N375: cerebral cortex

What do association fibers connect?

Regions within the same hemisphere.

Difference between short vs long association fibers?

Short = within one lobe; long = across multiple lobes.

What do commissural fibers connect?

Left ↔ right hemispheres.

Main commissural structure?

Corpus callosum.

What do projection fibers connect?

Cortex ↔ subcortical structures (thalamus, brainstem, SC) through internal capsule

Where do projection fibers travel?

Through the internal capsule.

What is somatotopy in the posterior limb of the internal capsule?

Face → arm → trunk → leg (moving posteriorly).

What fibers run in the anterior limb of the internal capsule?

Thalamocortical projections.

What runs in the genu of the internal capsule?

Corticobulbar tracts (face/head motor control).

What runs in the posterior limb of the internal capsule?

Corticospinal tract.

Lesion to posterior limb causes what?

Contralateral paralysis of limbs.

Order of CST descent from cortex to SC?

Corona radiata → internal capsule → cerebral crus → pons → medullary pyramids → spinal cord.

Where are CST upper motor neurons located?

Layer V of motor cortex (giant pyramidal cells).

Which layer is just under the pia and contains few cells?

Layer I

Which layer contains small neurons for intracortical connections?

Layer II.

Which layer contains medium neurons and gives association + commissural fibers?

Layer III.

Which layer receives most thalamocortical input?

Layer IV.

What inputs terminate in Layer IV?

DCML, STT, trigeminothalamic, and all thalamic sensory projections.

Which layer is the major OUTPUT layer?

Layer V.

What cells are in Layer V?

Giant pyramidal cells forming CST, projections to brainstem, BG.

What does Layer VI do?

Sends projection + association fibers (feedback to thalamus).

What concept did stimulation/ablation studies reveal?

Functional localization—different cortical areas perform specific tasks.

Who mapped motor and sensory homunculi?

Wilder Penfield.

Where is Broca’s area located?

Left inferior frontal gyrus.

What is Broca's function?

Motor planning for speech production.

Damage to Broca’s area causes what?

Expressive aphasia: good comprehension, cannot produce fluent speech.

Where is Wernicke’s area located?

Left posterior superior temporal gyrus.

What does Wernicke’s area do?

Language comprehension (understanding spoken + written language).

Damage to Wernicke’s area causes what?

Receptive aphasia: fluent but nonsensical speech, poor comprehension, unaware of deficits.

What is the arcuate fasciculus?

White matter tract connecting Wernicke ↔ Broca.

What does arcuate fasciculus damage cause?

Conduction aphasia: normal spontaneous speech + comprehension, but cannot repeat phrases.

Function of the primary auditory cortex (A1)?

Hear and localize sound.

Summary of language?

1. Primary auditory cortex: hear + localize the sound

2. Wernicke: understand the meaning

3. Association fibers (long association from parietal → frontal)  to arcuate fasciculus send language plan

4. Brocas: formulate motor speech plan

5. Execute speech: M1 → UMN → LMN (speech muscles) 

What does primary sensory cortex do?

Receives raw sensory input; has topographic maps.

Role of association cortex?

Interprets sensory input; gives meaning to perception.

What happens if association cortex is damaged?

Sensation remains intact, but meaning is lost.

What is agnosia?

Inability to recognize objects despite normal sensation.

What is aphasia?

Disorder of language comprehension or production.

What is apraxia?

Motor planning disorder: can move but cannot perform purposeful sequences.

What functions depend on prefrontal association cortex?

Personality, behavior, executive function.

Lesions to limbic association cortex produce what?

Emotional + psychiatric disturbances (e.g., schizophrenia, depression).

What is tactile agnosia?

Can feel an object but cannot identify it. Due to damage of posterior parietal association cortex

What is astereognosia?

Failure to recognize objects by touch; “tactile amnesia.” due to damage of posterior parietal association cortex

What is cortical neglect (hemineglect)?

Ignoring one side of the body/world; severe form of parietal lesion (often right-sided).

What are the two major visual processing streams?

Dorsal (“where”) and ventral (“what”) pathways.

Pathway of dorsal stream?

V1 → posterior parietal cortex.

Damage to dorsal stream causes what?

Optic ataxia—difficulty reaching/grasping objects.

Pathway of ventral stream?

V1 —> inferotemporal cortex

What do lesions in auditory association cortex cause?

Auditory agnosias—can hear but cannot interpret.

What is amusia?

Inability to recognize music; usually right auditory hemisphere lesion.

What happens with bilateral auditory association destruction?

Hear sounds but cannot respond meaningfully.

Relationship between olfactory and gustatory association areas?

They intermix and project to limbic system → emotion-linked taste/smell.

What is the highest level of motor control? What areas involved?

Identification — deciding why and what to do. Prefrontal cortex + association cortex + posterior parietal cortex.

What is the second level of motor control? What areas involved?

Planning — converting idea → specific muscle sequence. Premotor cortex + supplementary motor area (SMA).

What is the lowest level of motor control?

Execution — sending commands from M1 → SC → LMN → muscles.

What is the effect of a UMN lesion?

Disinhibition of LMN → spastic paralysis.
Muscles overly contracted; little movement.

What is the effect of a LMN lesion?

Loss of LMN activity → flaccid paralysis.
Muscles completely relaxed; no tone.

What is a central motor program?

Pre-built movement pattern stored in CNS that runs automatically.

Why is central motor program called a “black box”?

You don’t consciously control each tiny movement detail.

What does the rubrospinal tract do?

Facilitates flexion of upper limbs (arm flexor pathway).

What do reticulospinal + vestibulospinal tracts do?

Facilitate extension of limbs (extensor pathways).

Where is the lesion in decorticate posturing?

Above the red nucleus (cortex or internal capsule).

Arm position in decorticate?

Flexed — rubrospinal tract still intact.

Leg position in decorticate?

Extended — reticulospinal + vestibulospinal tracts dominate.

Prognosis of decorticate relative to decerebrate posturing?

Better — higher (less severe) lesion.

Where is the lesion in decerebrate posturing?

Below the red nucleus (lower brainstem).

Arm position in decerebrate?

Extended — rubrospinal tract is dead → no arm flexion.

Leg position in decerebrate?

Extended — extensor tracts unopposed.

Prognosis relative to decorticate?

Worse — lower lesion affecting brainstem nuclei.