Cerebral Cortex and Thalamocortical Relationships (Week 4--Houser)

Question 1

Diencephalon

Answer 1

1) Thalamus: one on each side of 3rd ventricle
2) Subthalamus (subthalamic nuclei mentioned in basal ganglia)
3) Hypothalamus: immediately ventral to the thalamus
4) Epithalamus: habenular nucleus and pineal

Question 2

Divisions of the thalamus

Answer 2

Anterior: relay nucleus (thalamus to cortex and back)

Medial: relay nucleus; goes to reticular activating system; mediodorsal (MD) nucleus

Lateral: relay nucleus; VA, VL, LD, LP, VPL, VPM

Internal medullary lamina: thin band of myelinated fibers separating divisions

Intralaminar nuclei: small nuclei located within internal medullary lamina

Question 3

Which regions and pathways have “relays” in the thalamus?

Answer 3

Almost all!

Sensory pathways

Connections from cerebellum

Connections from basal ganglia

Connections from cerebral cortex

Question 4

What determines which regions of the cortex are specialized for which functions?

Answer 4

Connections with the thalamus are important in determining function of each cortical area

Specific types of sensory and motor information are relayed to specific regions of the thalamus then neurons of thalamus project to specific regions of the cortex

Question 5

Major functional areas of the cerebral cortex and Brodmann’s areas and lobes

Answer 5

Primary motor = area 4 = frontal lobe

Primary somatosensory = areas 3, 1, 2 = parietal lobe

Primary visual = area 17 = occipital lobe

Primary auditory = area 41 = temporal lobe

Question 6

Additional motor areas of the cortex

Answer 6

Supplementary motor area (SMA) = area 6 = frontal lobe = programming/planning complex set of movements and may be activated by “mental rehearsal” (more medial/midline on cortex)

Premotor cortex = area 6 = frontal lobe = involved in preparing for a movement, ready set go, and movements guided by external stimuli such as reaching for a visible object (more lateral on cortex)

Question 7

“Association cortex”

Answer 7

The remainder of the brain

Determined by types of info integrated (associated) in the region

Prefrontal cortex, language areas?

Question 8

Prefrontal cortex

Answer 8

“Executive functions”

“Association area” because receives inputs from many other regions of the cortex (as well as thalamus and limbic areas)

Rostral to motor areas and much of the frontal lobe

Dorsal and lateral prefrontal cortex: planning, problem solving, working memory, maintaining attention

Orbital and medial prefrontal cortex: self-control, suppressing inappropriate responses

Question 9

Language areas (Perisylvian language zone)

Answer 9

Broca’s area: motor or expressive speech area; posterior part of inferior frontal gyrus, opercular and triangular parts (areas 44, 45)

Wernicke’s area: sensory or receptive spech area; posterior part of superior temporal gurys and parietal-occipital-temporal junction

Connected through arcuate (superior longitudinal) fasciculus

Question 10

Which hemisphere are language areas located in?

Answer 10

Left hemisphere in nearly all right handed people and half of left handed people

Called dominant hemisphere if that’s where language is controlled

Question 11

Aphasia

Answer 11

Disturbance of formulation or comprehension of language, not a disorder of hearing, vision or motor control

Broca’s aphasia

Wernicke’s aphasia

Global aphasia

Conduction aphasia

Question 12

Broca’s aphasia

Answer 12

“Broken Boca”

Nonfluent, motor or expressive aphasia (with intact comprehension)

Question 13

Wernicke’s aphasia

Answer 13

“Wernicke’s is wordy but makes no sense–what?”

Fluent, sensory or receptive aphasia (with impaired comprehension)

Question 14

Global aphasia

Answer 14

Wernicke’s and Broca’s areas are damaged

Nonfluent aphasia with impaired comprehension

Severe loss of all language skills

Question 15

What happens if you have damage to the non-dominant (right) hemisphere?

Answer 15

Lesion to right parietal association cortex can produce contralateral neglect syndrome

Neglect the left side of body or space

Note: lesion on left (dominant) hemisphere does not produce severe neglect on right side

Question 16

What sorts of functional deficits occur if there is damage to the parietal-occipital-temporal association cortex?

Answer 16

Agnosia: inability to recognize object with a particular sense even though that sense is intact (no loss in vision but can’t recognize an object by sight)

Apraxia: inability to perform a learned motor skill on command even though no primary motor deficits (can perform same motor activity in different context); occur most commonly when lesion in dominant hemisphere

Question 17

Basic pathway for visual information

Answer 17

Pathway referred to as geniculocalcarine tract or optic radiations

1) Ganglion cells in retina of each eye terminate in different layers of lateral geniculate nucleus (6 layers total)
2) Neurons of LGN project to primary visual area (striate cortex) on medial surface of cortex and surrounding the calcarine fissure (sulcus)

Question 18

Superior colliculus

Answer 18

Not in the direct visual path from retina to cortex

Associated with visual function because part of reflex center for orienting the eyes and head in response to visual stimulation (so must receive visual stimuli)

Some retinal fibers and collaterals from retinogeniculate axons and info from primary visual cortex and other, nonprimary, regions of visual cortex send stimuli to superior colliculus

Question 19

Contralateral or ipsilateral relationship between visual field and visual cortex?

Answer 19

Contralateral

Crossing of axons from nasal retina in optic chiasm so left visual field represented in right visual cortex

Question 20

Cortical afferents

Answer 20

Info comes into the cortex from thalamus, other cortical areas and chemically defined nuclei in brainstem (NE, serotonin, dopamine) and basal forebrain (cholinergic)

Question 21

Basal nucleus of Meynert

Answer 21

Cholinergic neurons here (in forebrain) project to virtually all areas of cerebral cortex

Neurons here also are lost in patients with Alzheimer’s disease

Question 22

Septal nuclei and nucleus of diagonal band

Answer 22

Cholinergic neurons here project to hippocampal formation

Neurons in these brain regions degenerate in patients with Alzheimer’s disease, and loss is accompanied by marked reductions in choline acetyltransferase (enzyme that synthesizes ACh, marker for cholinergic neurons)

Question 23

Cortical efferents

Answer 23

Projections from the cortex go to numerous subcortical regions such as the thalamus, striatum, brainstem and spinal cord

Also project to other cortical areas: same region of the opposite hemisphere (callosal fibers) and to other regions of the same hemisphere (association fibers)

Question 24

Posterior limb of internal capsule

Answer 24

Both motor and somatosensory areas are grouped together here so damage will produce both motor and sensory problems

Question 25

What will damage to motor areas of cerebral cortex or descending axons from these areas cause?

Answer 25

Upper motor neuron signs: paralysis, paresis, spasticity

Depends on location and extent of lesion

Not usually paralysis of limbs, but voluntary movements cannot be “fractionated” into normal patterns

Question 26

Explanation for spasticity

Answer 26

Not completely understood

In general, loss of proper balance of facilitation and inhibition at both alpha and gamma motor neurons

Inhibitory control of motor neurons by descending pathways is decreased

Not damage of corticospinal tract alone but damage to multiple descending pathways that could include projections from cortex to brainstem motor centers (red nucleus, reticular formation, etc) or pathways within the brainstem itself

Question 27

Two cell types of cerebral cortex

Answer 27

1) Pyramidal cells: apical dendrites coming out the top of cell body toward pial surface of cortex and basilar dendrites from base of cell body horizontally; project to thalamus, brainstem, spinal cord, other cortical areas; primary projection/output of cerebral cortex; excitatory actions on targets
2) Non-pyramidal cells (stellate or granule cells): small, axons of most remain within cerebral cortex so they’re interneurons or local circuit neurons; most use GABA and are inhibitory but not all; basket cells contact cell bodies and proximal dendrites of pyramidal neurons to control excitability of output neurons (remember, this is an influential place to be!); cnahdelier cells form axo-axonic contacts with axon initial segments of pyramidal neurons (one neuron can have wide effect on function!)

Question 28

Layers of the cerebral cortex

Answer 28

Neocortex has 6 layers

Each of the 6 layers has combination of pyramidal and nonpyramidal neurons (but different predominance in each layer)

Olfactory region and hippocampus have only 3 layers

Question 29

6 layers of the cerebral cortex

Answer 29

Molecular layer: not many cell bodies (Layer I)

External granular layer (Layer II)

Externalpyramidallayer: project to cerebral cortex of opposite hemisphere (Layer III)

Internalgranularlayer:receive information (Layer IV)

Internalpyramidallayer:project information; project to subcortical regions (striatum or spinal cord) (Layer V)

Multiformlayer: wide variety of cell types (Layer VI)

Question 30

Layers in primary motor vs. sensory areas of cortex

Answer 30

In primary motor area, have poorly developed layer IV; “agranular”

In sensory areas, have poorly developed layer V; “granular cortex”

Question 31

Laminar organization in the cerebral cortex

Answer 31

There is specificity in the sites to which neurons in the different layers project

Question 32

Columnar organization in the cerebral cortex

Answer 32

Axons and apical dendrites have vertical organization and there is synaptic interaction between different layers

Info comes in from afferent cell, goes to interneuron in layer IV which sends projections to spines of a pyramidal cell –> pyramidal cell sends projections down/out but also sends collaterals to GABAergic neurons that can inhibit pyramidal cells on either side of this functional column –> more specificity

Vertical columns (functional columns) from cortical surface to white matter

Neurons in given column respond to same stimulus (Ex: some columns activated by touch and others by position of joint)

Prominent feature of visual cortex

“Sharpened” by inhibitory neurons that influence other neurons at edges of columns (basket cells)

This is how we get functional specifications within the areas we’ve been talking about

Question 33

Conduction aphasia

Answer 33

Lack of communication between Broca’s and Wernicke’s areas

Question 34

Blood supply territories of the brain

Answer 34

Middle cerebral artery: lateral hemisphere surfaces, temporal and frontal lobes, striatum and globus pallidus (specifically deep branches of MCA/lenticulostriate arteries); affects Broca’s, Wernicke’s, primary motor and primary somatosensory, auditory

Anterior cerebral artery: front and center; oflactory bulbs, primary motor and primary somatosensory, corpus callosum

Posterior cerebral artery: back, part of temporal lobe and occipital lobe, visual, thalamus

Question 35

Thalamocortical relationships

Answer 35

Basal ganglia and cerebellum –> VA and VL of thalamus –> motor cortex of cerebral cortex

Spinothalamic tract and medial lemniscus and trigeminal lemniscus –> VPL and VPM of thalamus –> somatosensory cortex of cerebral cortex

Substantia nigra, ventral pallidum, olfactory cortex, amygdala –> MD of thalamus –> prefrontal cortex of cerebral cortex

Optic tract –> LG –> primary visual cortex of cerebral cortex

Inferior colliculus and lateral lemniscus –> MG –> primary auditory cortex

Question 36

Optic radiations and Meyer’s Loop

Answer 36

From LGN, upper fibers take care of lower visual field and lower fibers take care of upper visual field

Question 37

Terminology for visual field defects

Answer 37

Side of field: right vs. left visual field

Region of field: homonymous (same field for each eye) vs. temporal (bitemporal) vs. nasal (binasal)

Amount of field: hemianopia (half) vs. quadratanopia (superior or inferior)

Question 38

What are fibers that eventually make up internal capsule called higher up?

Answer 38

Centrum semiovale –> corona radiata –> internal capsule

Question 39

3 parts of internal capsule

Answer 39

Anterior limb: frontopontine fibers, anterior thalamic radiation (sensory??)

Genu: corticobulbar fibers?

Posterior limb: corticopontine fibers, superior thalamic radiation, corticofungal fibers, corticospinal fibers (motor??)

Question 40

Pyramidal vs. granular cell

Answer 40

Pyramidal cells are projecting (layer V)

Granular cells are receiving (layer IV)

Question 41

Basket cells

Answer 41

GABAergic interneurons in the cerebral cortex, but also in the cerebellum, hippocampus

Project to pyramidal cells right near the body of the pyramidal cell, which is an influential place to be!