CNS Mod 2 TOPIC 2 – FUNCTIONAL TOPOGRAPHY OF THE MOTOR AND SENSORY AREAS

neocortex

Largest part of human cerebral cortex (90%); has six neuronal layers; present only in mammals; represents ~75% of total brain gray matter

allocortex

Evolutionarily older cortex; has 3-4 layers; found predominantly in limbic system cortex

cytoarchitecture

Organization of the six layers of the neocortex

molecular layer (layer I)

Outermost neocortex layer; contains nonspecific afferent fibers from within cortex or from thalamus

external granular layer (layer II)

Dense layer composed of small cells

external pyramidal layer (layer III)

Contains pyramidal cells

internal granular layer (layer IV)

Thin layer of cells similar to layer II; receives specific afferent fibers from thalamus

internal pyramidal layer (layer V)

Contains pyramidal cells that are fewer in number but larger in size; these cells project to distal structures (brainstem and spinal cord)

fusiform layer (layer VI)

Also called multiform layer; consists of irregular fusiform cells whose axons enter adjacent white matter

pyramidal neurons

Main output neurons of cerebral cortex; have apical dendrite reaching toward cortical surface and basilar dendrites extending horizontally; axons form projection and association fibers; axons project to spinal cord brainstem and other cortical areas

stellate neurons

Star-shaped neurons with dendrites extending in all directions; are interneurons whose axons remain within cortex

fusiform neurons

Found in deeper layers (layer VI); have large dendrite ascending toward cortical surface; axons form projection and association fibers similar to pyramidal cells

cortical column

Vertically-oriented group of neurons with similar functions extending from superficial to deep layers; contains ~10

somatosensory cortex column

All neurons in a column are activated by a single type of sensory receptor and receive inputs from similar part of body

visual cortex column

All cells within a column receive input from same part of retina and are tuned to respond to stimuli with similar orientations

Brodmann areas

Classification system based on cytoarchitectonics; 52 areas each with common cytoarchitecture; used as reference base for localization of physiologic and pathologic processes

Brodmann area 4

Primary motor cortex (M1); located in precentral gyrus

Brodmann area 6

Premotor cortex and supplemental motor area (SMA); located anterior to primary motor cortex

Brodmann area 8

Frontal eye field; motor area for visual attention and voluntary eye movements

Brodmann area 44 and 45

Broca's area; motor production of speech; located in dominant hemisphere (usually left)

Brodmann areas 3 1 and 2

Primary somatosensory cortex (S1); located in postcentral gyrus

Brodmann areas 5 and 7

Posterior parietal cortex (PPC); visuospatial perception spatial attention integration of multisensory input

Brodmann area 17

Primary visual cortex (V1); located in posterior pole of occipital lobe on medial side

Brodmann area 18

Visual association area; receives stimuli mainly from area 17; located more medially

Brodmann area 19

Visual association area; can receive stimuli from entire cerebral cortex; located more laterally

Brodmann area 41

Primary auditory cortex (A1); located in superior temporal gyrus

Brodmann area 42

Auditory association cortex; located in surrounding parts of temporal lobes

Brodmann area 22

Wernicke's area; language comprehension; highly connected with Broca's area; located on dominant hemisphere (left)

Brodmann areas 39 and 40

Multimodal association areas; allow higher-order information processing through converging projections from different modality-specific areas

primary motor cortex (M1)

Located in precentral gyrus (Brodmann area 4); contains upper motor neurons (UMN) that send impulses to brainstem and spinal cord for execution of movement (actual act of movement)

premotor cortex

Located anterior to primary motor cortex (Brodmann area 6); involved in direct and indirect control of movement; sends axons to spinal cord for proximal and trunk muscles; sends inputs to primary motor cortex for preparation and sensory guidance of movement

supplemental motor area (SMA)

Located anterior to precentral gyrus; runs more superior and medial than premotor cortex; together with premotor cortex involved in planning of movement; selection of appropriate motor plans/sequences; coordination of two sides of body for complex motor output

frontal eye field

Brodmann area 8; motor area involved in control of visual attention and voluntary eye movements

Broca's area

Brodmann areas 44 and 45; motor area important for speech production; located just anterior to motor cortex that controls lips and tongue; located in dominant hemisphere (usually left)

motor homunculus

Somatotopic map-like representation of body on primary motor cortex; lips tongue face hands on lateral/lower hemisphere; arm trunk hip on lateral/upper hemisphere; foot lower leg genitals on medial section; relative size indicates amount of cortex devoted to that area (large hand size reflects fine finger control)

primary somatosensory cortex (S1)

Located in postcentral gyrus (Brodmann areas 3 1 and 2); receives and processes touch temperature pain and conscious proprioceptive information; also provides some motor control

secondary somatosensory cortex

Also called sensory association cortex; processes higher-order sensory information

posterior parietal cortex (PPC)

Brodmann areas 5 and 7; important for visuospatial perception; spatial attention; integration of somatosensory visual and auditory input; connections to motor cortex for movement planning

multimodal association areas

Brodmann areas 39 and 40; allow higher-order information processing; example includes converging projections of visual image of dog sound of bark and feel of fur

sensory homunculus

Somatotopic map on primary somatosensory cortex; very similar to motor homunculus

primary auditory cortex (A1)

Brodmann area 41; located bilaterally in superior temporal gyrus; receives auditory information

auditory association cortex

Brodmann area 42; located in surrounding parts of temporal lobes; involved in auditory processing and perception of sounds; gives meaning and comprehension to sounds

Wernicke's area

Brodmann area 22; key role in speech comprehension (language comprehension); highly connected with Broca's area; located on dominant hemisphere (usually left)

primary visual cortex (V1)

Brodmann area 17; receives visual input from retina; located in posterior pole of occipital lobe on medial side; V1 in left hemisphere receives visual information from right visual field

visual association area area 18

Brodmann area 18; receives stimuli mainly from area 17; located more medially

visual association area area 19

Brodmann area 19; can receive stimuli from entire cerebral cortex; located more laterally

neocortex composition

Contains ~80% excitatory glutamatergic neurons and ~20% inhibitory GABAergic neurons; receives inputs from neurons in all major neurotransmitter systems