NEO-CORTEX NEURON TYPES • In most of the cerebral cortex (neocortex), neurons are arranged in five layers with a sixth layer consisting of synapses on the surface. • Pyramidal and stellate cells predominate over all regions of the neo-cortex. • Pyramidal cellsare the output cells of the cerebral cortex. • Pyramidal cells activate other neurons. • Stellate cellsprovideconnection between afferent (Granule cells) neuron and efferent neuron (Pyramidal cell) • Granule cells receive afferents and form intracortical networks.
INTERNEURONS OF NEO-CORTEX • There are many different types of interneurons. • Interneurons have short dendrites and axons. • Interneurons create network interactions between neighboring neurons. • They are mostly inhibitory • Example of Interneurons: • Chandelier cells, • Double bouquetcells, • Spiny stellate cells, • Basket cells
PYRAMIDAL CELL • Pyramidal cells vary in size. • Pyramidal cell consists of conical cell body (>30 µm in diameter) with apical and basal dendrites and an axon • Axon of the pyramidal neuron leaves the base of the cell to enter white matter. • Pyramidal cells are found in layers 3, 5, and 6 • These cells have relatively few dendriticprocesses, however they contain a large number of dendritic spines covering the extent of these processes. • They have long, insulated axons.
PYRAMIDAL CELL • The apex of the dendritic arrangement is directed towards the cortical surface. • Dendrites from this cell synapse through more superficial layers (V to I) of the cerebral cortex. • Axon of the pyramidal course through corticospinal tracts to enervate motor neurons, • They also form recurrent axon collaterals projecting upon neurons located in more superficial layers. • Pyramidal cells may receive up to 200,000 synaptic inputs
OTHER PYRAMIDAL NEURONS TYPES • BETZ CELL: • Betz cells (Large Pyramidal Cell) are found in Layer V. • MEYNERT CELL (Large Pyramidal Neuron): • This cell is excitatory. • Meynert is located two layer of cortex. • Cell bodies of outer Meynert is found in 4B. • The "inner" Meynert cell bodies are regularly spaced at the layer 5/6 border.
PYRAMIDAL CELL (MEYNERT) • Meynert cell (excitatory); large pyramidal cell first described by Meynert; the "outer" Meynert cell bodies are found in 4B and are regularly distributed relative to pyramidal cell cones (Peters and Sethares, 1991; see below). • These cells project to area middle temporal (MT) (V5 Visual Area). • The "inner" Meynert cell bodies are regularly spaced at the layer 5/6 border. • These cells are also found in layer 6. • These cells typically have asymmetric, lateral dendritic arborizations, are highly direction selective, and are also known to project to area MT.
STELLATE (MARTINOTTI) CELL • Stellate (Martinotti) cell issmall, round cell body (<10 µm in diameter). • Martinotti cells are located in all lamina of the cortex except layer I. • These cells are estimated to have in the range of 10,000 to 50,000 synapses along the extent of their dendritic processes. • Stellate cells aremainly found in layers 2 and 4.
These cells are very small in comparison to the stellate and pyramidal cells, They are the most numerous within the human brain at an estimated 10 billion cells. Granule cells are located primarily in the somatosensory region of the neo-cortex known for sensory input. Axonsof the granule cell arborize extensively, forming synaptic connections with multiple stellate cells. GRANULE CELL
NEO-CORTEX SIGNAL PATHWAY • The afferent (input) signals relayed via thalamic pathways to the parietal lobe (somatosensory region). • Within this sensory receptive region, granule cells are the most numerous. • The location of granule cells(parietal lobe) and their connectivemap indicate that granule cells perform the task of expanding afferent stimuli to higher order arrangement. • Higher order signals again vastly increase the memory storage capacity and learning capabilities of downstream cortical (stellate and pyramidal) cells.
NEO-CORTEX SIGNAL PATHWAY • The granule cell operating in conjunction with stellate and pyramidal cells establish a three staged processing structure (granule->stellate->pyramidal). • Incoming signals are received by granule cells through principal fiber tracts. • These signals originating from the brain stem, hippocampus, thalamus, etc. • Axonal signals from granule cells are received by stellates, which subsequently project their axons onto the dendritic synapses of nearby pyramidal cells. • Axons from the pyramidal cells course down the cerebrospinal tract to enervate motor neurons, completing the control loop from the input stage (senses) to output (motor movement). • Two types of afferent projection fibers from the thalamus enter the neocortex: • Specific afferents: modality specific input; • Terminate in inner granule cell layer (IV) • Non-specific afferents: backgroundexcitation; • Terminate in molecular layer (I).
TEMPORAL LOBE & HIPPOCAMPUS • In the medial edge of the temporal lobes, the neuronal layers are reduced to two,one pyramidal and one granular (the dentate gyrus). • These layers are wound around one another and form a structure that is called hippocampus (sea horse) or Ammon's horn because of its peculiar shape. • The hippocampus receivesextensive afferents from association cortex and limbic areas, and projects to the thalamus, hypothalamus, and cortex. • The subiculum and entorhinal cortex are interposed between the hippocampus and the temporal neocortex.
TEMPORAL LOBE & HIPPOCAMPUS • The hippocampus is important for the process of declarative memory, • The memory for facts and events, • But is not a storage depot for old memories.
KORSAKOFF AMNESIA • Bilateral lesions of the hippocampus (and medial dorsal nucleus of the thalamus) cause Korsakoff amnesia, • A disorder is characterized by inability to remember, new things is relative preserved on the established memories.
CEREBELLUM • Function • Gross Anatomy • Histology: cell types • Synaptic circuits • Differ from the spinal cord in these ways: • Gray matter lies to the exterior with white underlying it; • Tissue of both kinds of cortex is folded into gyres for the cerebral cortex and folia in the cerebellum; • Nerve cells are of various types and are disposed in layers parallel to the pial surface, thusCerebellar cortex (Pia). • Molecular layer (cell processes, but few cells). • Purkinje cell layer. • Granule cell layer (densely packed small neurons) (underlying white matter).
CEREBELLUM GROSS ANATOMY • Cortex • Gray matter • White matter • Deep nuclei • Gray matter • The basic functional design of the cerebellum is that of aninteraction between two sets of quite different neuronalelements, • Those of the cortex and those centrally locatedcerebellar nuclei. • The cerebellar nucleireceive collaterals from the climbingand mossy fibers and are the main targets for the Purkinjecell axons. • The cerebellar cortex receives two types of afferents, theclimbing and mossy fibers, • Generates a single outputsystem, the axons of the Purkinje cells.
CEREBELLUM NEURONAL CIRCUITRY • The basic circuit is present in all vertebrates: • Purkinje cells and two inputs: • 1) Monosynaptic input, the climbing fiber (CF) • 2) Disynaptic input, the mossy fiber/parallel fibers (MF/PF) • Purkinje cell bodies are arrange in a single sheet (the Purkinje celllayer: PCL), • The cerebellar cortex is divided in two main strata: • 1) the level peripheral to the PCL known as molecular layer (ML) • 2) The layer deep to the PCL (towards the white matter), thegranule cell layer (GCL)
Cortex: Molecular Layer Stellate cells Basket cells Purkinje cell Layer Purkinje cells Granule cell Layer Granule cells Golgi cells Lugaro cells Unipolar Bushy cells CEREBELLAR HISTOLOGY • Nuclei: Deep cerebellar nuclei • Main Output: Purkinje Cells • Two main inputs: • Climbing fibers fromthe inferior olive • Mossy fibers fromthe brainstem
CEREBELLAR NEURONS • Purkinje cells • Out of the cerebellar cortex • Project to the deep cerebellar nuclei • Fan dendritic arborization • Dendritic spines • Receives one monosynaptic input (Climbing Fiber) • Adisynaptic input (Mossy or Parallel Fiber) • GABAergic • Calbindin positive
CEREBELLAR INTERNEURONS (INTRINSIC INPUT) • Granule cell Layer • Granule cells • Golgi cells • Lugaro cells • Unipolar Bushy cells • Molecular Layer • Basket cells • Stellate cells
GRANULER LAYER; GRANULE CELLS • Cell body and dendrites are located in the granule cell layer • Smallest cells in the cerebellum. • Oval round soma (5-8 µm) • Short dendrites that end on an expansion • Axon ascend to the ML through the PCL • Axons (T shape: split into two branches with opposite directions) make “en passant” synaptic contact with dendrites and dendritic spines in the molecular layer. • These fibers are • precisely array parallel to each other along the length of the folia; parallel fibers. • 1 PC can receives as many as 200,000 PF (humans)
GRANULAR LAYER; GOLGI CELLS • There are two sizes: • Large ones (9-16µm in diameter) whichare found in the upper of the GCL and • Small ones (6-11µmdiameter) which are found in lower half of the GCL. • They have extensive radial dendritic trees that extendthrough all the layers of the cortex. • They receive input from PF and CF • Mossy fibercollaterals in the GCL. • Their axons branch in the GCL, where they terminate ongranule cell dendrites in the cerebellar glomeruli. • They are GABAergic neurons • Other Granular Layer Cells • Lugaro cells • Unipolar Bushy cells
MOLECULER LAYER • Stellate cells • In the outer 2/3 of the ML.Few dendrites and short axon. • GABAergic • Basket cells: in the deep parts of the ML, near the PCL. • Theiraxons extend along the PCL at tight angles to the direction ofthe PF. • They spread over a distance equal to 20 PC with. • Thehorizontal segment of a basket cell axon send off groups ofcollaterals that descend and embrace (hold) the PC soma and initialsegment. • Also contact the dendritic shaft. • GABAergic
CEREBELLAR TRACT • Climbing fibers • From the contralateral inferior olive • Myelinated/ Collaterals to the deep cerebellar nuclei • Dendritic shaft and spines on main dendritic trees of Purkinje cells • Purkinje neurons only receive 1 CF (adult). Excitatory • Mossy fibers • From brainstem (ipsilateralvestibular nuclei) • Distribute in the GCL • Have 20-30 rosettes • Synaptic contact on granule cell and • Golgi cell dendrites;as glomerulus • Excitatory
CEREBELLAR NEURONS • Flask-shaped Purkinje cell and one of itslarge primary dendrites (D). Note theclosely packed chromatic nuclei of granulecells (GC). The irregular light spaces(arrows) here and there are glomeruli. • Elaborate dendritic tree of a Purkinje cell(PC) which spreads fan-like at right anglesto long axis of the folium. • Note that primaryand secondary branches (open arrow) havea smooth surface whereas tertiary branches(solid arrow) are spinous. • Basket cell axon collaterals (BCA)traversing the molecular layer (ML) of thecerebellum and descending to formsynaptic networks (baskets) about thesoma of Purkinje cells. • Basket cells arelocated deep within the molecular layer. • GCL, granular cell layer; CWM, central whitematter.
Glial cells • Embryonic/young cerebellumcontains one type of glialcells • Adult cerebral cortexcontains 4 types of glial cells Radial glia(Bergmann glia)
BRAIN STEM • (a) Resembles the spinal cord in having nerve cell bodies grouped in nuclei and nerve fibers in tracts. • (b) Some special nuclei of the brain stem and hypothalamus are: • (i) The reticular formation is an extensive system of groups of neurons serving many vital tasks, but whose nuclear organization is hard to discern. • (ii) Neurons of the substantianigra contain melanin pigment and dopamine. • (iii) Certain hypothalamic nuclei have neurosecretory neurons
SPINAL CHORD • l Enclosed in CT meninges with pia extending in at the ventral fissure with the anterior spinal artery. • 2 The ependyma-linedcentral canal lies centrally. • 3 Surrounding the canal in a butterfly shape is grey matter (grey to the naked eye when fresh and unstained). • 4 Horns of grey matter partly separate three columns ofwhite matter;dorsal (posterior), lateral, and ventral (anterior) columns (or funiculus).
SPINAL CHORD • 5White matter is composed of nerve fibers, many thickly myelinated, running mainly up or down the cord. • Generally, fibers projecting to or from a particular brain region run together in a tract. • 6Grey matter has groups of multipolar nerve cell bodies, nerve fibers entering and leaving the grey matter, and preterminal fiber branches (poorly myelinated, hence the grey color in the fresh, unstained cord). • 7 Glial cells and blood vessels are in both white and grey matter. Grey matter is more vascular. The oligodendrocyte is the principal glial cell of white matter.
SPINAL CHORD • 8Roots of nerve fibers enter the cord on the dorsal sides; other roots leave on the ventral sides. • 9Substantiagelatinosa lies at the extreme margin of the dorsal horn of grey matter. • 10 The multipolar neurons include: • Large Golgi Type I neurons lie in the Anterior Horn. • Motor neurons, whose axons pass out of the cord to join peripheral nerves and serve skeletal muscles; • Short axoned interneuron/ Renshaw cells (Golgi type II).
PERIPHERAL NERVOUS SYSTEM • The peripheral nervous system is divided into two major parts: • The SomaticNervous System • TheAutonomic Nervous System. • 1. Somatic Nervous System • The somatic nervous system consists of; • Peripheral nerve fibers that send sensory information to the central nervous system • Motor nerve fibers that project to skeletal muscle. • The cell body is located in either the brain or spinal cord and projects directly to a skeletal muscle
PERIPHERAL NERVOUS SYSTEM • 2. Autonomic Nervous System • The autonomic nervous system is divided into three parts: • The sympathetic nervous system, • The parasympathetic nervous system • The enteric nervous system. • The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands. • The preganglionic neuron is located in either the brain or the spinal cord. This preganglionic neuron projects to an autonomic ganglion. • The postganglionic neuron then projects to the target organ. • Notice that the somatic nervous system has only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons.
ENTERIC NERVOUS SYSTEM • The enteric nervous system is a third division of the autonomic nervous system • The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, gall bladder).
In the Peripheral Nervous System, neurons can be functionally divided in 3 ways