal and stellate cells and collateral branches of pyramidal and fusiform cells.

Tangenial Fibers

• Most concentrated in layers 4 & 5

• They are referred to as the outer and inner Bands of Baillarger

Bands of Baillarger

Well developed in sensory areas

Stria of Gennari

In the visual cortex, the outer band of Baillarger, which is so thick that it can be seen with the naked eye, is known as the:

Striate Cortex

Visual cortex in the walls of the calcarine sulcus is sometimes called the:

Molecular layer (plexiform layer)

• Most superficial layer

• It consists of a dense network of tangentially oriented nerve fibers.

• It is where the large number of synapses between different neurons occur.

• Afferent fibers originating and occasional horizontal cells of Cajal are also present.

External granular layer

• This layer contains large numbers of small pyramidal cells and stellate cells.

• Dendrites: terminates in the molecular layer

• Axons: enter deeper layers and terminate/pass on to enter the white matter of the cerebral hemisphere.

External pyramidal layer

• Composed of pyramidal cells, whose cell body size increases from the superficial to the deeper borders of the layer.

Internal granular layer

• Composed of closely packed stellate cells with a high concentration of horizontally arranged fibers known collectively as the external band of Baillarger.

Ganglionic layer (internal pyramidal layer)

• This layer contains very large and medium-size pyramidal cells.

• A large number of horizontally arranged fibers form the inner band of Baillarger.

• →

Motor cortex (precentral gyrus)

Contains very large pyramidal cells called Betz cells

3%

Betz cells make up about how many % of corticospinal (pyramidal) tract projection fibers?

Multiform layer (layer of polymorphic cells)

• Mainly fusiform cells.

• Also contains modified pyramidal cells (triangular or ovoid bodies).

• Martinotti cells are prominent.

• Contains many nerve fibers entering or leaving the white matter.

6 layers

How many layers does homotypical cortex have?

Heterotypical cortex

What is cortex called if it does not have all 6 layers?

Granular type and Agranular type

What are the two types of heterotypical cortex?

Granular type

• Well-developed layers 2 & 4 (stellate cells, thalamocortical input)

• Poorly developed layers 3 & 5 (merge into single granular layer)

• Found in postcentral gyrus, superior temporal gyrus, hippocampal gyrus

Agranular type

• Poorly developed layers 2 & 4 (almost absent)

• Large, densely packed pyramidal cells in layers 3 & 5

• Found in precentral gyrus and frontal lobe areas (linked to motor function/ efferent fibers)

• Electrophysiology

• Histochemistry

• Immunocytochemistry

• EEG

• PET

• MRI

Which research techniques improved knowledge of cortical structure and function?

Cortical organization

• Organized into vertical functional units (columns), 300-600 µm wide.

• Each column extends through all six layers.

• Components: afferent fibers, internuncial neurons, efferent fibers.

• Function: In sensory cortex, each column has specific sensory function.

Signal pathways

• Afferent → direct to efferent OR via vertical chains of interneurons.

• Spread occurs via short-axon granular cells (to nearby columns) or horizontal cells of Cajal (to distant columns).

• This lateral spread aids in understanding sensory input.

Precentral (motor) area

• Located in precentral gyrus + parts of frontal gyri + paracentral lobule.

• Histology: Granular layers absent, pyramidal cells prominent.

• Contains giant pyramidal cells of Betz (up to 120 µm x 60 µm).

• Most concentrated in superior precentral gyrus & paracentral lobule.

• Fewer Betz cells anteriorly/inferiorly.

Corticospinal & corticobulbar fibers

Mostly arise from small pyramidal cells, not Betz cells

Brodmann's division

Considered oversimplified and misleading

Anatomical location

Cortical areas are named by _______ until better terminology is developed

Motor Speech Area

(Broca's area, Brodmann areas 44 & 45)

Location: inferior frontal gyrus near lateral fissure.

Dominance: usually left hemisphere (right in some individuals).

Function: formation of words via connections to primary motor areas

Prefrontal Cortex

(Brodmann areas 9-12)

Location: anterior to precentral area; includes superior, middle, inferior frontal gyri, orbital gyri, medial frontal gyrus.

Connections: to thalamus, hypothalamus, corpus striatum, cerebellum (via frontopontine fibers), opposite hemisphere (via corpus callosum).

Functions: concerned with the makeup of the individual's personality. This area plays a role as a regulator of the person's depth of feeling. It also exerts its influence in determining the initiative and judgment of an individual.

Parietal lobe

Contains the primary somesthetic area (SI) in the postcentral gyrus, which processes sensations like touch and temperature from the opposite side of the body.

Somesthetic association area

The superior parietal lobule integrates different sensory inputs, allowing for the recognition of objects by touch alone without needing to see them

Occipital lobe

Contains the primary visual area (Brodmann area 17) located along the calcarine sulcus, identifiable by its thin granular cortex.

Superior retinal quadrants

Project to the superior calcarine wall, while inferior quadrants project to the inferior wall.

Macula lutea

Occupies a large portion of area 17, with peripheral vision represented anteriorly.

Secondary visual areas (18 and 19)

Surrounds area 17, integrating visual input with past experiences for recognition.

Occipital eye fields

Within these areas, coordinate reflexive eye movements, while the frontal eye fields control voluntary scanning

Temporal lobe

• It is the primary auditory area (Brodmann 41, 42), lies in the gyrus of Heschl within the lateral sulcus, with area 41 as granular cortex and area 42 as an association area.

• It enables understanding of written and spoken language by integrating auditory, visual, and somatic information.

Unilateral lesion

Causes partial deafness in both ears, more severe in the contralateral one.

Secondary auditory area (area 22)

It is in the superior temporal gyrus interprets sounds integrates them with other sensory input.

Arcuate fasciculus.

Wernicke's area, in the dominant hemisphere, connects to Broca's area via the _________

Taste area (Brodmann 43)

Located at the lower end of the postcentral gyrus, superior wall of the lateral sulcus, and insula.

The vestibular area

Lies near the postcentral gyrus region for facial sensation, opposite the auditory area.

Insula

• lies deep within the lateral sulcus, forming its floor and visible only when the sulcus is opened

• is thought to play a role in planning and coordinating speech articulation.

Primary sensory (granular) and Motor (agranular) areas

Form only a small part of the cortex, while most of it is homotypical cortex with six layers

Association areas

These larger regions, called __________, handle integration, behavior, and interpretation sensory input.

• Prefrontal

• Anterior temporal

• Posterior temporal

Three main association areas exist:

Anterior temporal cortex

Aids in storing past sensory experiences

Posterior parietal cortex

• Integrates vision, touch, and proprioception into concepts like size, form, and texture (stereognosis)

• It also builds body image awareness and spatial orientation, with each hemisphere representing the opposite side of the body.

Cerebral Dominance

The two cerebral hemispheres appear anatomically similar, with contralateral pathways and commissures allowing information transfer.

Left-hemisphere dominance

Over 90% of adults are right-handed, reflecting :

96%

Around how many % are left-hemisphere dominant for speech.

Left pyramid

Research shows more corticospinal fibers cross from the ________ , explaining righthand dominance.

The speech area

this is larger in the left hemisphere in adults, but newborn hemispheres start as equipotential.

Hemispheric dominance

Develops during childhood, becoming fixed after the first decade, explaining children's adaptability after brain injury.

Tumors, strokes, surgery, or head injuries

Studies of cerebral cortex function in humans come from patients with

Primary motor cortex

• Lesions of the __________ in one hemisphere cause paralysis of the contralateral extremities, especially affecting fine and skilled movements

• If both areas are destroyed, complete contralateral paralysis occurs.

Jacksonian epileptic seizure

Arises from irritation of the primary motor area

Convulsions

Begin in the body part represented in the irritated motor cortex region

Seizures

May remain localized (face, foot) or spread to other regions depending on cortical irritation

Muscle spasticity

A small lesion in the primary motor cortex (area 4) that causes little change in muscle tone

Frontal Eye Field

• A lesion in one _____________ impacts eye movement.

• A destructive lesion causes eyes to deviate toward the lesion's side.

• This makes it impossible to turn the eyes to the opposite side.

Expressive Aphasia

Damage to the left inferior frontal gyrus, known as Broca's area, causes _________

Broca’s Aphasia

This condition affects speech production without impairing language comprehension.

Receptive Aphasia

• Damage to Wernicke's area causes _____________, a condition where a person can't understand spoken or written language

• While their speech remains fluent, they are unaware that the words they use are often incorrect or nonsensical.

• This is because Broca's area, which controls speech production, is not affected.

Global aphasia

Lesions affecting both Broca's and Wernicke's areas lead to ______________, causing a loss of both speech production and comprehension.

Insular damage

Patients with ____________ struggle to pronounce phonemes in the correct order, often making sounds that are close to the intended word but are not quite right.

Angular gyrus

Damage to the __________ part of the posterior parietal lobe, severs the connection between the visual association area and Wernicke's area.

Dominant Angular Gyrus

This specific lesion leads to a condition where patients lose the ability to read (alexia) and write (agraphia).

Prefrontal Cortex

• Damage to the___________ doesn't cause a loss of intelligence, but it impairs judgment and personality.

• A person with this damage may lose initiative, become euphoric, and act carelessly.

• They also stop conforming to accepted social behavior.

Schizophrenia

Reduced dopamine innervation to the prefrontal cortex may contribute to symptoms of _________

Frontal lobotomies and leukotomies

• Were once surgical procedures used to reduce emotional responses in patients.

• By severing connections in the frontal lobe, these operations lessened introspection and the ability to consider the future.

Thalamus

Relays sensory signals to the cerebral cortex for detailed analysis like spatial intensity recognition.

Primary somesthetic area

Damage to the ___________ of the cortex leads to sensory disturbances on the opposite side of the body, particularly in the limbs.

Astereognosis

Damage to the superior parietal lobule impairs the ability to integrate touch and pressure, leading to a condition called