Peripheral Nervous System PDF

(007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 - Surrounded by several layers of connective tissue w/c...

(007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 - Surrounded by several layers of connective tissue w/c partition the nerve into several nerve (axon) bundles or OUTLINE fascicles I. INTRODUCTION - Inside a nerve track will be a groupings or bundles of II. CONNECTIVE TISSUE LAYERS IN PNS fascicles; inside the fascicles, they contain many axons or III. NERVE FIBERS nerve fibers. A. Myelinated Nerve Fiber B. Myelin Formation C. Non-myelinated Nerve Fiber II. CONNECTIVE TISSUE LAYERS IN THE PNS IV. PERIPHERAL NERVES A. Spinal nerves & Spinal nerve roots B. Cranial Nerves Epineurium V. SENSORY GANGLIA VI. AUTONOMIC GANGLIA - Outermost connective tissue layer - Strong sheath that binds all fascicles together VII. PERIPHERAL NERVE PLEXUS - Consists of dense irregular connective tissue that completely VIII. RECEPTOR ENDINGS surrounds the PN A. Non-encapsulated Receptors B. Encapsulated Receptors Perineurium IX. NEUROMUSCULAR SPINDLES - Thinner connective tissue layer - Extends into the nerve - Surrounds one or > individual nerve fascicles - w/in each fascicle are individual axons & their supporting Schwann cells I. INTRODUCTION Endoneurium - loose vascular connective tissue layer of thin reticular fibers - surrounds each myelinated axon or a cluster of unmyelinated PERIPHERAL NERVOUS SYSTEM axons associated w/ Schwann cell Consists of - Inside the epineurium, fascicles are covered with 1. Neurons perineurium. 2. Supporting cells - Each fascicle contains many axon nerve fibers, and each 3. Nerves nerve fibers are covered in endoneurium. 4. Axons Located outside of the CNS - In the PNS nerve fibers are grouped into bundles to form Include: nerves. Except for very thin nerves containing only a) CN from the brain unmyelinated fibers, nerves have a whitish, glistening b) Spinal nerves from the SC appearance because of their myelin and collagen content. c) Associated ganglia Axons and Schwann cells are enclosed within layers of connective tissue. Immediately around the external lamina Ganglia (sing. Ganglion) of the Schwann cells is a thin layer called the endoneurium, consisting of reticular fibers, scattered - Small accumulations of neurons & supporting glial cells fibroblasts, and capillaries. Groups of axons with Schwann surrounded by a connective tissue capsule cells and endoneurium are bundled together as fascicles by a sleeve of perineurium, containing flat fibrocytes with Nerves of the PNS their edges sealed together by tight junctions. From two to six layers of these unique connective tissue cells regulate - Contain both sensory & motor neurons w/c transmit diffusion into the fascicle and make up the blood-nerve information between the peripheral organs & the CNS barrier that helps maintain the fibers’ microenvironment. Externally, peripheral nerves have a dense, irregular fibrous - The PNS has a mediating role between the outside of the coat called the epineurium, which extends deeply to fill the environment as well as the inside of the body to the CNS space between fascicles. Very small nerves consist of one fascicle. Small nerves can be found in sections of many Neurons of the peripheral nerves organs and often show a winding disposition in connective tissue. - Located either w/in the CNS or outside of the CNS in - Peripheral nerves establish communication between different ganglia centers in the CNS and the sense organs and effectors (muscles, glands, etc.). They generally contain both Peripheral nerve afferent and efferent fibers. Afferent fibers carry information from internal body regions and the environment - Composed of numerous axons of various sizes to the CNS. Efferent fibers carry impulses from the CNS to PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 effector organs commanded by these centers. Nerves Lanterman clefts) the major dense lines temporarily possessing only sensory fibers are called sensory nerves; disappear. those composed only of fibers carrying impulses to the effectors are called motor nerves. Most nerves have both - Faintly seen ultrastructurally in the light staining layers sensory and motor fibers and are called mixed nerves, are the intraperiod lines that represent the apposed usually also with both myelinated and unmyelinated axons. outer bilayers of the Schwann cell membrane. Membranes of Schwann cells have a higher proportion III. NERVE FIBERS of lipids than do other cell membranes, and the myelin sheath serves to insulate axons and maintain a Axon or Dendrite of a nerve cell constant ionic microenvironment most suitable for *Nerve tracts action potentials. Between adjacent Schwann cells on - Bundles of nerve fibers found in the CNS an axon the myelin sheath shows small nodes of - Ex: long tracts of brain and spinal cord (corticospinal Ranvier or nodal gaps where the axon is only partially tract/ spinothalamic tract) covered by interdigitating Schwann cell processes. At - these nodes the axolemma is exposed to ions in the *Peripheral nerves interstitial fluid and has a much higher concentration of - Bundles of nerve fibers found in the PNS voltage-gated Na+ channels, which renew the action - Ex: sciatic nerve, radial nerve, popliteal nerve potential and produce saltatory conduction (L. saltare, to jump) of nerve impulses, their rapid - Nerve fibers are analogous to tracts in the CNS, containing movement from node to node. The length of axon axons enclosed within sheaths of glial cells specialized to ensheathed by one Schwann cell, the internodal facilitate axonal function. In peripheral nerve fibers, axons are sheathed by Schwann cells, or neurolemmocytes segment, varies directly with axonal diameter and - The sheath may or may not form myelin around the axons, ranges from 300 to 1500 μm. depending on their diameter. A. Myelinated nerve fiber Surrounded by Myelin sheath Myelin sheath - Segmented, discontinuous layer interrupted at regular intervals by the Nodes of Ranvier (interruption region in the myelin sheath) - Each segment measures 0.5 – 1.0 mm - One schwann cell produces one myelin sheath on one axon - As axons of large diameter grow in the PNS, they are engulfed along their length by a series of differentiating neurolemmocytes and become myelinated nerve fibers. The plasma membrane of each covering Schwann cell fuses with itself at an area termed the mesaxon and a wide, flattened process of the cell continues to extend itself, moving circumferentially around the axon many times. The multiple layers of Schwann cell membrane unite as a thick myelin sheath. Composed mainly of lipid bilayers and membrane proteins, myelin is a large lipoprotein complex that, like cell membranes, is partly removed by standard histologic procedures. Unlike oligodendrocytes of the CNS, a Schwann cell forms myelin around only a portion of one axon. With high- magnification TEM, the myelin sheath appears as a thick electron-dense axonal covering in which the concentric membrane layers may be visible. The prominent electron-dense layers visible ultrastructurally in the sheath, the major dense lines, represent the fused, protein-rich cytoplasmic surfaces of the Schwann cell membrane. Along the myelin sheath, these surfaces periodically separate slightly to allow transient movement of cytoplasm for membrane maintenance; at these myelin clefts (or Schmidt- CNS: PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 - Oligodendrocytes may form & maintain myelin sheaths for ≈60 nerve fibers (axons) PNS: - Schwann cell form & maintain for each segment of one nerve fiber B. Myelin formation/MYELINATION Myelin sheaths begin to form before birth & during the 1st Major Dense Line (darker) year postnatally - 2.5 nm thick - around 4 months or 16 weeks up to the time the individual - Consists of 2 inner CHON layers of the plasma can stand or walk on his own membrane In the PNS: Minor Dense Line (lighter) - 10 nm thick Nerve Fiber indents the side of a Schwann cell - Made up of lipid Schwann cell plasma membrane forms MESAXON Schwann cell rotates on the axon wrapping it in spiral manner Wrappings become tight with maturation Thickness of the myelin depends on the number of spirals or turns of Schwann cell membrane (≈50 turns) In the CNS: Each lamella (lamination)measures 13 – 18 nm thick *Oligodendrocytes - Formation of myelin sheaths - 1 oligodendrocyte make myelin sheaths of ≈60 nerve fibers - Myelination occurs by the growth in length by the process of the oligodendrocyte, the process wrapping itself around the axon - Unlike the schwann cells, they will not turn around the axon, instead they will send their processes. And engulf the axon. - Oligodendrocytes looks like an astrocyte PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 - no nodes of Ranvier formed, no major and minor dense line formed in non-myelinated nerve fibers - THE PURPOSE OF MYELINATION IS FOR SPEEDY CONDUCTION OF IMPULSE IN ORDER TO RAPIDLY TRAVEL ALONG THE AXON. BECAUSE OF MYELINATION AND FORMATION OF NODES OF RANVIER THERE WILL BE A JUMPING MECHANISM OF THE IMPULSE. - Unlike the CNS where many short axons are not myelinated at all but run free among the other neuronal and glial C. Non-myelinated nerve fibers processes, the smallest-diameter axons of peripheral nerves are still enveloped within simple folds of Schwann cells. In these unmyelinated fibers the glial cell does not form the Examples: multiple wrapping of a myelin sheath. - In unmyelinated fibers, each Schwann cell can enclose 1. Smaller axons of the CNS portions of many axons with small diameters. Without the 2. Post ganglionic axons of the autonomic part of the thick myelin sheath, nodes of Ranvier are not seen along nervous system unmyelinated nerve fibers. Moreover, these small-diameter 3. Fine sensory axons associated w/ the reception of axons have evenly distributed voltage-gated ion channels; pain their impulse conduction is not saltatory and is much slower than that of myelinated axons. IV. PERIPHERAL NERVES *Cranial nerves- arise inside the cranium *Spinal nerves- arise from the spinal cord Peripheral nerves consist of axons from motor neurons (in the spinal cord), sensory neurons, and autonomic neurons (in Axons & dendrites are invaginated ganglia); all the axons are enclosed within a series of Schwann in the Schwann cell cytoplasm cells, but only large (myelinated) axons have myelin sheaths and nodes of Ranvier. Each PN consists of parallel bundles of nerve fibers: o Efferent or afferent axons o Myelinated or non-myelinated Nerve Trunk - Surrounded by a dense connective tissue sheath called Epineurium Nerve Fascicles - Each is surrounded by a connective tissue sheath called Perineurium Nerve Fibers - Is covered by loose delicate connective tissue called Endoneurium PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 - Endoneurium is a thin connective tissue layer immediately surrounding Schwann cells in peripheral nerves, containing a few nonfenestrated capillaries and much reticulin. - Groups of axons (with Schwann cells and endoneurium) are surrounded by perineurium, consisting of layered, squamous fibroblastic cells joined by tight junctions to make a blood- nerve barrier. - In large peripheral nerves, groups of axons are subdivided as fascicles, each of which is surrounded by perineurium. - Surrounding the perineurium is a thick, outermost layer of dense irregular connective tissue, the epineurium. A. Spinal nerves & spinal nerve roots 31 pairs of SN Passing through intervertebral foramina in the vertebral column Each spinal nerve is connected to the SC by 2 roots: *Anterior Root (efferent fibers) -conduct motor impulse *Posterior Root (afferent fibers) Sensory fibers - Conveying information to the CNS - Cell bodies in the DRG *Fiber diameter of Beta- 5-12 B. Cranial nerves 12 pairs of CN Afferent nerve fibers (pure sensory) *Olfactory (CN I) *Optic (CN II) *Vestibulocochlear (CN VIII) Efferent nerve fibers (pure motor) *Oculomotor (CN III) *Trochlear (CN IV) *Abducent (CN VI) *Accessory (CN XI) *Hypoglossal (CN XII) Both Afferent & Efferent (mixed motor & sensory) *Trigeminal (CN V) *Facial (CN VII) *Glossopharyngeal (CN IX) *Vagus (CN X) PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 glycogenolysis to bring about the stored glucose during the figh tor flight mode. ▪ This will help you in times of danger. o PARASYMPATHETIC-in times of pleasure uses ACETYLCHOLINE as neurotransmitter ▪ Miosis – pupillary constriction ▪ Eat, defecate, urinate, relax, cry, ▪ GI will have a lot of activities. -All of cranial nerves arise from the brain stem except for 2: CN1 & 2. They are both direct extension of the brain V. SENSORY GANGLIA Nerves with Sensory Ganglia: *CN V, VII, IX, & X *Posterior Sensory nerve roots Neurons are UNIPOLAR Cell bodies: round or oval in shape Cell body is surrounded by layer of flattened cells called Capsular Cells or Satellite Cells (similar in structure to Schwann cells) - Satellite cells are in the PNS Ganglia, which can be either sensory or autonomic, contain neuronal cell bodies and their satellite cells and are surrounded by connective tissue continuous with that of nerves. VI. AUTONOMIC GANGLIA *Sympathetic *Parasympathetic At a distance from the brain & SC At the sympathetic trunks, in pre-vertebral autonomic plexuses, & ganglia close to viscera VII. PERIPHERAL NERVE PLEXUS Neurons are MULTIPOLAR Cell bodies: irregular in shape Axons: small in diameter (C Fibers) & unmyelinated (pass to viscera, blood vessels, & sweat glands) Bundles of nerve fibers (+) Capsular Cells or Satellite Cells Divide into branches that join neighboring peripheral nerves forming network of nerves (Nerve Plexus) It belongs to the autonomics OPPOSING FORCES Nerve Plexus o SYMPATHETIC – In times of trouble, uses - Allows individual nerve fibers to pass from one peripheral NOREPINEPHRINE as neurotransmitter nerve to another ▪ Mydriasis – pupillary dilatation ▪ Increase HR, SV, no urination/ - Permits redistribution of the nerve fibers within the Defecation because of the inhibition of different peripheral nerves the bladder contraction, and there is PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 - at the root of the upper limbs - Cervical and Brachial Plexus VIII. RECEPTOR ENDINGS - individual receives impressions from the outside world and - Composed of 5 nerve roots the C5, C6, C7, C8, T1 from within the body by special sensory nerve endings or Robert Tan Drinks Cold beer (ROOTS, TRUNKS, DIVISION, receptors CORDS) The nerves will continue until the fingertips Five Basic Functional Types of Sensory Receptors 1. Mechanoreceptors - at the root of the lower limb - respond to mechanical deformation - Lumbar and Sacral Plexus - e.g. after an accident you will perceive the impact because you have mechanoreceptors - 2. Thermoreceptors - respond to changes in temperature - e.g. You will know that water is hot or cold PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 3. Nociceptors - respond to changes in any stimuli that bring about damage to the tissue - e.g. Make you detect that pricking your finger is painful - NOCI-means pain 4. Electromagnetic receptors - sensitive to changes in light intensity & wave length - rods and cones of the eye 5. Chemoreceptors - respond to chemical changes associated with taste and smell - O2 and CO2 concentration in the blood ANATOMICAL TYPES OF RECEPTORS - Sensory Endings Structural Basis A. NON-ENCAPSULATED RECEPTORS: 1. FREE NERVE ENDINGS B. ENCAPSULATED RECEPTORS - widely distributed in the body - skin, cornea, alimentary tract, connective tissues 1. MEISSNER’S CORPUSCLES - afferent fibers are either myelinated or non- myelinated - located dermal papillae of the skin, nipple, and - terminal endings devoid of myelin sheath external genitalia - Unmyelinated Nerves - ex palm of the hand, sole of the foot - ovoid in shape - consists of stack of modified flattened Schwann 2. MERKEL’S DISCS cells transversely arranged across the long axis of the corpuscle - hairless skin (fingertips), hair follicles - very sensitive to touch - nerve fibers passes to the epidermis and - rapidly adapting mechanoreceptor terminates as disc-shaped expansion - enable you to distinguish between two pointed - dark staining epithelial cell in the epidermis cell structures placed close together on the skin (TWO- (Merkel Cell) POINT TACTILE DISCRIMINATION) - TACTILE DOMES- cluster of Merkel’s disc, found - reduction in number as the person grows older in the epidermis between hair follicles - slowly adapting touch receptors that transmit 2. PACINIAN CORPUSCLES information about the degree of PRESSURE exerted on the skin - widely distributed in the body - ex. holding a pen - abundant in dermis, subcutaneous tissue, ligaments, joint capsules, pleura, peritoneum, - if somebody punch you, you know that somebody nipples and external genitalia punch you because of this receptor - rapidly adapting mechanoreceptor 3. HAIR FOLLICLE RECEPTORS - sensitive to VIBRATION - respond up to 600 stimuli per second - nerve fibers wind around the follicle in in its outer 3. RUFFINI’S CORPUSCLES connective tissue sheath below the sebaceous gland - located dermis of hairy skin - bending of the hair stimulates the follicle receptor - large unmyelinated nerve fibers - rapidly adapting mechanoreceptors - stretch receptors - slowly adapting mechanoreceptor - This will make you feel pain during exercises Can also function as thermoreceptors PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 > KRAUSE END BULBS - cylindrical or oval bodies consisting of a capsule, formed by the expansion of connective-tissue sheath, containing an axis-cylinder core - detect changes in temperature like Ruffini’s - location: conjunctiva (eye), mucous membrane (lips/tongue), epineurium (nerve trunks) STRETCH REFLEX Stretching a muscle → elongation of intrafusal fibers (muscle IX. NEUROMUSCULAR SPINDLES spindle); stimulation of annulospiral and flower spray endings →impulse reach the SC (afferent neurons); synapse with large alpha motor neurons (motor horn) → impulses now pass via motor nerves; Muscular Spindles stimulate extrafusal muscle fibers →muscle spindles→ muscle - located in the skeletal muscles contracts - numerous toward the tendinous attachment of the muscle Doesn’t reach the CNS. It is just a reflex - 1 to 4 mm in length - surrounded by a fusiform capsule of connective tissue A reflex is a primitive response of the body - within the capsule: 6 to 14 INTRAFUSAL muscle fibers - outside the spindle: EXTRAFUSAL muscle fibers During intrauterine life, reflexes are in existent why? Because one purpose of reflexes is for you to survive. 2 Types of reflexes: a. Physiological b. Pathological PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.  (007) PERIPHERAL NERVOUS SYSTEM DR. STEVE S. ARELLANO | 10/14/2020 4. Which structure contains trabeculae around which cerebrospinal fluid (CSF) flows? a. Arachnoid mater b. Ependyma c. Dura mater d. Pia mater e. Gray matter ANSWERS: 1.d 2. a 3.e 4. a Reference: Junquiera’s Basic Histology Text and Atlas, 14th edition (Anthony L. Mescher) TEST YOURSELF 1. A report from a hospital pathology laboratory indicates that a microscope slide with a small specimen of neural tissue contains “numerous GFAP-positive” cells. What is the most likely source of this specimen? a. A region of white matter b. A sensory ganglion c. An autonomic ganglion d. A region of gray matter e. Pia mater 2. In the choroid plexus water from capillaries is transported directly into the cerebrospinal fluid by what structure(s)? a. Ependyma b. Astrocytes c. Cells of the arachnoid mater d. Lining of the central canal e. Microglial cells 3. What term applies to collections of neuronal cell bodies (somata) in the central nervous system? a. Ganglia b. Neuroglia c. Nodes d. White matter e. Nuclei PREPARED BY: KOUSHIK, S., LAMSIS, V., MATEO, K., MAYRENA, K., PEREZ, K. PIMENTEL, B., PISCO, G.