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Nerve Tissue
Organisation of the Body
| Question | Answer |
|---|---|
| Nervous system | Central nervous system - Brain and spinal cord Peripheral nervous system - all neuronal elements outside brain and spinal cord, Cranial and spinal nerves contain sensory and motor neurons Associated ganglia and supporting Schwann/satellite cells |
| Divisions of the nervous sytems | CNS and PNS PNS splits into motor afferents and sensory efferents These divide into autonomic and somatic Autonomic divides into sympathetic, enteric and parasympathetic |
| Development of PNS | Neural crest stem cells originate from edge of neural tube Migrate away to peripheral organs and ganglia outside CNS CNS arises from neural tube |
| Development of neural crest cells | Neural crest stem cells specify, migrate and terminally differentiate Melanocytes are like neurons and originate from this neural crest. These confer pigmentation |
| Motor component of PNS | Somatic motor - motor innervation of skeletal muscle Visceral motor - motor innervation of smooth muscle, cardiac muscle and glands (splits into sympathetic etc) |
| Sensory component of PNS | Somatic sensory - touch, pain, pressure, vibration, temperature and proprioception in skin as well as hearing, equilibrium, vision and smell Visceral sensory - Stretch, pain, temperature, chemical changes, irritation, hunger and taste (less conscious) |
| Types of peripheral neurons | Sensory - convey sensory info to CNS from the environment and within the body Motor - Convey information from the CNS to muscles or glands |
| Sensory neurons | Single unipolar neurons with a central cell body Involved in reception of sensory info Peripheral branch originates in destination in body Cell bodies located in clusters called ganglia next to the spinal cord Axons terminate at interneurons in CNS |
| Types of receptor | Thermoreceptors - changes in temperature Photoreceptors - light Chemoreceptors - chemicals Mechanoceptors - pressure, touch, vibrations Nociceptors - pain |
| Sensory endings of neurons | Pacinian corpuscle - coarse touch, pressure, vibration Ruffini's corpuscle - stretch Merkel's disks - touch Free nerve endings - touch, pain, temperature |
| Baroreflex | Sensory nerves that run along the vagus control blood pressure Sensory neurons have ganglia projecting into aortic arch - contain baroreceptors that detect volume of aortic arch |
| Location of sensory cell bodies | Found in ganglia All cell bodies located here so creates a bulge |
| Types of motor neurons | Single neurons - somatic NS e.g. NMJ Two neuron network - Sympathetic and parasympathetic NS - cell bodies in spinal cord project onto a ganglia, not the effector |
| Somatic neurons | Motor neurons of the somatic nervous system (voluntary) are a single neuron network Monosynaptic |
| Autonomic neurons | Motor neurons of the autonomic nervous system (involuntary) are a two neuron network Disynaptic Parasympathetic use ACh Sympathetic use noradrenaline |
| Cell bodies of autonomic nervous system | Cell bodies onto which autonomic neurons synapse found in ganglia Sympathetic - paravertebral ganglia (sympathetic trunks) e.g cervical, celiac and mesenteric ganglia Parasympathetic - terminal ganglia e.g. Meissner's and Auerbach's plexus |
| Autonomic nervous system divisions | Sympathetic - supplies motor component to muscles, viscera, cardiac and smooth muscle Parasympathetic - viscera, cardiac and smooth muscle, no Musculo-skeletal supply Enteric - intrinsic nervous system of gut |
| Origination of sympathetic system | Originates from the thoracic spinal cord and upper lumbar spinal cord Connect to paravertebral ganglia or sympathetic trunks (two chain ganglia extending along the vertebral column) Cervical ganglia - head and thorax Celiac/mesenteric ganglia - gut |
| Origination of parasympathetic system | Originates from medulla and mid brain and in sacral position of spinal cord Second neuron found in small terminal ganglia near effector organs Terminal ganglia of sacral nerve neurons - plexus of submucosa Meissner and Auerbach plexus |
| Neurons | Basic cell of nervous system Has axons and dendrites Conducts impulses Has structural and functional polarity |
| Structure of neurons | Cell body - nucleus and organelles (metabolic centre) Axon - long process stretching from cell body (signal from cell body) Dendrites - short cell processes (increase contact between axons) - to cell body Synapses - junction between axons and dendrites |
| Cell bod of neurons | Large nucleus and contains finely dispersed chromatin indicative of synthetic activity Cytoplasm had abundant RER When stained with basic dyes RER appears as clumps of basophilic material called Nissl bodies |
| Specialisation of axons | Long slender projections from cell body Conducts electrical impulses away from neuron cell body Transmits signals to other cells Microscopic in diameter May extend up to a metre As bundles they make nerves |
| Dendrites | Short branched projections from cell body Conduct electrical impulses to cell body Have short trunk ramifies into several smaller and smaller branches Have dendritic spines (plastic structures) involved in motivation, learning and memory |
| Asymmetry of neurons | Structural polarity - one domain specialised to receive incoming signals the other for sending signals Functional polarity - unidirectional impulse propagation |
| Role of cytoskeleton in neurons | Maintains neural form Microtubules - transport Neurofilaments - maintain axonal structure Microfilaments - changes in cell shape and scaffold for signal transduction systems Can identify these with immunocytochemistry - double fluorescence |
| Axonal transport and neuronal polarity | Neurons have bidirectional transport Microtubules are the main skeletal tracks for transport Towards cell body- retrograde Away from cell body - anterograde |
| Role of molecular motors in transport | Mechanochemical enzymes - Kinesin (anterograde) and dynein (retrograde) Transport in two directions Anterograde - organelles, growth factors, neurotransmitters Retrograde - endocytosis produces to endosomes in cell body |
| Myelination of neurons | Myelin mixture of phospholipids and proteins PNS neurons myelinated by schwann cells Plasmalemma of schwann cells is myelin Schwann cells wrap around axons Important for rapid AP propagation |
| How does myelination begin | Numerous layers are wrapped around axon Each turn forms a lamella of myelin Mesaxon is where 2 layers of plasma membrane pf Schwann cells meet to form Edge to edge contact to form major dense line |
| Role of schwann cells in all axons | One schwann cell produces myelin for one node of one axon Non-myelinated axons bury themselves in schwann call cytoplasm Oligodendrocytes in CNS wrap myelin around several axons at once |
| Role of myelin | Provides support Facilitates fast axonal conduction Insulates the axon to reduce electrical capacitance of axons |
| Nodes of Ranvier | Myelin sheath has gaps along its path called nodes of ranvier Inter digitating processes of schwann cells cover node Lamella of myelin end on axon in a staggered fashion |
| Saltatory conduction | Clustering of voltage gated Na and K ion channels at nodes of ranvier action potential signal jumps along the axon from node to node Na and K ion channels with Caspr2 mediate repolarisation enabling ion flow Results in rapid wave of depolarisation |
| Faster propagation in myelinated neurons | AP propagation in myelinated neurons faster than in unmyelinated neurons due to saltatory conduction Nodes of ranvier can be thought of as a digital electronic amplifier |
| Composition of myelin | 70% phospholipid 30% membrane proteins Myelin proteins are the products of a set of specialised genes expressed by myelinating cells Major proteins - protein 0, myelin basic protein and phospholipid protein Have adhesive properties |
| Pelizaeus-Merzbacher disease | Large range of mutations involving PLP gene leads to varying degrees of physical and mental retardation 40 different mutations in PMD-severity depends on location Spontaneous PLP point mutations in mice and rats (jimpy mice) lead to neurological defects |
| Demyelination due to PLP mutation | In jimpy mice majority of axons remain unmyelinated and myelin sheaths formed are thin relative to axon diameter In wild type mouse axons are surrounded by thick myelin sheaths |
| Charcot-Marie-Tooth disease | Most common inherited neurological disorder Autosomal dominant/recessive - heterogenous disease Demyelinating or axonal neuropathy - mutations in myelin or axon genes Occurs in adolescence or early childhood Muscle weakness, hand and arm + lower limb |
| Autoimmune disease - loss of myelin | Autoimmune demyelinating neuropathies like Guillain Barre syndrome Caused by antibodies to glycosphingolipids Usually triggered by acute infection Myelin regeneration occurs but axon damage can occur Many patients are paralysed and unable to breathe |
| How are neurons arranged in nerves | Collection of axons linked together by support tissue into anatomically defined trunk Nerve - bundles (fascicles) of axons, schwann cells and myelin Support cells and blood vessels present |
| Support tissue of nerve trunk - Epineurium | Outer sheath binds individual fascicles into a nerve trunk with type I collagen and fibroblasts Prevents interference from conduction of nerve impulses in neighbouring axons |
| Support tissue of nerve trunk - Perineurium | Surrounds fascicles bundles of axons Tight junctions between cells protect nerve fibres |
| Support tissue of nerve trunk - Endoneurium | Surrounds axons and Schwann calls Longitudinally orientated type III collagen fibre, fibroblasts and capillaries |
| Neuronal relay stations | Ganglia - collection of cell bodies Also contain efferent and afferent axons, support cells (satellite cells), blood vessels and support tissue |
| Dorsal root ganglia | Contain cell bodies of afferent sensory neurons Cell bodies have defined nucleus, nucleolus and visible schwann cells |
| Autonomic ganglia | Host cell bodies of post-ganglionic nerves Cell bodies dispersed Fewer satellite cells Neurons are multipolar Numerous unmyelinated axons Sympathetic ganglia can have lipofuschin granules Parasympathetic ganglia - few cell bodies clumped in support |
| Synapse | Part of the synapse on axon side is presynaptic (bouton) terminal Part on adjacent cell is post synaptic terminal Between these terminals is the synaptic cleft (10-50 nm) Synaptic vesicles contain neurotransmitter Transmitter receptors |
| Neurotransmission | Arrival of nervous impulse at pre-synaptic axon Initiates opening of VGCC and Ca influx Triggers release of neurotransmitter in synaptic cleft Neurotransmitters binds to receptors on post synaptic membrane Stimulate/inhibit membrane |
| Neuromuscular junction | A single axon can innervate several muscle fibres via axon collaterals When neuron fires, all muscle fibres will contract simultaneously This aggregation of muscle fibres is a motor unit Each skeletal muscle fibre has a single NMJ |
| Autonomic neuroeffector junctions | In ANS synapses between efferent neuron and effector organ is a neuroeffector junction Neurotransmitters are released at swellings on the axon called varicosities Transmitter released at all these Release is widespread and slower than typical |
| Myasthenia Gravis | Caused by autoantibodies blocking the AChR so signal not transmitted Affects control of voluntary muscle Drooping eyelids, Impaired speech, breathing difficulties, blurred vision, muscle weakness |
| Blood test for antibodies against AChR | Cobra toxin binds to AChR alpha bungarotoxin can be tagged by fluorescent dyes or radioactivity Antibodies against AChR bind to it and cause precipitation Fluorescent dyes or radioactivity measured Antibodies in 85% of MG patients |
| Mutations in proteins at NMJ | Dominant mutations in NMJ genes cause disease Use gene silencing to silence mutated allele Allele specific silencing allows selective down regulation of pathogenic dominant allele - allow expression of normal wild type allele |
| Regeneration of nerve axons | Peripheral nerve axons - able to regenerate throughout life - can reattach limbs Central nerve axons - Ability to regenerate decreases from birth - spinal cord injuries do not regenerate |
| Nerve regeneration process | Distal part atrophies when damaged - Wallerian degeneration Schwann cells divide and form hollow tube enclosed by endoneurium Proximal end of nerve fibre sends out sprouts towards tubes Sprouts are attached by growth factors produced by Schwann cells |
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