PHYSIOLOGY)OF)THE)NERVOUS)SYSTEM · Referred’Pain’ ......
Transcript of PHYSIOLOGY)OF)THE)NERVOUS)SYSTEM · Referred’Pain’ ......
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PHYSIOLOGY OF THE NERVOUS SYSTEM
Table of Contents
Neural Development ................................................................................................................................................ 2 Principles of Nerves ................................................................................................................................................. 3 Parts of a Neuron .................................................................................................................................................................. 3 Sympathetic Nervous System ........................................................................................................................................... 3 Parasympathetic Nervous System .................................................................................................................................. 3 Central Nervous System ..................................................................................................................................................... 4 Peripheral Nervous System .............................................................................................................................................. 4 Enteric Nervous System ..................................................................................................................................................... 4
Types of Nerves ......................................................................................................................................................... 5 Neurotransmitters ............................................................................................................................................................... 5
Membrane and Action Potential .......................................................................................................................... 6 Voltage-‐Gated Channels ..................................................................................................................................................... 6 Action Potential Process .................................................................................................................................................... 7 Action Potential Propagation ........................................................................................................................................... 8
Neurotransmission ................................................................................................................................................... 9 Synapsing ................................................................................................................................................................................ 9 Summation ............................................................................................................................................................................ 10
Neural Integration ................................................................................................................................................. 11 Modulation of Transmitter Release ............................................................................................................................. 11
Spinal Cord ............................................................................................................................................................... 12 Physical Structure of Spinal Cord ................................................................................................................................. 12 Dorsal column-‐medial lemniscus v Spinothalamic Tract .................................................................................... 13
Spinal Reflexes ........................................................................................................................................................ 14 Two types of Nociceptive Fibres .................................................................................................................................... 14 Pain Transmission ............................................................................................................................................................. 14 Referred Pain ....................................................................................................................................................................... 14 Reflexes .................................................................................................................................................................................. 14
Neuromuscular Junction ..................................................................................................................................... 15
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NEURAL DEVELOPMENT Gastrulation (wk 3): formation 3 primary germ layers, epiblast cells migrate through primitive streak
Endoderm (interior): lining of digestive tract, related organs (stomach, colon, liver, lungs etc) Mesoderm: muscle, CT, skeletal system, circulatory system, organs (heart, blood, kidney, spleen) Ectoderm: CNS, skin
Neuralation (21-‐28 days): transformation of neural plate à neural tube, • Notochord (mesodermal mass) induces epiblastic neural plate to elevate and involute/curl up • Neural folds at either side rise up to form neural groove (leaving a dip in middle à neural tube) • Ectoderm invaginates forming groove, cells on either side = neural crest (form spinal ganglia). Neural tube closes centrally then ‘zips up’ including at top and bottom.
• pseudostratified cells lining inner lumen of neural tube à apolar neuroblasts à neurons • neural crest: cells lining edge of neural folds à migrate around body à ganglia Failure of neural tube to fuse rostrally: anencephaly (failure of bone to form around brain), caudally: spina bifida (spinal cord not completely encased) Cephalic end (top) of neural tube: divides into 3 dilations (to form brain)
Somites: mesodermal masses (derived from paraxial region) distributed along two sides of neural tube. Consists of sclerotome (à CT, cartilage, bone) and dermomyotome (à dermatome = deep layers of dermis and myotome = muscle). Nerves grow to somites through chemotaxis (grow towards secreted chemicals)
Nervous System
Central Nervous System (CNS)
Brain Spinal Cord
Peripheral Nervous System (PNS)
Somatic Autonomic
Enteric (directly donctrolls
gastrointestinal system)
Sympathetic (Fight or Flight)
-‐> thoracolumbar
Parasympathetic (Rest and digest) -‐>craniosacral
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PRINCIPLES OF NERVES Nerve Fibre: process and transmit information through electrical and chemical signalling. Nerves are:
• excitable (polarity of membrane can change) • conductible (depolarisation propagates along
membrane) • secretory (release chemical neurotransmitters)
Synapse: joining of an axon with another nerve fibre, which allows the signal to continue to be propagated. Axosomatic (cell body) = inhibitory, axodendritic = excitatory, axoaxonic (axon à axon) Parts of a Neuron:
• Dendrites: receive impulse to be propagated • Soma/Perikaryon: cell body, contains nucleus. Merges with axon
at axon hillock • Axon: carries signal from soma to terminal, inside = axoplasm.
Conduction velocity proportional to diameter of axon • Myelin Sheath: lipid-‐based electrical insulation of neuron,
increasing speed of conductance 100 fold. Myelin secreted by glial cells (Schwann = PNS, oligodendrocytes = CNS). Glial cells wrap tightly around axon multiple times, with layers of myelin between each coil.
• Neurilemma: outermost nucleated cytoplasmic layer of glial cells • Nodes of Ranvier: depressions in sheath
Glial / Neuroglia cells: non-‐neuronal cells that maintain homeostasis, secrete myelin, and provide support and protection for neurons. As nervous tissue has no ICM, glial cells provide a microenvironment for neuronal activity. Astrocyte: star shaped glial cells in the brain and spinal cord. Most abundant cell in brain. Plexus: network of intersecting (intermingle but don’t join) nerves. Most nerves both motor and sensory. Dorsal Root Ganglia: swellings of cell bodies of afferent spinal nerves. Have no synapses, all located in intervertebral foramina.
Sympathetic Nervous System: ‘fight or flight’, thoracolumbar outflow à presynaptic preganglionic cell bodies in intermediolateral columns of spinal cord à postganglionic cells in sympathetic trunk (hence short presynaptic nerve) Parasympathetic Nervous System: ‘rest and digest’, craniosacral outflow à presynaptic preganglionic cell bodies in grey matter of brainstem + sacral/tail segment of spinal cord à postganglionic cell peripherally located (hence long presynaptic nerve) **SNS and PS can have complementary action (PS ‘point’ erection and S ‘shoot’ ejaculation), independent action (blood vessels SNS only) or opposing action (S dilates pupils, PS constricts pupils)
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Central Nervous System: Brain + Spinal Cord Grey matter: closely packed neural cell bodies (central in spinal cord, peripheral in brain) White matter: mostly contains long, myelinated axon tracts + glial cells, connect one part of brain to another or to spinal cord (central in brain, peripheral in spinal cord) à Projection tracts: up and down à Association tracts: front and back à Commissural tracts: from one cerebral hemisphere to another (across midline) Meninges: connective tissue layer around brain providing support + protection (Dura, Arachnoid, Pia matter) Cerebral Spinal Fluid: circulates within subarachnoid space and ventricles of brain
Peripheral Nervous System: peripheral nerves 12 cranial nerve pairs (ordered rostral = head to caudal = tail) originating in brain stem innerving parts of head 31 spinal nerve pairs; each associated with spinal cord segment
• 8 cervical (C8) above C1-‐7 and T1: head, neck, shoulders • 12 thoracic (T12) below T1-‐12: form intercostal nerves (not plexus), pass around trunk to upper limbs • 5 lumbar (L5) below L1-‐5: form lumber plexus, supply lower limbs • 5 sacral (S5) below S1-‐5: form sacral plexus, hips, lower limbs etc • 1 coccygeal: supplies lower limb and pelvis
Enteric Nervous System: subdivision of ANS that directly controls gastrointestinal system in vertebrates. Operates independently of brain and spinal cord.