Cerebrum

Motor and Sensory Areas of the Cortex

-Central sulcus separates motor and sensory areas -Motor areas *Precentral gyrus (primary motor cortex) of frontal lobe: directs voluntary movements -Sensory areas *Postcentral gyrus (primary sensory cortex) of parietal lobe: receives somatic sensory information (touch, pressure, pain, vibration, taste, and temperature)

Frontal Lobe

A large part of the frontal cortex rostral to the central sulcus is related to the control of movements, primarily on the opposite side of the body. These areas include primary motor cortex (Brodmann area 4), premotor cortex (area 6), the frontal eye field (area 8), and the motor speech areas of Broca (area 44 and 45). Premotor cortex Just anterior to area 4 is the premotor cortex (area 6). Neurons here are particularly active prior to the activation of area 4 neurons, so it is thought that the premotor cortex is involved in the planning of motor activities. Damage here results in an apraxia, a disruption of the patterning and execution of learned motor movements. Individual movements are intact, and there is no weakness, but the patient is unable to perform movements in the correct sequence. Prefrontal cortex This area is involved in organizing and planning the intellectual and emotional aspects of behavior, much as the adjacent premotor cortex is involved in planning its motor aspects.

Split brain syndrome

A structure known as the corpus callosum connects the left and right hemispheres of the brain and enables communication between them. Dysfunction or absence of this structure can result in a condition known as split-brain syndrome, in which each hemisphere of the brain functions independently. Split-brain syndrome is associated with conditions such as alien-hand syndrome, which is characterized by involuntary and uncoordinated yet purposeful movement of the hands.

Gait apraxia

Characterized by diminished cadence, wide base, short steps, and shuffling progression; it is reminiscent of parkinsonian gait. ● A frontal lobe sign seen in normal-pressure hydrocephalus with gait apraxia, dementia, and incontinence.

Broca (motor) aphasia

Characterized by good comprehension; effortful, dysarthric, telegraphic, and nonfluent speech; poor repetition; and contralateral lower facial and upper limb weakness. ● Results from a lesion in the frontal lobe, in the inferior frontal gyrus (brodmann 44 and 45).

Motor and Sensory homunculus

Indicates that the upper limb and head are demonstrated on the lateral surface of the cortex. The pelvis and the lower limb are represented on the medial surface of the hemispheres. Therefore, the motor and sensory functions of the lower limb are supplied by the anterior cerebral artery while the motor and sensory functions of the upper limb and head are supplied by the middle cerebral artery.

Gerstmann Syndrome

It is a cognitive impairment that results from damage to a specific area of the brain, the left parietal lobe in the region of the angular gyrus. It may occur after a stroke or in association with damage to the parietal lobe. It is characterized by four primary symptoms: a writing disability (agraphia or dysgraphia), a lack of understanding of the rules for calculation or arithmetic (acalculia or dyscalculia), an inability to distinguish right from left, and an inability to identify fingers (finger agnosia).

Sagittal sinus thrombosis

It is often associated with one of the hypercoagulable states. It occurs with increased frequency in pregnant women and within the first few weeks post partum. Obstruction of venous drainage usually causes elevated intracranial pressure. Back pressure in cortical veins can cause parasagittal hemorrhages. In addition, the increased venous pressure can decrease cerebral perfusion, leading to infarcts. Seizures are common. Patients often have headaches and papilledema, and they may have depressed level of consciousness. More subtle radiological signs of sagittal sinus thrombosus include increased density of the sagittal sinus on CT due to coagulated blood, or increased signal on MRI. In suspected sagittal sinus thrombosus, regardless of whether these subtle radiological findings are present, a more definitive study should be performed, such as magnetic resonance venography (MRV) or a conventional angiogram. Treatment usually involves anticoagulation therapy, although this is controversial when hemorrhage has occurred. Seizures and elevated intracranial pressure should be treated as well, when present. Venous thrombosis can also occur less commonly in other intracranial venous sinuses, in the deep cerebral veins, or in a major cortical vein, leading to infarcts or hemorrhage in the territories of these vessels.It is often associated with one of the hypercoagulable states.

Cerebrum

It is the largest part of the forebrain. It is highly developed in human. It is derived from the telencephalon. Each hemisphere has a covering of gray matter, the cortex and internal masses of gray matter, the basal nuclei, and a lateral ventricle. They are connected by the corpus callosum. Consists of the neocortex (90%) and the allocortex (10%). Neocortex (isocortex) a six-layered cortex. Allocortex (heterogenetic) three-layered and includes two types: 1. Archicortex: includes the hippocampus and the dentate gyrus. 2. Paleocortex: includes the olfactory cortex.

Layers of cerebral cortex

Layer I the molecular layer: is the outermost layer of the cerebral cortex and consists primarily of horizontally running nerve fibers. Layer II, the external granular layer: is made of densely packed granule cells with dendrites extending into molecular layer and axon passing deeper layers. Axons of these neurons from association projections that interconnect different parts of hemisphere. Layer III, the external pyramidal layer: it consist of moderate sized pyramidal cells. Axons of these neurons from commissural projections that interconnect the 2 hemispheres via the corpus callosum. Layers IV, the internal granular layer: consist mostly of small stellate cells. It represents the major sites of termination of ascending cortical inputs from the thalamus. Layers V internal pyramidal layers: contains medium to large pyramidal cells. Betz cells, the largest neurons in the CNS, are found in layer V of the primary motor cortex, which is located on the precentral gyrus. Layer VI the multiform layer: consist of pyramidal and fusiform cells.

Blood supply of the cerebrum

Middle cerebral artery (MCA) supplies: The lateral surface of the frontal, parietal, and upper temporal lobes The posterior limb and genu of the internal capsule Most of the basal ganglia Anterior cerebral artery (ACA) Supplies: 1. Medial surface of frontal and parietal lobes 2. Anterior four-fifths of corpus callosum· 3. Anterior limb of internal capsule Posterior cerebral artery (PCA) Supplies: 1. Occipital lobe 2. Lower temporal lobe 3. Splenium 4. Mid brain

The Dominant Hemisphere

Ninety five percent of human population is right handed and has speech and language functions localized in the left or dominant hemisphere and responsible for propositional language (grammar, syntax, semantic) speech and calculation. The right hemisphere is usually non dominant and responsible for 3 dimensional or spatial perception or nonverbal ideation. Most left-handed people show language functions bilaterally, although a few, with strong left-handed preferences, show right-sided speech and language functions. The cerebral dominance is determined by Wada Test. Sodium amobarbital (Amytal) is injected in carotid artery. If patient becomes aphasic, the anesthetic was administered to the dominant hemisphere.

Dysprosodies

Nondominant hemispheric language deficits that affect the emotionality of speech (inflection, melody, emphasis, and gesturing).

Broadmann's areas

On the basis of variations in the histology of layers I-VI, Brodmann divided the cerebral cortex into 47 areas. Most of Brodmann areas are used synonymously with functionally specific areas.

Temporal Lobe

Primary auditory cortex On its superior and lateral aspect, the temporal lobe contains the primary auditory cortex. Auditory cortex (areas 41 and 42) is located on the 2 transverse gyri of Heschl, which cross the superior temporal lobe deep within the lateral sulcus. Much of the remaining superior temporal gyrus is occupied by area 22 auditory association cortex, which receives a considerable projection from both areas 41 and 42 and projects widely to both parietal and occipital cortices. Patients with unilateral damage to the primary auditory cortex show little loss of auditory sensitivity but have some difficulty in localizing sounds in the contralateral sound field. Area 22 is a component of Wernicke area in the dominant hemisphere, and lesions here produce a Wernicke aphasia.

Parietal Lobe

Primary somatosensory cortex The postcentral gyrus corresponds to Brodmann areas 3, l, and 2 and contains primary somatosensory cortex. There is a similar somatotopic representation of the body here, with head, neck, upper limb, and trunk represented on the lateral aspect of the hemisphere, and pelvis and lower limb represented medially. These areas are concerned with discriminative touch, vibration, position sense, pain, and temperature. Lesions in somatosensory cortex result in impairment of all somatic sensations on the opposite side of the body, including the face and scalp. Posterior parietal association cortex Just posterior and ventral to the somatosensory areas is the posterior parietal association cortex, including Brodmann areas 5 and 7. Wernicke area The inferior part of the parietal lobe and adjacent part of the temporal lobe in the dominant (left) hemisphere, known as Wernicke area, are cortical regions that function in language comprehension. At a minimum, Wernicke area consists of area 22 in the temporal lobe but may also include areas 39 and 40 in the parietal lobe. Areas 39 (the angular gyrus) and 40 (the supramarginal gyrus) are regions of convergence of visual, auditory, and somatosensory information.

Expressive dysprosody

Results from a lesion that corresponds to the broca area but is located in the nondominant hemisphere. ● Patients cannot express emotion or inflection in their speech.

Receptive dysprosody

Results from a lesion that corresponds to the wernicke area but is located in the nondominant hemisphere. ● Patients cannot comprehend the emotionality or inflection in the speech they hear.

Ideational apraxia

The inability to demonstrate the use of real objects (e.g., Smoke a pipe [a multistep complex sequence]). ● A misuse of objects owing to a disturbance of identification (agnosia). ● Results from a lesion in the wernicke area.

Construction apraxia

The inability to draw or construct a geometric figure (e.g., The face of a clock). Called hemineglect if the patient draws only the right half of the clock. The lesion is located in the right inferior parietal lobule.

Apraxia

The inability to perform motor activities in the presence of intact motor and sensory systems and normal comprehension.

Ideomotor apraxia

The loss of the ability to perform intransitive or imaginary gestures, resulting in the inability to perform complicated motor tasks (e.g., Saluting, blowing a kiss, or making the v-for-peace sign). ● May be typified by facial apraxia, which is also known as buccofacial or facial-oral apraxia, the most common type of apraxia. ● Results from a lesion in the wernicke area.

Occipital Lobe

The occipital lobe is essential for the reception and recognition of visual stimuli and contains primary visual and visual association cortex. Visual cortex The visual cortex is divided into striate (area17) and extrastriate (areas18 and 19). Area 17, also referred to as the primary visual cortex, lies on the medial portion of the occipital lobe on either side of the calcarine sulcus. Its major thalamic input is from the lateral geniculate nucleus. Visual association cortex Anterior to the primary visual or striate cortex are extensive areas of visual association cortex. Visual association cortex is distributed throughout the entire occipital lobe and in the posterior parts of the parietal and temporal lobes. These regions receive fibers from the striate cortex and integrate complex visual input from both hemispheres. From the retina to the visual association cortex, information about form and color, versus motion, depth and spatial information are processed separately.

Watershed infarcts

When a cerebral artery is occluded, ischemia or infarction occurs in the territory supplied by that vessel, with regions near other vessels relatively spared. In contrast, when the blood supply to two adjacent cerebral arteries is compromised, the regions between the two vessels are most susceptible to ischemia and infarction. These regions between cerebral arteries are called watershed zones (Figure 10.10). Bilateral watershed infarcts in both the ACA-MCA and MCA-PCA watershed zones can occur with severe drops in systemic blood pressure. A sudden occlusion of an internal carotid artery or a drop in blood pressure in a patient with carotid stenosis can cause an ACA-MCA watershed infarct, since the MCA and ACA are both fed by the carotid. Can produce proximal arm and leg weakness ("man in the barrel" syndrome) because the regions of homunculus involved often include the trunk and proximal limbs. In the dominant hemisphere, watershed infarcts can cause transcortical aphasia syndromes. MCA-PCA watershed infarcts can cause disturbances of higher-order visual processing. Watershed infarcts can also occasionally occur between the superficial and deep territories of the MCA

Conduction aphasia

● Involves the transection of the arcuate fasciculus; the arcuate fasciculus interconnects broca speech area with wernicke speech area. ● Characterized by good comprehension, poor repetition, and fluent speech.

Clinical Presentation of split brain syndrome

patients have been studied by presenting stimuli selectively to one or the other hemisphere and comparing the subject's responses with them. For instance, A stimulus presented briefly to one visual field or placed in one hand is accessible only to the opposite hemisphere (because the projections are contralateral and all commissural connections have been severed). Objects in the right visual field or right hand are recognized and named easily by the 'verbal' left hemisphere. In contrast, patients cannot name, and appear to lack knowledge of, objects placed in the left visual field or left hand, because these are available only to the 'non-verbal' right hemisphere. However, the object has undoubtedly been identified correctly, because the person can later pick it out from a selection of objects. These functional specializations are relative and apply to people with left hemisphere language representation. Subsequent studies have added more detail and complexity. Overall, split-brain work has been central in establishing the extent and nature of functional asymmetries, and its importance was highlighted by Sperry's 1981 Nobel Prize.

Wernicke (sensory) aphasia

● Characterized by poor comprehension, fluent speech, poor repetition, and quadrantanopia also marked by paraphasic errors such as non sequiturs (latin—does not follow logically what is said previously), neologisms (words with no meaning), and driveling speech. ● Results from a lesion in the posterior temporal lobe, in the superior temporal gyrus (brodmann 22).

Transcortical motor aphasia

Involves good comprehension, good repetition, and nonfluent speech.

Functional anatomy of cerebrum

Divided into left and right hemispheres by a longitudinal fissure. The most conspicuous features on the surface of each hemisphere are numerous folds called gyri, which greatly increase the surface area of the cortex. The intervening grooves between the gyri are called sulci. The central sulcus is located between the precentral gyrus anteriorly, which is the primary motor cortex, and a postcentral gyrus posteriorly, which is the primary somatic sensory cortex. Each cerebral hemisphere is divided into lobes, which are named for the skull bones overlying each one. The frontal lobe is important in voluntary motor function, motivation, aggression, the sense of smell, and mood. The parietal lobe is the major center for the reception and evaluation of sensory information, except for smell, hearing, and vision. The frontal and parietal lobes are separated by the central sulcus. The occipital lobe functions in the reception and integration of visual input and is not distinctly separate from the other lobes. The temporal lobe receives and evaluates input for smell and hearing and plays an important role in memory. Its anterior and inferior portions are referred to as the "psychic cortex," and they are associated with such brain functions as abstract thought and judgment. The temporal lobe is separated from the rest of the cerebrum by a lateral fissure, and deep within the fissure is the insula, often referred to as a fifth lobe.

Aphasias

Impaired or absent communication by speech, writing, or signs (i.e., loss of the capacity for spoken language). Results from lesions in the dominant hemisphere.

Intrinsic organization of the cerebral cortex

Thalamocortical and intracortical projections terminate mainly in layer IV, and monoaminergic projections are distributed mainly to more superficial layers. Cortical afferents terminating in layer IV can either excite or inhibit pyramidal cells in layer V, which contribute significantly to the outputs of the cerebral cortex. The major outputs to the spinal cord, cranial nerve motor nuclei, other brainstem structures, thalamus, and neostriatum arise in layers V-VI, whereas projections to other regions of cortex either on the ipsilateral or contralateral side arise from layer III.

Fibers of the Cerebrum

The gray matter on the outer surface of the cerebrum is the cortex, and clusters of gray matter deep inside the brain are nuclei. The white matter of the brain between the cortex and nuclei is the cerebral medulla. The cerebral medulla consists of nerve tracts that connect the cerebral cortex to other areas of cortex or other parts of the CNS. These tracts fall into three main categories: (1) Association fibers, which connect areas of the cerebral cortex within the same hemisphere; (2) Commissural fibers, which connect one cerebral hemisphere to the other; and (3) Projection fibers, which are between the cerebrum and other parts of the brain and spinal cord.