Histology - Nervous Tissue PDF

HISTOLOGY LOGO Nervous Tissue UNIVERSITY OF SANTO TOMAS - FACULTY OF MEDICINE AND SURGERY UNIT 4 | SHIFT 2 | TERM 1 | A.Y. 2024-2025 Lecturer: Dr. Gellaco C01.1...

HISTOLOGY LOGO Nervous Tissue UNIVERSITY OF SANTO TOMAS - FACULTY OF MEDICINE AND SURGERY UNIT 4 | SHIFT 2 | TERM 1 | A.Y. 2024-2025 Lecturer: Dr. Gellaco C01.1 Divisions of the Nervous System UNIT 04: NERVOUS TISSUE TABLE OF CONTENTS I. The Nervous System A. Divisions of the Nervous System II. Nervous Tissue III. Neuron A. Histological Features of Neurons B. Structures in Neurons C. Classifications of Neurons 1. According to Number of Processes 2. According to Axonal Length 3. According to Function D. Parts and Processes E. Organelles and Inclusions IV. Axonal Transport by Microtubules Figure 01 - 1: Divisions of the Nervous System V. Types of Neurons A. According to No. of Processes 1. Central Nervous System B. According to Length of the Axon ➔ Contains the brain and spinal cord C. According to Function ◆ Also includes the brain stem VI. Ganglia (midbrain, pons, medulla VII. Myelin Sheaths oblongata) and cerebellum VIII. Nerve Fascicles and Nerve Trunk ◆ Integration center of the body; IX. Synapse commands the course of action X. Nerve Endings / Receptors 2. Peripheral Nervous System A. Exteroreceptors in Epithelium ➔ Made up of receptors and nerves (motor B. Exteroreceptors in Connective Tissue and sensory) found outside the CNS C. Neuromuscular Spindle ➔ Contains ganglia - nerves running as XI. Spinal Cord relay stations XII. Slide Review ➔ Receptors - function to sense stimuli XIII. Micrographs and Figures LEGEND: 📑 - From Previous Transcript (changes in the external and internal environment) ◆ Receptors transform these stimuli into signals and send C01. The Nervous System (E.T.R) these as impulses via afferent ➔ Maintains body homeostasis for proper body neurons into the CNS functioning ◆ CNS will integrate this ➔ Provides rapid and precise communication information and send signals between different parts of the body through efferent neurons to ➔ Follows the general pathway: make a response. Stimulus → Receptors → Afferent neuron → ➔ Nerves in the PNS are either afferent or Integration center → Efferent neuron → Effector efferent organ → Response ◆ Afferent neurons - carry sensory input to CNS LEAPMED ‘26 1 HISTOLOGY - NERVOUS TISSUE ◆ Efferent Neurons - carry motor ➔ Presence of nerve processes: output; innervate effectors ◆ Nervous tissue is composed of neurons with processes that are either dendrites Two Divisions of Efferent Neurons: and axons 1. Somatic Nervous System: ◆ Number of nerve processes determines ➔ Innervates skeletal muscles (nerve the cell’s shape fibers directly coming from the spinal ◆ Most nervous tissue is multipolar in nerve) shape ➔ Responsible for voluntary movement 2. Autonomic Nervous System ➔ Innervates visceral organs, smooth muscles, cardiac muscles, and glands ➔ Responsible for involuntary actions ➔ Further divided into three: ◆ Sympathetic nervous system - fight or flight response ◆ Parasympathetic nervous system - rest and digest ◆ Enteric nervous system - gastrointestinal tract C02. Nervous Tissue (E.T.R) Figure 02 - 2: Nervous Tissue with structures staining Nervous Tissue basophilic ➔ Made up of multipolar cells which are basophilic in staining ➔ Presence of Fish-eye nucleus ◆ Nervous tissue is basophilic due to the ◆ In electrographs, nervous tissue is presence of Nissl bodies that take up composed of nerve cells with a basic stains vesicular nucleus and prominent Nissl bodies are negatively charged, taking up positive dyes ◆ 📑 nucleolus This signifies that the cell is highly metabolic ◆ Nervous tissue is the only basophilic staining cell among other cells of the ◆ Nucleus is pale-staining, while body (both cell nucleus and cytoplasm nucleolus is dark-staining stains basophilic) This is due to the presence of euchromatin in the nucleus, which functions for intense metabolic activity Nucleolus is eccentrically located. ➔ Has a property of excitability ◆ Nerve cells are capable of transiently changing their membrane permeability from a stimulus ◆ Excitability is observed in both nerve cells and muscle cells ◆ Allows communication of information between neurons and other cells Figure 02 - 1: Electrograph of a Nerve Cell Communication between two ➔ AH - Axon Hillock ➔ Processes neurons occurs in centers called ➔ NB - Nissl Bodies ◆ A - Axons ➔ NU - Nucleolus ◆ D - Dendrites synapses LEAPMED ‘26 2 HISTOLOGY - NERVOUS TISSUE Remember: PRA (Process, Receive, Away) C03. Neurons (E.T.R) Perikaryon - process Dendrites - receive Axons - away ➔ No matter how many processes a neuron has, there is only one axon and the rest are dendrites. C03.3 Parts and Processes Figure 03 - 1: Nerve cells with prominent histological features (stained basophilic) C03.1 Histological features of a Neuron 1. Vesicular nucleus ➔ Nucleus contains more euchromatin than heterochromatin ➔ Euchromatin functions for intense cellular metabolism 2. Nissl bodies in the cytoplasm and dendrites ➔ Nissl bodies are masses of Rough ER Figure 03 - 5: Neuron’s main parts and processes ➔ Nissl bodies are scattered all over the cytoplasm and dendritic structures, ➔ The two processes of a nerve cell are dendrites which makes the tissue stain basophilic and axons. 3. Absence of Nissl bodies in the Axon Hillock, ◆ There will always be only one axon and Axon, and Terminal Boutons one or more dendrites. ➔ Since Nissl bodies are Rough ER ◆ Axons are longer structures that do not functioning for protein synthesis, there possess Nissl bodies. are no events of protein synthesis in these structures. Dendrites ➔ Proteins are transported to these ➔ Extensions of the cell body structures using axonal transport ➔ Shorter; more abundant ➔ Extensive branchings near the cell body ➔ Characterized by the presence of gemmules C03.2 Structures in a Neuron (dendritic spines) 1. Cell Body ◆ Gemmules - surface extensions in a ➔ Processes information from the two dendrite processes ➔ Cellulipetal ➔ Also known as perikaryon ◆ Information is moving towards its 2. Processes structures a. Dendrites - receive and bring ◆ Like centripetal force that moves one information from the environment to the towards the center i.e. inwards perikaryon b. Axons - sends/brings information out of Axons the neuron to other neurons or effector ➔ Lacks Nissl body, starts from the axon hillock tissues ➔ Longer; fewer (only one axon per nerve cell) ➔ Few branches far from the cell body LEAPMED ‘26 3 HISTOLOGY - NERVOUS TISSUE ◆ Branches are called axon collaterals ➔ No gemmules ➔ Cellulifugal ◆ Information is moving away from its structure ◆ Like centrifugal force that moves one away from the center (fictitious force) C03.4 Organelles and Inclusions in Neurons Figure 03 - 7: Nissl Bodies Similar to most cells, neurons have the following organelles and inclusions: 1. Large central nucleus and prominent nucleolus 2. Prominent mitochondria for cellular respiration 3. Golgi complex 4. Nissl bodies (RER) 5. Neurofibrils 6. Microfilaments and Microtubules 7. Lysosomes 8. Glycogen granules and Lipofuscin Figure 03 - 8: Nissl Bodies of a Neuron are clusters of RER Figure 03 - 6: Organelles of a neuron 2. Golgi Complex ➔ Packages and concentrates secretory materials synthesized by the RER Prominent Structures: ◆ Sometimes, proteins are also 1. Nissl Bodies temporarily stored in this ➔ RER structures that produce proteins structure ◆ Proteins are needed for cell ➔ Located in the same places with RER membrane components (extrinsic intrinsic, 3. Neurofilaments transmembrane) ➔ Intermediate filaments of the neuron ➔ Basophilic masses ➔ Functions to maintain cell shape and cytoskeleton and to serve as structures where chemical and electrical signals pass through ➔ 8-12 nm in diameter LEAPMED ‘26 4 HISTOLOGY - NERVOUS TISSUE ➔ Run parallel to the microfilaments and C04. Axonal Transport by Microtubules (K.J.A.) microtubules The neuron’s axon and axon terminals are 4. Microtubules and Microfilaments dependent on the cell body for its protein needs ➔ Contribute to the cytoskeleton of due to its lack of Nissl bodies. neurons These proteins / organelles travel down and up ➔ Works with neurofilament to maintain the axon via microtubules (structural organelle cell shape and rigidity with wide enough diameters for proteins to pass ➔ Important in axoplasmic transport through) ➔ 24 nm in diameter ○ This type of transport is termed “axonal ➔ “Neurofilaments are where electrical transport” or “axoplasmic transport” signals pass through. Microtubules are where vesicular proteins and organelles Table 04 - 1: 2 Types of Axonal Transport pass through.” (Gellaco, 2024) Anterograde or Retrograde Transport 5. Mitochondria Orthograde Transport ➔ Provides energy (ATP) for the reception From: Nerve cell body From: Terminal boutons and transmission of impulses To: Terminal boutons To: Nerve cell body ➔ Abundant in both presynaptic and postsynaptic neurons Proteins in both types pass through microtubules 6. Centriole Motor Protein: Kinesin Motor Protein: Dynein ➔ No function in neurons since neurons (coiled structure) are terminally differentiated cells ➔ Only present in young neurons 📑 Can be fast or slow Transport: 📑 Possible route for infectious agents to reach 7. Lipofuscin pigments ➔ Slow transport the CNS (ex. Rabies ➔ Wear and tear pigments ◆ Larger virus, poliomyelitis virus, ➔ Characterized by reddish-brown cytoskeletal and Clostridium tetani) colors filaments ➔ Forms due to accumulation of the Poliomyelitis virus products of autophagocytosis ➔ Fast transport ➔ Viral infection ◆ Vesicles causing upper containing respiratory tract smaller (in symptoms comparison) ➔ Enters the spinal proteins cord RETROGRADELY ➔ Destroys/infects anterior horn cells; thus, this spinal cord area will have no neurons sending innervation to skeletal muscles Figure 03 - 9: Lipofuscin pigments = denervation atrophy 8. Melanin pigments (decreasing size) ➔ Accumulates in the substantia nigra of = limping = the midbrain smaller limb LEAPMED ‘26 5 HISTOLOGY - NERVOUS TISSUE Clostridium tetani (Atchison, 2018) ➔ This toxin is transported to the CNS retrogradely, where it blocks the release of GABA and glycine leading to a spastic form of paralysis Figure 04 - 2: Motor Proteins Dynein and Kinesin ➔ 📑 Rabies goes to the spinal cord retrogradely, then to the cerebral cortex and C05. Types of Neurons (K.J.A.) salivary glands anterogradely. C05.1 ACCORDING TO THE NUMBER OF a. Anterograde transport - reason for PROCESSES why rabies is found in saliva as a Determines the shape of a neuron means for a rabid person to bite/infect another ➔ Multipolar b. Once it enters the spinal cord, the ◆ Many processes virus easily travels upward and goes ◆ Typical neuron; most common (99% of to the brain (cerebral rabies) neurons) ◆ One axon and at least two dendrites Reason for possible Used in the injection of ◆ Found in the cerebral cortex and the injection of nerve growth nerve growth factors to anterior horn cell of the spinal cord factors from the nerve cell the terminal button that ◆ Ex: Pyramidal neurons body to terminal boutons will flow to the cell body, which sustains the growth ➔ Bipolar and nutrition of the ◆ 2 processes: 1 dendrite, 1 axon neuron ◆ Present in special sensory organs ◆ The nerve cell body is spindle-shaped Remember: DR. AKO Ex: retina (vision), taste buds ➔ (DYNEIN - Retrograde); (taste), organ of Corti (hearing), ➔ (Anterograde-KINESIN-Orthograde) olfactory neuroepithelium (smell) ➔ Pseudounipolar ◆ Initially a single process from the perikaryon that branches into two One branch extends to the peripheral ending, the other toward the CNS ◆ Dendraxon Y or T-shaped process A portmanteau of dendrite and axon Figure 04 - 1: Axonal Transport Types and its Motor ◆ Found in cerebrospinal ganglion (also Proteins called the spinal ganglion or dorsal root ganglion) Ganglion: clusters of neuronal cell bodies in the PNS ◆ Nerve cell body is globular in shape LEAPMED ‘26 6 HISTOLOGY - NERVOUS TISSUE ◆ Sensory in function reach diff. skeletal ➔ Unipolar muscles ◆ 1 process from the cell body (axon that extends into dendrites) ◆ Only occurs in invertebrates (e.g., flies), but NOT in humans Figure 05.2 - 1: Golgi Cell Type I Figure 05.1 - 1: Types of Neurons According to the Number of Processes C05.2 ACCORDING TO THE LENGTH OF THE AXON Figure 05.2 - 2: Golgi Cell Type II Table 05.2 - 1: Types of Neurons According to the Length of the Axon Golgi Cell Type I Golgi Cell Type II C05.3 ACCORDING TO FUNCTION Very long axon Short or no axons ➔ Sensory (Afferent) Neurons ◆ Convey sensory information to the CNS Medium to large cells; Small to medium cells; ◆ Found at the periphery of the body to mostly multipolar few sparsely branching the CNS dendrites ◆ These neurons are usually concentrated in ganglia, and their dendrite branches Examples: Examples: extend to the skin or sensory organs 1. Pyramidal cells 1. Cerebral cortex and act as sensory receptors (either of the cerebral directly or indirectly) cortex 2. Cerebellar cortex - Sole output for cerebrum 3. Retina 2. Purkinje cells of the cerebellar 📑 Granule interneurons cells and cortex - Sole output for cerebellum Figure 05.3 - 1: Sensory (Afferent) Neurons 3. Motor cells of ➔ Motor (Efferent) Neurons the spinal cord ◆ Convey information from CNS to - Anterior horn muscles, glands, or some other cells that “effector” to produce a certain action LEAPMED ‘26 7 HISTOLOGY - NERVOUS TISSUE ◆ Most of the neurons in the spinal cord Fine Nissl bodies Intermediate-sized Nissl and many of those in the brain are uniformly dispersed in the bodies at the periphery motor neurons cytoplasm Presence of numerous Few satellite small satellite cells/amphicytes cells/amphicytes Associated with thick Associated with myelinated fibers unmyelinated nerve fibers Figure 05.3 - 2: Motor (Efferent) Neurons 📑 a.k.a. Spinal ganglion 📑Innervation of smooth, or cerebrospinal ganglion cardiac muscle and ➔ Interneurons or Association Neurons glands. Not directly into ◆ All other neurons are interneurons Found in the posterior effector organs but ◆ Also called “connector” neurons root of the peripheral passes through ◆ They interconnect neurons with other nerves ganglions: neurons Sympathetic ◆ Nearly all neurons in the CNS are First order neuron of all ganglion interneurons (restricted to the CNS) sensory modalities before ○ Located ◆ Act as bridges between sensory and entering the spinal cord lateral to the motor neurons or relays impulses to from the neck down (ex. spine various functional centers of the CNS Pain, temperature Parasympathetic changes, touch, pressure, ganglion vibration) ○ Located in the wall of the organ it supplies Figure 05.3 - 3: Interneurons or Association Neurons Figure 06 - 1: L/M of pseudounipolar dorsal root ganglion cell with labeled C06. Ganglia (K.J.A.) 📑 structures Figure 06 - 2: L/M of an 📑Cells of the CNS are called neurons autonomic Take note of: ganglion cell Cells of the PNS are called ganglia - Uniform distribution of fine Nissl bodies Table 06 - 1: Types of Ganglion Cells - Numerous satellite cells Dorsal Root Ganglion Autonomic Ganglion Sensory function Motor function C07. MYELIN SHEATHS (D.M.J.J.) Pseudounipolar Multipolar 📑Axons (fibers) may acquire one of two coverings: ensheathment)📑 ➔ Neurilemma or sheath of Schwann (initial LEAPMED ‘26 8 HISTOLOGY - NERVOUS TISSUE ◆ Source of myelin in the peripheral nervous system ◆ Myelinating agent ◆ Provides structural and metabolic support ◆ If an axon has a Schwann cell surrounding it, it is considered unmyelinated (Type C fiber) ➔ 📑 Myelin (possible ensheathment beneath neurilemma) ◆ Concentric deposition of Schwann cell Figure 07 - 1: L/M long section of myelinated nerve inner and outer membrane wrapped fibers spirally around the axon ➔ a = axon ◆ Found underneath the neurilemma ➔ ▲: Node of Ranvier ◆ Since cell membranes are generally ➔ b = Myelin/neurilemma with outermost lipid in structure (phospholipid bilayer), endoneurium myelin is considered a lipid ◆ Appears as a colorless, empty, foamy, ◆ Acts as an insulator around the axon, bubbly network because the lipid thus electrical potential cannot travel solvent used for slide preparation from through it fixation dissolves or washes off lipid ◆ Fibers can be categorized depending on content 📑 thickness of myelin A fibers - thickest, fastest ➔ Axons (fibers) may be categorized depending on 📑 conduction B fibers - lightly myelinated, slowest conduction among the presence of myelin sheaths: ◆ Myelinated Fibers ◆ Unmyelinated Fibers 📑 myelinated fibers C fibers - not myelinated ◆ Only forms in segments called the Table 07 - 1: Myelinated vs Unmyelinated Fibers internode Myelinated Fibers Unmyelinated Fibers Only one myelin sheath per 📑 internode Large diameter axon Small diameter axon Nodes of Ranvier ○ Found between Presence of myelin Absence of myelin internodes where sheaths sheaths myelin is NOT present Myelin sheaths Axon is simply ○ Only covered by are also enveloped by the membrane loop from enveloped by the cytoplasm and major dense lines of the cytoplasm and neurilemma of myelin neurilemma Schwann cells ○ Rich in positive sodium ions Faster transmission Slower Transmission ○ There is no resistance to ion flow Saltatory conduction Continuous conduction ○ Interrupts insulation at due to nodes of Ravier Slow conduction intervals Fast conduction Impulse travels ○ Discontinuity that allows Impulse jumps slowly along axon action potentials to jump from node to node from node to node Thicker and length Ex. C fibers 📑 (saltatory conduction) longer myelination = faster conduction LEAPMED ‘26 9 HISTOLOGY - NERVOUS TISSUE 📑 A-alpha fibers have faster conduction than B fibers Found in the brain, spinal Found in the nerves of cord, cranial nerves, and ANS and small pain fibers spinal nerves Figure 07 - 3: Oligodendrocytes and Schwann cells ➔ Jelly Roll Hypothesis ◆ Named due to concentric rolling/layering ◆ Proposes a process of how myelin sheaths are formed ◆ Process: 1. Axon indents the Schwann cell 2. Axon becomes enveloped by a fusion of the inner and outer cell membranes of the Schwann cells Mesaxon ○ Pair of parallel plasma membranes of Schwann cell Figure 07 - 2: E/M of peripheral nerve with myelinated ○ Marks the point of and unmyelinated fibers edge-to-edge contact by (U: unmyelinated; M: myelinated) the Schwann cell Yellow arrow - Myelin (Schwann cell membrane); appears black under E/M because ○ 📑encircling the axon Extension of Schwann cell surface lipids take up osmium tetroxide membrane, grows and Green arrow - Schwann cell nucleus forms a spiral sheet Blue arrow - Unmyelinated fiber; only around the axon surrounded by sheath of Schwann 3. Inner and outer membrane of Schwann cell wraps itself spirally and ➔ Myelin sheaths are formed by one of these cells: concentrically envelopes around the ◆ Oligodendrocytes axon in the middle Found in CNS As successive layers become Forms multiple sheaths around more compact, myelin sheath different axons form ○ One cell can myelinate 50 internodes NOTE: Axons only become myelinated when multiple ◆ Schwann Cell Schwann cells wrap their cytoplasm around the axon. Found in PNS When a Schwann cell only encases its cytoplasm on the Myelinates a single axon axon (no wrapping/layering), it is considered unmyelinated LEAPMED ‘26 10 HISTOLOGY - NERVOUS TISSUE Figure 07 - 4: Jelly Roll Hypothesis Figure 07 - 6: Phases of Myelin Formation in Peripheral Nerve Fibers One may observe 2 lines when looking at a cross-section of the myelin sheaths: ➔ Major Dense Lines ◆ ~2-3 nm wide ◆ Darker staining lines ◆ Formed by the apposition of the closely condensed intracellular (cytoplasmic) Figure 07 - 5: Development of Myelin Sheath ➔ There are 4 distinct phases during the process ◆ 📑 surfaces In E/M, they form membrane/external loops at the region near Node of of Jelly Roll hypothesis: Ranvier A. Schwann cell has surrounded the axon, It is the only covering of the but external surfaces of the plasma Node of Ranvier membrane have not yet fused in Marks the end of one internode mesaxon and the beginning of another B. Mesaxon has fused into a five-layered structure and spiraled once around the ➔ Intraperiod Lines axon ◆ 4 nm wide C. A few layers of myelin have formed but ◆ Formed by tightly apposed extracellular are not compacted completely surfaces of myelin sheaths There are cytoplasm trapped in zones where the cytoplasmic membrane surface has not yet fused D. Layers compacts completely, and myelin sheaths have been completely formed Schwann cell cytoplasm forms ring inside and outside of the sheath LEAPMED ‘26 11 HISTOLOGY - NERVOUS TISSUE Figure 07 - 9: Longitudinal Section of Myelinated Nerve Fibers C8. NERVE FASCICLES AND NERVE TRUNK (D.M.J.J) Figure 07 - 7: Diagram of Myelinated Nerve Fiber ➔ Nerve Fiber ◆ Endoneurium: outer covering of nerve fiber ◆ Circular structures in the cross-section of nerve fascicles ◆ Structures present in a cross-section (L/M): Axon: dot in the center Myelin: white region Figure 07 - 8: Cross-sectional diagram of myelin, ○ as lipid content is emphasizing origin of intraperiod line dissolved/washed off Neurilemma: dark outer layer ➔ Myelin Incisures/Schmidt-Lanterman Clefts Endoneurium: outermost thin ◆ aka medullary segments connective tissue covering ◆ Small pockets of cytoplasm left behind during Schwann cell myelination ➔ Nerve Fascicle process ◆ Also known as nerve bundles ◆ Help sustain the growth and function of ◆ Bundles of several nerve fibers compact myelin ◆ Perineurium: outermost covering of They were originally thought of nerve fascicle (robust collagenous as mere artifacts capsule) ➔ Nerve Trunk ◆ Bundle of several nerve fascicles ◆ Equivalent term for nerves found in the gross anatomy (e.g., radial nerve, ulnar nerve) ◆ Epineurium: outer covering of the nerve trunk ➔ Vasa Nervorum ◆ Blood vessel located in/near/supplying a nerve structure LEAPMED ‘26 12 HISTOLOGY - NERVOUS TISSUE Figure 08 - 1: Cross-Section of Nerve Fibers or Fascicles Figure 08 - 4: Diagram of the Hierarchical Structures of Nerve Fiber, Fascicle, and Trunk C9. SYNAPSE (D.M.J.J) ➔ Synapse ◆ Junction between two neurons that allow a signal to pass between them ◆ Found where neurons connect with other neurons ◆ Provides neurons with continuous ◆ 📑 communication The synapse is primarily chemical An action potential reaches the Figure 08 - 2: Cross-Section of Part of Nerve Trunk presynaptic terminal that causes neurotransmitter release into the synaptic cleft which then attach to the NT receptors on the postsynaptic side The axon that propagates the transmission terminates as a bulbous swelling called terminal ◆ 📑 boutons Synaptic transport is unidirectional (presynaptic terminal to postsynaptic terminal) Figure 08 - 3: Cross-Section of Vasa Nervorum and Part of Nerve Trunk Figure 09 - 1: Types of Neuronal Synapses ➔ Axo-dendritic - axon → dendrite LEAPMED ‘26 13 HISTOLOGY - NERVOUS TISSUE ➔ Axo-somatic - axon → cell body ➔ Axo-axonic - axon → axon ➔ Dendro-dendritic - dendrite → dendrite Structures: ◆ 📑 ➔ Presynaptic Terminal Induces opening of calcium channels when stimulated by a wave of depolarization; calcium influx promotes neurotransmitter release ◆ Contains: Mitochondria Vesicles with NT Golgi apparatus cisternae that recycle vesicles Figure 09 - 2 and 3: Diagram of a Synapse ➔ Synaptic Cleft ◆ 20-30 nm ◆ Narrow intercellular gap of uniform width ◆ Where neurotransmitters are released ➔ Postsynaptic Terminal ◆ Differentiated from the presynaptic terminal by comparing the amount of vesicles 📑 ◆ Has postsynaptic density 📑 aka synaptic web Dense network of fine filaments Provides structural stability to ◆ Contains: 📑 the apposed membranes Seen black/dark in E/M Mitochondria Specialized receptors Figure 09 - 4: E/M of a Synapse ➔ Green arrow - Synaptic vesicles; presynaptic terminal ➔ Blue arrow - Golgi apparatus cisternae ➔ Yellow arrow - Synaptic cleft ➔ Red arrow - Postsynaptic terminal ➔ Orange arrow - Mitochondria ➔ Neuromuscular Junction (NMJ) ◆ Type of axo-somatic synapse; also known as myoneural junction ◆ Conducts motor impulses from the peripheral nervous system to the muscles, causing muscle contraction LEAPMED ‘26 14 HISTOLOGY - NERVOUS TISSUE Figure 09 - 4: Diagram of the Pathway of Impulses through the PNS towards the muscles Physiological Events at the Neuromuscular Junction: 1. Action potential reaches the motor end plate of Figure 09 - 5: Diagram of the Physiological Events that the neuron cause Muscle Contraction 2. Voltage-gated calcium channels open, and calcium enters the neuron ➔ Myasthenia Gravis 3. Vesicles are stimulated to migrate to the end ◆ Most common disorder affecting the plate and release acetylcholine into the neuromuscular junction of skeletal neuromuscular junction muscles Acetylcholine - neurotransmitter of ◆ Usually affects muscles of the eye, somatic neuromuscular junctions throat, and extremities 4. The acetylcholine will bind to special receptors on the postsynaptic membrane, which will trigger an electrical signal leading to muscle contraction 📑 ◆ Autoimmune in nature Auto-antibodies damage or occupy postsynaptic acetylcholine receptors in eye ➔ Muscle Contraction ◆ Initiated by the arrival of a nervous signal reaching the synapse of the 📑 muscles Leads acetylcholine to decreased binding and neuromuscular junction decreased neuromuscular transmission Physiological Events that lead to Muscle ◆ Manifestation: Fluctuating Contraction: weakness/ptosis that is more prominent 1. Acetylcholine initiates an electrical signal in the muscle cell membrane 2. Electrical signal spreads across the muscle cell ◆ 📑 in the afternoon Treatment: anticholinergic agents, acetylcholinesterase inhibitors membrane and enters into T-tubules T-tubules - invaginations of the plasma membrane 3. Electrical signal spreads from T-tubules to the sarcoplasmic reticulum, where calcium is released into the cytoplasm of the muscle cell 4. Calcium binds to the troponin molecule, initiating muscle contraction LEAPMED ‘26 15 HISTOLOGY - NERVOUS TISSUE organ Vater Pacinian corpuscles End bulb of Krause C10.1 EXTEROCEPTORS IN EPITHELIUM ➔ Merkel Cells and Tactile Discs ◆ Slowly adapting-discriminatory touch ◆ 📑 and pressure receptors For continuous, prolonged pressure (e.g., writing: awareness when holding ◆ 📑 ballpen for a long time) Works together with Meissner’s corpuscles for discriminatory touch* and pressure (e.g., awareness of subchanges on pen holding/shifting your position from time to time) Figure 09 - 6: Diagram of the Mechanism behind Myasthenia Gravis *NOTE: Meissner’s corpuscles are the PRIMARY sense receptors for two-point tactile discrimination. Merkel C10. NERVE ENDING / RECEPTORS (D.M.J.J) cells are only secondary to it, only functioning when it (E.I.C) comes to prolonged periods of pressure. ➔ ➔ 📑 📑 Structures that receive the impulses Detects changes in the internal and external ➔ 📑 environments Transduction - translate/convert stimulus into nerve impulses that would be recognized by cells and the CNS Table 10 - 1: Main Types of Nerve Receptors 📑Interocepto rs Proprioceptors Exteroceptors 📑 Detects changes in the Information on orientation, Free nerve endings at the visceral skeletal position, surface of skin environment muscle tension, for pain, and movement pressure, touch, and thermal sense 📑 Chemorecep tors, Neuromuscular spindle and Skin receptors: Free nerve Baroreceptors Golgi tendon endings organs Merkel discs Meissner’s Figure 10.1 - 1: Diagram of the Merkel cells and Tactile corpuscles discs Ruffini’s end ➔ Free Nerve Endings LEAPMED ‘26 16 HISTOLOGY - NERVOUS TISSUE ◆ Most numerous/common (in epithelium) Nociceptor - free nerve endings that perceive pain ◆ Consists of sensory dendrites that may be stimulated by a variety of stimuli “Unang-unang wala dito si Merkel, ang more common ◆ Each receptor monitors a specific area kasi is Pacinian” - (Gellaco, 2024) known as the receptive field ◆ Very versatile OVERVIEW It can subserve any modality (these exteroceptors are found deeper compared to and transmit any impulse C10.1 in the epithelium; usually found in the dermis) Touch, thermal sense, pain (nociceptors), pressure ➔ Meissner Corpuscle ➔ Vater Pacinian Corpuscle ➔ Ruffini’s End organ ➔ End bulb of Krause MEISSNER’S CORPUSCLE (micrograph) - Fast-adapting (a) receptor - Good for discriminatory touch - For vibration but in low levels (10-50Hz) - Responsible for (b) (c) you knowing which certain fingers are in Figure 10.1 - 2: Diagram of free nerve endings contact with something (two-point C10.2 EXTEROCEPTORS IN CONNECTIVE TISSUE discrimination) - Ensheathed; like laminations - “Nakatayo yung receptor, longitudinal laminations” (Gellaco, 2024) (d) Figure 10.2 - 2-5: Meissner Figure 10.2 - 1: Schematic Representation of the Corpuscle Different Sense Organs in the Skin LEAPMED ‘26 17 HISTOLOGY - NERVOUS TISSUE C10.3 NEUROMUSCULAR SPINDLE NOTE: look for longitudinal laminations pointed in (a) to easily identify the Meissner Corpuscle; areas without laminations are just elevations called papillary dermis VATER PACINIAN (micrograph) - Hypodermis - Fast-adapting - Distinction: rapid vibration (200Hz) - “Pabilog yung laminations” (Gellaco, 2024) Figure 10.3 - 1: Neuromuscular Spindle Exteroreceptors in the muscle Small organs 2-10mm long, half a mm thick, not seen in the naked eye Embedded in the skeletal musculature RUFFINI’S END ORGAN (micrograph) Subserve proprioception Responds to both the changes in muscle length - Dermis, deep and velocity of change skin Innervated by both sensory and motor fibers - More common in ○ Example: Knee-Jerk Reflex (hitting the soles of the feet patella, shortens the muscle - Detects tension [contraction] involuntarily and quickly) Responsible for regulation of muscle tone via the stretch Encapsulated END BULB OF KRAUSE (micrograph) Made up of specialized skeletal muscle fibers called intrafusal muscle fibers, which run - Encapsulated parallel to the extrafusal muscle fibers. receptor - In oropharynx and conjunctiva C11. SPINAL CORD (not discussed; taken from - Responsible for prev. trans) cold temperature Outer White Matter Inner Grey Matter Non-basophilic Basophilic (Butterfly-shaped) Myelinated nerve fibers Contains nerve cell bodies and proximal processes Predominance of fibrous Predominance of LEAPMED ‘26 18 HISTOLOGY - NERVOUS TISSUE 4. Nerve Fascicle w/ Vasa Nervorum (1 astrocytes and protoplasmic micrograph, 1 drawing) oligodendroglia astrocytes 5. Auerbach Plexus (1 micrograph, 1 drawing) 6. Dorsal Root Ganglion (1 micrograph, 1 drawing) SPINAL CORD (micrograph) NEURON C12. SLIDE REVIEW OUTLINE 1. Neuron (1 micrograph, 1 drawing) 2. Meissner Corpuscle (1 micrograph, 1 drawing) 3. Pacinian Corpuscle (1 micrograph, 1 drawing) LEAPMED ‘26 19 HISTOLOGY - NERVOUS TISSUE MEISSNER CORPUSCLE PACINIAN CORPUSCLE LEAPMED ‘26 20 HISTOLOGY - NERVOUS TISSUE NERVE FASCICLES WITH VASA NERVORUM AUERBACH PLEXUS (AUTONOMIC) LEAPMED ‘26 21 HISTOLOGY - NERVOUS TISSUE DORSAL ROOT GANGLION C13. MICROGRAPHS & FIGURES Autonomic Ganglion Cell LEAPMED ‘26 22 HISTOLOGY - NERVOUS TISSUE Synapse (E/M) & Neuromuscular Junction (NMJ) LEAPMED ‘26 23 HISTOLOGY - NERVOUS TISSUE CHAPTER SUMMARY (prev. trans) Category Detail Others Neurons Cell Body Cells specialized in carrying electrical signals as communication Dendrites Branched processes receiving incoming signals from synapses and at sensory receptors Axons Often long, always solitary outgoing process; may branch at destination Schwann cells Myelination Schwann cell and wraps the cell Oligodendrocyte membrane around the axon many times, protecting, insulating and speeding LEAPMED ‘26 24 HISTOLOGY - NERVOUS TISSUE transmission pressure and (oligodendrocyt other es in CNS) Merkel Merkel cell in Non-Myelinated Unmyelinated; cell-neurite basal epidermis axon protected and adjacent as above but nerve ending; without myelin slow-adapting wrapping; discriminatory slower touch and pressure Synapses Axon endings at receptor which neurotransmitter Exteroreceptors Meissner Encapsulated chemicals are in Connective Corpuscle body in papillary released to pass Tissue dermis; the signal to the fast-adapting next cell or end discriminatory Organ touch and vibration (10–50 - Motor end Hz) receptor plates - Special Vater Pacinian Encapsulated synapse Corpuscle bodies in deep between skin; fast axon and adapting for muscle rapid vibration cells (200 Hz) Peripheral Neuronal Supporting cells Ruffini’s End Found in soles Nerves Processes in a wrapped, Organ of feet; detects protected tension structure traversing tissue End Bulb of In oropharynx Krause and conjunctiva; Ganglia Nerve cell Spinal; cell responsible for bodies and body of sensory cold support cells nerves, dorsal temperature external to CNS spinal Sympathetic; Receptor in Neuromuscular Complex along vertebral muscle Spindle encapsulated column structure; Parasympathet monitors tension ic; in end organs in muscles; receptor for Exteroreceptors Free nerve Free nerve tendon reflex in Epithelium endings endings: common sensory REFERENCES receptors for pain, temperature, touch, LEAPMED ‘26 25 HISTOLOGY - NERVOUS TISSUE Previous trans Gellaco, M.L., MD.(2024, September 25). Nervous Tissue Lecture. LEAPMed 2024-2025. Slide Review Pictures from Regadio & Gino-gino FREEDOM WALL LEAPMED ‘26 26