CEREBRUM – Higher Integrative Functions
The cerebrum is related to and concerned with higher brain functions. These important aspects include:
1. Perception of sensory impulses (van de Graaff 1999).
2. Instigation of voluntary movement.
3. Storage of memory.
4. Though processes (language, calculation).
5. Reasoning ability.
6. Instinctual and limbic (emotional) functions.
The theory of this theme is considered under the following headings:
1. Structure of cerebrum (hemispheres, fissures, sulci, gyri, falx, ventricles, cerebral cortex.
2. Cerebral hemispheres and lobes (frontal, parietal, temporal, occipital and insula). Functions of each lobe.
3. White and grey matter.
4. Functional cortical localization (Zones of Brodmann).
5. Cerebral dominance.
6. Motory and sensory homunculus.
7. Association, projection and commissural fibres.
8. Internal capsule.
9. The diencephalon (thalamus, hypothalamus) and basal ganglia are dealt with in theme 5 (subcortical grey matter).
CEREBRAL HEMISPHERES
Lateral Characteristics: Macro
1. Frontal, occipital, parietal and temporal lobes.
2. Sulci (furrows) and gyri (eminences).
Prent 2.26 pg 53 3. Central sulcus (Rolandic fissure) dividing postcentral (sensory area) and precentral (motory area) gyrus.
1. Lateral sulcus (Sylvian fissure) divides frontal from temporal lobe.
2. Temporal gyri (superior, middle, inferior).
3. Insula: Burried cortex covered by temporal and frontal cortex - visualised by turning the parietal, temporal and frontal operculum back.
7. Calcarine fissure (occipital lobe) – primary visual cortex.
Medial Characteristics: Sagittal section
1. Sulci: Central, parieto-occipital, calcarine, collateral, Cingulate.
2. Gyri: Cingulate, inferior temporal, parahippocampal, lingual. Cuneus and precuneus.
3. Corpus callosum: Components (consists of white matter).
a. Splenium (posterior)
b. Genu (anterior)
c. Trunk (anterior). Also forms the forceps major and forceps minor.
4. Lateral wall and features of third ventricle: components to be identified.
a. Septum pellucidum/thalamus, anterior commissure.
b. Fornix, infundibulum, mammillary body, tuber cinereum, rostrum.
c. Choroid fissure, optic chiasm, lamina terminalis, hypothalamus
d. Uncus.
Inferior Characteristics: Study your atlas
1. Collateral sulcus, parahippocampal gyrus.
2. Calcarine sulcus, lingual gyrus, dentate gyrus.
3. Piriform lobe and rhinal sulcus.
FUNCTIONS OF CEREBRAL LOBES (After Van de Graaff 1999)
Frontal lobe:
1. Voluntary motory control of skeletal muscle (motory cortex).
2. Personality.
3. Verbal communication/language.
Parietal lobe:
1. Sensory or somatesthetic interpretation: post-central gyrus.
2. Understanding speech; word formulation; thought expression.
3. Expression of emotions.
4. Interpretation of textures and shapes.
Temporal lobe:
1. Analysis and interpretation of auditory sensation.
2. Memory; auditory and visual experience.
Occipital lobe:
1. Eye: movement integration and focus.
2. Conscious perception of vision.
3. Correlation of previous visual experiences.
Insula:
1. Memory: other integrative function.
Functional Cortical localization: Important Zones after Brodmann (Cytoarchitectoral zones)
1. Parietal lobe: Primary somatic sensory area (areas 3, 1, 2): postcentral gyrus - somatotropic representation and mapping (sensory homunculus) of body on the lateral and medial surface of the hemisphere. Posterior to this zone is the general sensory association area. Receives sensation from the contralateral side of the body.
2. Occipital lobe: Striate cortex: Visual area (area 17) calcarine sulcus. Also visual association cortex (area 18, 19).
3. Temporal lobe: Auditory area: (area 41, 42) superior surface of temporal lobe/gyrus. Also auditory association cortex (area 22).
4. Taste area: Postcentral gyrus in lateral fissure/insula area.
5. Frontal lobe: Primary motor area (area 4) precentral gyrus. Anterior lies the premotor area. Gives rise to 80% of the corticospinal (pyramidal) tract. Also premotor cortex (area 6) ie cell bodies of the primary motor pathways.
6. Motor speech area (Broca’s area 44): frontal lobe anteriorly above lateral fissure at the posterior end of the inferior frontal gyrus. Damage to this area results in motor aphasia.
7. Sensory speech area: Parietal and temporal lobes (C-shaped) close to the auditory association cortex - at the posterior aspect of the lateral sulcus.
Specific Cerebral Cortical Functions
1. Dominant hemisphere (95% of persons - left hemisphere).
2. Specific cortical functional areas.
a. Pre- and motor cortex (motor function).
b. Planning of movements and elaboration of thoughts.
c. Broca’s area (speech): i.e. word formation (95% of persons in left hemisphere). Fall-out leads to motory dysphasia (expressive aphasia)
d. Somatic sensory cortex (sensory function).
e. Spatial co-ordination surroundings.
f. Vision.
g. Wernickes Area: Most important area for language, comprehension and intelligence (95% of persons in left hemisphere). Fall-out leads to sensory dysphasia (receptive aphasia).
h. Emotions, behaviour, auditory, motivation, thought processes, memory communication.
I. Limbic area function: behaviour, emotions, motivation.
j. Supplementary motory, sensory and auditory functions.
Examples of Cerebral Functional Localization According to Brodmann
Brodmann Area
Function
1. 8 (frontal)
Frontal eyefield (eye movements)
2. 44 and 45 (frontal)
Broca’s motor speech area (control lips and tongue).
3. 3,1,2 (parietal)
Primary sensory area (sensory homunculus) - situated in the postcentral gyrus. Receives fibres from the VPL and VPM thalamic nuclei.
4. 17 (occipital)
Primary visual cortex.
5. 18,19 (occipital)
Visual association areas.
6. 41 (temporal)
Primary auditory cortex (superior temporal gyrus).
7. 42 (temporal)
Associative (secondary) auditory cortex.
8. 22 (temporal)
Comprehension and language
9. 4 (frontal)
Primary motor area (pre-central gyrus)
Motory and Sensory Homunculus:
Represents face, limbs, trunk.
1. Motory: Precentral gyrus (principal motor area); area 4.
2. Sensory: Postcental gyrus (principal sensory area); area 3-1-2 (somesthetic area). Both reflect somatotropic representation (note size aportioned to each organ).
Important Functions of the Brain in General
1. Sensory integration (from receptors, auditory and visual stimuli).
2. Information storage - memory.
3. Generation of thought processes, ambition.
4. Motor co-ordination.
5. Execution of human desires.
6. Vegetative integration via the autonomic nervous system.
4. Affect: firmness, benevolence, cautiousness, combativeness, love, self-esteem, music, friendship, acquisitive, time, locality, calculation, destructiveness, etc.
8. Dominance.
General Facets of the Somatic Sensory Axis: Ascending Stimuli
1. Stimuli from sensory receptors enter the spinal cord via the following afferent receptors.
a. Skin; pain, pressure (Pacinian) touch (Meissner).
b. Muscle: spindle and tendons (Golgi).
c. Joints: Kinesthetic receptors.
d. Visual, auditory.
2. Relay in medulla, pons, cerebellum, bulboreticular formation and thalamus.
3. Relay from the thalamus to the somesthetic area in the sensory cortex. Direct fibres also pass from the thalamus to the motor cortex. A highly complex to- and -from neuron network exists between the sensory cortex and the thalamic nuclei.
General Facets of the Motor Axis : Descending Stimuli
1. Stimuli originate in the motory cortex and descend by two routes:
a. Directly to relay stations in the spinal cord. Relay then by alpha motor fibres to motor-end plates in skeletal muscle thus controlling contraction.
b. From the motory cortex to the basal ganglia (caudate nucleus, putamen, globus pallidum and subthalamic nuclei). (A complex feed back relay system exists between the putamen, thalamic nuclei and motory cortex). From the basal ganglia further relay occurs in the bulboreticular formation (in the medulla) and at various spinal cord levels. From the spinal cord stimuli pass via gamma motor fibres to skeletal muscles.
Important Levels of CNS Functional Integration
1. Spinal cord level i.e. spinal reflexes (walking, withdrawal, vessel tone, extension).
2. Lower brain level i.e. subconscious reflexes that control blood pressure, respiration, equilibrium, feeding, salivation.
3. Higher cortical level i.e. thought process. Always functions in conjunction with lower brain centers.
Summary of Important Surface Features of the Cerebral Hemispheres: See Atlas.
1. The cerebral hemispheres are furrowed by sulci and bridged by ridges or folds called gyri.
2. Identify the: Lateral sulcus or Sylvian fissure.
3. Identify the: Central sulcus or Rolandic fissure.
4. The two hemispheres are divided into four lobes: frontal, parietal, occipital and temporal.
5. The insula is covered by the opercula.
6. A massive white band of white matter joins the two hemispheres.
7. The cerebral hemispheres are made up of white and grey matter.
8. Two important dural septa must be recognized:
a. Falx cerebri: extends down into the longitudinal fissure between the cerebral hemispheres.
b. Tentorium cerebelli: separates the occipital lobes from the cerebellum.
9. The diaphragma sella forms an incomplete lid over the hypophysis in the sella turcica. The pituitary stalk passes through the opening in the diaphragma.
10. Identify the pre- and postcentral gyri responsible for the primary motory and sensory cortices.
Cerebral Association and Commissural Fibres: Projections of the Cerebral Cortex
1. Association: Fibres joining one part of the hemisphere to another or to the opposite hemisphere. These fibres run in the white matter.
2. Commissural: Interlink identical areas of the two hemispheres.
3. Projection: Fibres that link subcortical nuclei in the cerebrum, brainstem and spinal cord.
Association Fibres: May be long or short.
1. Superior longitudinal fasiculus (links frontal and occipital lobes).
2. Inferior longitudinal fasiculus (links occipital and temporal lobes).
3. Arcuate fasiculus (links frontal lobe and occipito-temporal cortex).
4. Uncinate fasiculus (links frontal and anterior temporal lobes).
INTERNAL CAPSULE
Site: Between thalamus and lentiform nucleus.
Composition:
1. Anterior limb: Descending frontopontine fibres and thalamocortical fibres.
2. Genu and anterior two thirds of the posterior limb: corticospinal fibres.
3. Posterior limb: Descending corticospinal and corticobulbar fibres.
4. Retrolentiform section:
Important Fibres:
1. Thalamus to cerebral cortex (ascending fibres).
2. Cerebral cortex to thalamus, brainstem and spinal cord (descending fibres).
3. Other descending tracts to the corpus striatum, red nucleus, substantia nigra and inferior olivary nucleus.
External Capsule: Sited between putamen and claustrum.
Extreme Capsule: Sited lateral to claustrum.
Major Cerebral Commissures
1. Corpus callosum - fibres fan out as the
a. Tapetum (link temporal and occipital lobes).
b. Lateral fibres intersecting the corona radiata.
c. From the splenium fibres radiate as the occipital (major) forceps to the occipital lobe and from the genu anteriorly into the frontal lobe as the frontal (minor) forceps.
2. Components: body, genu, splenium.
3. Function: co-ordinates the activities of the two cerebral hemispheres.
4. Other: anterior and posterior commissures.
Laminar Structure of Neocortex
Components: Cortex varies in thickness from 2-4 mm.
1. Molecular layer: Pyramidal and Betz giant cells.
2. Outer granular layer.
3. Outer pyramidal layer.
4. Inner pyramidal layer.
5. Fusiform layer.
Labels: Basic Science: Neuroanatomy
