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32 Cards in this Set
- Front
- Back
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nervous tissue
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Composed of interconnected network of specialized cells called neurons and their support cells
– Neurons gather and process information and generate appropriate response signals |
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Fundamental property of neurons is their excitability
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– Electrical potential created in resting neurons by maintaining an ionic gradient across the plasma membrane
– Change in membrane permeability in response to stimulus reverses the ionic gradient and plasma membrane becomes depolarized – wave of depolarization along the plasma membrane referred to as an action potential – Followed by repolarization to re-establish resting potential |
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Anatomic division:
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– Central nervous system
• brain and spinal cord – Peripheral nervous system • ganglia and nerves that link brain or spinal cord with tissues outside the CNS |
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Functional division:
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– Somatic (voluntary)
• Effector organs are skeletal muscles – Autonomic (involuntary) • Sympathetic and parasympathetic • Effector organs include smooth muscle, cardiac muscle, and certain endocrine tissues (e.g. adrenal medulla, posterior pituitary gland) – Sensory (afferent) - input – Motor (efferent) - output |
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components
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– Nerve cells (neurons), including their extensive cytoplasmic processes
• Clusters or groups of nerve cell bodies • CNS: nuclei • PNS: ganglia • Organized collection of nerve cell processes that form a definable anatomic structure • CNS: tract • PNS: nerve – Support cells (glial cells) • CNS: • Astrocytes • Oligodendrocytes • Microglia • Ependymal cells • PNS: Schwann cells |
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cell body
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– Primarily in CNS, also PNS ganglia
– Round, centrally located nucleus • Dispersed chromatin • Prominent nucleolus Perikaryon (cytoplasm) • Nissl substance – large aggregates of rough ER visible as basophilic material by light microscopy – Extends into dendrites but not axons – Cytoskeleton • Intermediate filaments (neurofilaments; structural support) and microtubules (axonal transport) |
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dendrites
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– One to multiple
– Branched, tapered processes – end in specialized sensory receptors (primary sensory neurons), or synapse with adjacent neurons (from which receive stimuli) – Sites of information INPUT (AFFERENT) |
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axons
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– Single
– originate in cone-shaped region of the cell body, axon hillock – long cylindrical process, can be as long as 1 meter – Terminates on other neurons or on effector organs – Terminates in small branches which end in small swellings called terminal boutons – aka nerve fibers – Sites of information OUTPUT (EFFERENT) • Conduct action potentials that arise in cell body – Lack Nissl substance |
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basic types of neurons
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– Characterized by arrangement of axons and dendrites w/ respect to cell body
– Multipolar neurons • most common, numerous dendrites arising from the cell body • e.g. motor, intermediate and integratory neurons – Bipolar neurons • Single dendrite from one pole of cell body opposite origin of the axon • e.g. receptor neurons of sight, smell, balance – Pseudo-unipolar neurons • Single dendrite and the axon arise from common stem of the cell body • e.g. sensory ganglia in spinal nerve roots NOTE: Neurons are terminally differentiated (G0 cell cycle) and do NOT regenerate after cell death; axons and dendrites CAN regenerate after certain damages, provided cell body remains viable |
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nerve fibers
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• = Axons
• Myelinated or non-myelinated – Presence or absence of myelin sheath • Condensed, concentric layers of plasma membrane • Myelination in PNS vs. CNS – PNS: Schwann cells – CNS: Oligodendrocytes • Rate of conduction of action potentials proportional to axon diameter – Myelination increases conduction velocity • All axons in PNS are enveloped by Schwann cells – Provide structural and metabolic support – External surface of Schwann cell bounded by an external lamina |
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nonmyelinated
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• Non-myelinated:
– envelopment by simple longitudinal invaginations of Schwann cells plasma membranes – One Schwann cell envelops multiple axons – Small diameter axons, e.g. autonomic nerve fibers and small pain fibers |
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myelinated
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– Axons wrapped by variable numbers of concentric layers of the Schwann cell plasma membrane
– Inner leaflets fuse and cytoplasm is extruded to form multiple layers of membrane that surround the axon = myelin sheath – Specific proteins bind layers of cell membrane that form myelin – Differ between CNS and PNS – Large diameter nerve fibers |
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myelin sheath
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– Increase conduction velocity along axons
– Internode = single segment of myelin produced by one Schwann cell; oligodendroglia form multiple internodes – Nodes of Ranvier = short intervals between Schwann cells at which axon is not covered by myelin – Saltatory conduction – action potential travels by jumping from node to node; high lipid content of modified membrane layers insulate the underlying axon…prevents ion fluxes except at nodes of Ranvier |
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synapses
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• Highly specialized cellular junctions particular to neurons
• Neuron-to-neuron junctions = synapses • Neuron- to-muscle junctions = neuromuscular junctions or motor end plates • Types of synapses: axodendritic, axosomatic, axoaxonic • Conduction of impulse at a synapse is unidirectional, but response can be excitatory or inhibitory |
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terminal bouton
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– swellings at ends of axons
• non-myelinated • many mitochondria and synaptic vesicles (membrane-bound vesicles containing neurotransmitter) |
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synaptic cleft
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– narrow intercellular gap of uniform width (20-30 nm)
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Postsynaptic web (postsynaptic density)
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is network of microfilaments forming a desmosome-like structure that supports structural stability of postsynaptic membrane
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function
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• Arrival of action potential stimulates fusion of synaptic vesicles with presynaptic membrane and release of neurotransmitter by exocytosis into synaptic cleft
• Neurotransmitters diffuse across synaptic cleft to bind with receptors on postsynaptic membrane • Neurotransmitters inactivated by enzymatic cleavage or re-uptake into presynaptic terminal bouton |
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Motor end plates
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• Same basic structure as other synapses, except one motor neuron can innervate from few to a thousand muscle fibers
• Motor unit = motor neuron and muscle fibers it innervates • Motor end plate resides in depression in muscle cell surface known as sole plate • Postsynaptic membrane deeply folded to form parallel secondary synaptic clefts • Neurotransmitter of somatic neuromuscular junctions is acetylcholine • Ach receptors concentrated at margins of secondary synaptic clefts • Acetylcholinesterase located deeper in these clefts to deactivate Ach |
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peripheral nervous tissue
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• Peripheral nerves composed of any combination of afferent or efferent nerve fibers (axons) of either somatic or autonomic nervous systems
– cell bodies in CNS or peripheral ganglia • Each peripheral nerve composed of one or more bundles, i.e. fascicles, of nerve fibers |
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supporting/ct of peripheral nerves
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– Endoneurium – delicate fibrovascular tissue that surrounds and supports each individual nerve fiber and its associated Schwann cell
– Perineurium - collagenous connective tissue that surrounds each fascicle – Epineurium - fibrous sheath that binds fascicles together to form the nerve – Blood vessels course within epineurium and perineurium and ramify to generate capillaries located in endoneurium |
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ganglia
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are aggregates of cell bodies of neurons located outside CNS
– Satellite cells provide structural and metabolic support to neurons; similar embryologic origin to Schwann cells – Parasympathetic ganglion – located within or near effector organs, e.g. within walls of GI tract |
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Central nervous tissue
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• Central nervous system consists of brain and spinal cord
• Each can be divided into grey and white matter – Grey matter contains the neuron cell bodies and their processes embedded in a matrix of support cells, neuroglia – White matter consists of tracts of axons coursing within and between different regions of brain or spinal cord; also supported by neuroglia • Substantial number of axons are myelinated – appears white in fresh tissue |
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neuroglia
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– Supportive cells that constitute the non-neuronal cells of the CNS
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astrocytes
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• pack the interstices between neurons
• Highly branching with numerous processes • Provide mechanical support • Mediate exchange of metabolites between neurons and blood vessels • Form part of the blood-brain-barrier • have larger nuclei than oligodendrocytes and cytoplasmic processes which are not usually visible (blend with neuropil) • Most common glial cell in GREY matter • Long branched processes make up much of the neuropil • Astrocyte processes end on basement membranes of capillaries (perivascular feet) • Mediate metabolic exchange between neurons and blood • Those in white matter have few straight cytoplasmic processes fibrous astrocytes • Intermediate filament = glial fibrillary acidic protein (GFAP) |
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oligodendrocytes
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• CNS equivalent of Schwann cells
• Elaborate myelin sheaths around axons in CNS • Predominant type of neuroglia in WHITE matter • have small round condensed nuclei and unstained cytoplasm; tend to aggregate around neuron cell bodies • Artifactual perinuclear halo in paraffin-embedded sections • Aggregate around nerve cell bodies in grey matter, analogous to satellite cells • Short branched processes connect cell body to myelin sheaths • Process of myelination similar to that of Schwann cells…BUT, a single oligodendrocyte can provide myelination to up to 50 axons from same or different tracts • vs. Schwann cells -> only provide segment of myelin sheath for one axon (though can support multiple non-myelinated axons |
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microglia
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• Monocyte-macrophage lineage cell, with defense functions
• Derived from mesenchymal cells of mononuclear phagocyte system, aka macrophage-monocyte system • Small cell body with highly branched processes • Transform into large amoeboid phagocytes in response to tissue damage • Best identified by immunohistochemical staining, e.g. CD68 (stains cells of macrophage lineage) |
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ependymal cells
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• Specialized epithelial-like cells that line ventricles, central canal of spinal cord and choroid plexus
• Have junctional complexes but lack basement membrane • Cuboidal in shape; cilia and microvilli are present |
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choroid plexus
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• Arises from walls of ventricles
• Branching system of blood vessels that run in fronds covered by cuboidal epithelium derived from ependyma • Lining cells rest on a basal lamina • Tight junctions (zonula occludens) contribute to the blood-CSF barrier • Responsible for formation of cerebrospinal fluid |
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neuropil
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is network of axons and dendrites surrounding neurons and neuroglia
– Most fibers in neuropil lack myelin, hence eosinophilia |
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meninges
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Layers from outer to inner:
• Dura mater: “tough mother;” outermost layer comprised of dense fibroelastic CT; merges with periosteum of skull • AKA pachymeninges • Arachnoid layer: middle layer of fibrous tissue with cobweb-like strands that connect it to the underlying pia mater • Underlying subarachnoid space contains CSF and is connected to ventricular system • Pia mater: innermost layer of delicate collagen and elastin fibers; sit on underlying basement membrane • Arachnoid + Pia mater = leptomeninges – Glia limitans is the impermeable barrier formed by foot |
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Blood-Brain Barrier
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– Blood-brain barrier refers to properties of CNS capillaries, which are impermeable to large molecules from plasma
Endothelial cells are not fenestrated Endothelial cells secured to each other by tight junctions (zonula occludens), thereby sealing vascular space from neuroparenchyma Endothelial cells exhibit little or no pinocytosis Endothelial surface membranes harbor detoxifying enzymes -> Protection against toxins and infectious agents – Maintenance of this endothelium under control of astrocyte foot processes – Certain areas lack this barrier to allow for specialized functions - e.g. choroid plexus, pituitary gland, hypothalamic vomiting center |
