Cerebrum/Cerebral hemispheres and cortex

Cerebrum/Cerebral hemispheres and cortex:
Most superior and visible part of brain, making up 85% of brain mass. The two paired cerebral hemispheres (connected by mass of white matter, the corpus callosum) - cover the diencephalon and brain stem. Composed of outer layer (cortex) and inner white matter. Surface of cerebral hemispheres is characterised by fold of elevated ridges (gyri) and shallow grooves (sulci). Larger regions are seperated by fissures. The most prominent sulci and gyri are the longitudinal fissure (separates the two cerebral hemispheres) and the transverse fissure (separates the cerebellum below the two cerebral hemispheres).

Deeper sulci divide each hemisphere into four lobes – the frontal (anterior and superior; central sulcus is posterior border; mainly involved in voluntary motor functions), parietal (superior and posterior - between frontal and occipital; mostly involved with sensory reception and integration), occipital (posterior; visual centre), temporal (inferior; involved with hearing, smell, learning, memory and emotional behaviour)

Cerebral white matter
Most of volume is white matter - deep to gray matter – for communication between the cerebral cortex and the lower CNS centres. Made up mostly of myelinated fibres in large tracts and glia. Fibres are arranged in 3 tracts – projection tracts (carry information from cerebrum to rest of body), commissural tracts (cross from one hemisphere to other; most pass through corpus callosum), and association tracts (connect different parts of same hemisphere).

Cerebral cortex:
outer layer/sheet of cells above cerebellum; 2-3mm thick; 40% of mass of brain; 10-20 billion neurons
responsible for conscious functions such as perception, communication, memory, understanding and the initiation of movement
specific functions are localised in discrete areas of cortex (domains) – 3 types – motor ares, sensory areas and association areas; but, no functional area of brain functions alone.
each hemisphere is responsible for the sensory and motor functions of the contralateral side of the body

Motor areas:
• primary/somatic motor cortex located in posterior aspect of frontal lobe (in precentral gyrus)
• large neurons (pyramidal cells) with long axons that project to spinal cord control skeletal muscle  form the pyramidal/corticospinal tracts
• anterior to location of primary motor cortex is the premotor cortex – responsible for control of activities that have a pattern or are repetitive (eg typing)
• anterior to premotor area is Broca’s area – considered to be area of thinking prior to speaking or a voluntary motor activity

Sensory areas:
sensory areas are in parietal, temporal and occipital lobes
primary somatosensory cortex is in anterior part of parietal lobe (in postcentral gyrus) – receives impulses from sensory receptors
somatosensory association area (located posterior to primary somatosensory area) functions to integrate and analyse the different somatic sensory inputs  evaluates them (damage to this area affects ability to recognise objects by touch)
primary visual cortex and visual association areas are located at most posterior aspect of occipital lobe
primary auditory cortex is located in the superior aspect of temporal lobe
olfactory cortex are in the medial aspects of temporal lobes
gustatory (taste) cortex is located in parietal lobe

Association areas:
Prefrontal cortex in anterior portion of frontal lobe – involved in intellect, cognition and personality. Also plays a role in mood.
ill-defined area of temporal, parietal and occipital lobes is the ‘general interpretation’ area – appears to be a storage site for complex memory patterns associated with sensation
Language areas – associated with Wernicke’s area in posterior temporal lobe of one of the hemispheres, though other areas of brain are involved.
Visceral association areas – may be involved in conscious perception of visceral sensations (eg full bladder).

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