Outline of the next three lectures

Transcription

1 Outline of the next three lectures Lecture 35 Anatomy of the human cerebral cortex gross and microscopic cell types connections Vascular supply of the cerebral cortex Disorders involving the cerebral cortex Lectures 36 Functions of the association cortex Brain imaging Lecture 37 Language and the brain Hemisphere differences

2 Cerebral Cortex Yasushi Nakagawa Department of Neuroscience University of Minnesota

3 Goals 1. put together various parts of the cerebral cortex covered in previous lectures and labs. 2. review gross anatomy of the cerebral cortex -position of the cerebral cortex within the whole brain -Brodmann s areas -lobes 3. understand topographic organization of the sensory cortex 4. understand microscopic anatomy of the cerebral cortex -layers -major cell types in each layer -input and output connections 5. briefly overview disorders involving the cerebral cortex -major causes -vascular supply of the cerebral cortex

4 Where is the cerebral cortex? prosencephalon (forebrain) telencephalon diencephalon ventral telencephalon =part of basal ganglia -putamen -caudate -nucleus accumbens -globus pallidus dorsal telencephalon =cerebral cortex -neocortex -hippocampus -olfactory cortex The overall organization is conserved in all vertebrate brains. The six-layered organization is found only in mammalian neocortex.

5 Areas of the neocortex rostral caudal Brodmann s areas -proposed in early 20th century -based on thickness and cell density of each layer, as well as size, shape and arrangement of neuronal cell bodies ( cytoarchitecture ) -some areas have highly developed pyramidal layers (layer 2/3 and 5) and poorly developed layer 4 primary motor area (Area 4) -some areas have highly developed layer 4 and poorly developed pyramidal layers primary visual area (Area 17) primary somatosensory area (Area 3) primary auditory area (Area 41) -recent functional studies (fmri, etc.) have found that Brodmann areas are correlated with functional areas Area 17 Area 18 Brodmann (1909)

6 Lobes of the cerebral cortex caudal rostral lateral fissure central sulcus parieto-occipital line from Nieuwenhuys et al., (2008) The Human Central Nervous System Lobes are: -gross subdivisions of the cerebral cortex -divided on the brain surface by sulci and gyri -some areas are included in more than one gyri includes: frontal lobe parietal lobe occipital lobe temporal lobe limbic lobe

7 Frontal lobe caudal rostral lateral fissure central sulcus from Nieuwenhuys et al., (2008) The Human Central Nervous System bordered by: lateral fissure central sulcus includes: primary motor area caudal part of the precentral gyrus thalamic input: VA (ventroanterior) VL (ventrolateral) other motor areas premotor area supplementary motor area frontal eye fields Broca s area prefrontal cortex

8 Parietal lobe caudal parieto-occipital sulcus pre-occipital notch rostral bordered by: lateral fissure central sulcus (imaginary) parieto-occipital line includes: primary somatosensory area postcentral gyrus thalamic input: VP (ventroposterior) somatosensory association areas lateral fissure central sulcus parieto-occipital line from Nieuwenhuys et al., (2008) The Human Central Nervous System

9 Temporal lobe caudal parietooccipital sulcus rostral bordered by: lateral fissure (imaginary) parieto-occipital line includes: primary auditory area upper side of the superior temporal gyrus thalamic input: medial geniculate auditory association areas includes Wernicke s area pre-occipital notch lateral fissure central sulcus parieto-occipital line from Nieuwenhuys et al., (2008) The Human Central Nervous System primary auditory area (in Heschl gyrus; HG)

10 Occipital lobe caudal parietooccipital sulcus rostral bordered by: (imaginary) parieto-occipital line includes: primary visual area thalamic input: lateral geniculate visual association areas pre-occipital notch lateral fissure central sulcus parieto-occipital line from Nieuwenhuys et al., (2008) The Human Central Nervous System

11 Lobes in ventral view frontal lobe temporal lobe pre-occipital notch occipital lobe from Nieuwenhuys et al., (2008) The Human Central Nervous System

12 Lobes in medial view rostral caudal rostral caudal parietooccipital sulcus calcarine sulcus limbic lobe from Nieuwenhuys et al., (2008) The Human Central Nervous System arch-like structure on the medial aspect of the cerebral cortex (part of the limbic system) includes: hippocampus (32) amygdala parahippocampal gyrus (16) cingulate gyrus (3)

13 A topographic map is the ordered projection of a sensory surface, like the retina or the Topographic skin, or an effector system, like organization the musculature, to one more of structures the of brain the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems (Wikipedia). Topographic organization is particularly clear in primary sensory areas and the primary motor area of the cerebral cortex. In sensory systems, adjacent neurons often respond to similar sensory stimuli and send axons to closely located cortical targets. topographic organization of the sensory cortex: retinotopy (visual system)...location of the visual object somatotopy (somatosensory system)...location of the body surface tonotopy (auditory system)...frequency of sound

14 Topographic organization of the primary visual cortex (V1) optic chiasm optic nerve optic tract calcarine sulcus lateral geniculate nucleus optic radiation primary visual cortex Donkelaar (2011) Clinical Neuroanatomy Horton and Hoyt (1991) V1 calcarine sulcus CAUDAL V1: central visual field ROSTRAL V1: peripheral visual field UPPER V1: lower visual field LOWER V1: upper visual field LEFT V1: right visual field RIGHT V1: left visual field Partial legion of V1 leads to vision loss in a partial visual field. e.g.,a lesion in left upper part of V1 would lead to vision loss in right lower visual field.

15 Topographic organization of the primary somatosensory cortex (S1) paracentral lobule -medial surface -leg and foot, defecation, urination -contains both motor and somatosensory areas -lesions caused by -occlusion of anterior cerebral artery -parasagittal meningioma

16 Topographic organization of the primary auditory cortex (A1) Tonotopy encodes sound frequency. Unlike visual and somatosensory systems, auditory system is well crossed. Therefore, unilateral lesions rarely cause hearing loss. A bilateral lesion in A1 would lead to cortical hearing loss. Saenz and Langers (2014)

17 Microscopic anatomy of the cerebral cortex The cerebral cortex is radially organized into layers. The mammalian neocortex has six layers of neurons. Each layer contains a specific subset of neurons that have a unique pattern of input and output projections. The density and arrangement of a given type of neurons in each layer (this results in the microscopic appearance called cytoarchitecture) differ between areas. V1 V2

18 Six layers of the mammalian neocortex few completely stained neurons cell bodies myelinated axons -Layers can be observed with Nissl staining or myelin staining. -Each layer contains a distinct set of neurons. -Layer 1 (most superficial) contains few cell bodies -Layers 2/3 contains many pyramidal neurons that connect to other areas (contralateral or ipsilateral) of the neocortex -Layer 4 contains many neurons that receive sensory information from the thalamus -Layer 5 contains many large pyramidal neurons that connect to the brainstem and spinal cord -Layer 6 (deepest) contains many pyramidal neurons that connect to the thalamus (ipsilateral)

19 Cellular organization of the cerebral cortex Major neuronal types: 1. Pyramidal neurons (shown in A above) -large cell body (50-100µm in diameter) -long dendrite reaching the pial surface (apical dendrites) -Betz cells are particularly large cells in primary motor area that have direct synapses with motor neurons. -primary source of axons that leave the cortex -found in layers 2/3 (mostly intracortically projecting) and layer 5 (mainly subcortically projecting) 2 Spiny stellate neurons -small (< 30 µm across ) and star-shaped -no apical dendrites -exclusively located in layer 4 of primary sensory areas of the neocortex -major targets of thalamocortical axons -source of local axon connections 3 Local circuit neurons (interneurons) (shown in B above) -many types of inhibitory neurons -GABA is the neurotransmitter

20 Cellular organization of the cerebral cortex Canonical neocortical circuit Targeting specificity of inhibitory neurons Gao et al. (2013) (in primary sensory areas) thalamus layer 4 layer 2/3 layer 5/6 subcortical targets

21 Subcortical projections motor control (direct connections onto lower motor neurons or indirect connections via interneurons) corticospinal tract (from layer 5) to spinal cord corticobulbar tract (from layer 5) to brainstem regulate the firing of thalamic neurons corticothalamic tract (from layer 6) These projections all form the internal capsule within the ventral telencephalon.

22 Intracortical projections corpus callosum 1. Two major axon bundles connect the two sides of the cortex (commissural connections) -corpus callosum (dorsal part of the cortex) -anterior commissure (temporal lobes) 2. Long association fibers connect different cortical areas on the same side. anterior commissure 3. In addition, different cortical areas are indirectly connected by the cortico-thalamo-cortical loop via layer 5 neurons connecting with thalamic neurons that then project to different areas of the cortex.

23 Diseases involving the cerebral cortex Major causes -vascular ischemia hemorrhage -tumor malignant glioma meningioma -degeneration Alzheimer s disease frontotemporal dementia -others schizophrenia bipolar disorders autism and many other developmental disorders epilepsy

24 Vascular supply of the cerebral cortex anterior cerebral artery (ACA) internal carotid artery middle cerebral artery (MCA) posterior cerebral artery (PCA) basilar artery three major sources of blood supply to the cerebral cortex: -anterior cerebral artery (ACA) -middle cerebral artery (MCA) -posterior cerebral artery (PCA) vertebral artery

25 Vascular supply of the cerebral cortex ACA ACA MCA PCA medial surface lateral surface Branches of ACA, MCA and PCA supply blood to different parts of the cerebral cortex. Occlusions of some of these branches in ischemic strokes cause specific defects of cortical functions that are normally carried out by the affected cortical regions

26 Vascular supply of the cerebral cortex ACA ACA MCA PCA medial surface lateral surface -loss of vision in the left visual field can be caused by occlusion of... -occlusion of the branch #3 of MCA on the right side of the cortex can cause...

27 Subarachnoid hemorrhage (SAH) SAH is a bleeding into the subarachnoid space. -Subarachnoid space is filled with CSF as well as vasculature. Causes of SAH: -head injury -spontaneous rupture of cerebral aneurysm In the case shown above in pictures, the patient had headache, vomiting, sensory loss in the right half of the tongue and paralysis of the oculomotor nerve developed.

28 Developmental disorders of the cerebral cortex -Up to 40% of children with drug-resistant epilepsy (chronic condition of recurrent seizures) have a cortical malformation. -In many of these cases, seizures are caused by imbalance of excitation and inhibition (excitation >> inhibition) -Such imbalance can be due to: 1. deficient migration of excitatory neurons that form an aggregate within the cortex (too much excitation) or 2. reduced production or migration of inhibitory interneurons (too little inhibition) Development of interneurons in the cortex is also important for brain plasticity, and its abnormality is implicated in many disorders including schizophrenia and autism.