Nerve tissue 2 1. Neuroglial cells 2. Myelinogenesis 3. Nerve endings: sensory (afferent) receptors • free nerve endings • encapsulated receptors effector (efferent) endings Neuroglia . Glial cells – glioblastic origin: central – macroglia and microglia (in CNS) peripheral – in PNS . central gliocytes – neural tube: astrocytes oligodendrocytes ependymal cells microglial cells . peripheral gliocytes – neural crest: Schwann cells (neurolemmocytes) satellite cells of Cajal (syn: mantle cells or amphicytes) Prof. Dr. Nikolai Lazarov 2 Central gliocytes (Gr. astron . astrocytes – star) the most numerous glial cells in the brain origin – progenitor cells in the embryonic neural tube structural and metabolic support of neurons morphological substrate of the blood-brain barrier repair processes – astrocytic scar protoplasmic astrocytes more prevalent in the gray matter linked by gap junctions bundles of intermediate filaments – glial fibrillary acid protein (GFAP) fibrous astrocytes abundant in the white matter fewer but long and straight Prof. Dr. Nikolai Lazarov 3 Central gliocytes (Gr. oligos – small) . oligodendrocytes origin – progenitor cells in the embryonic neural tube found only in the CNS predominant in the white matter relatively few processes myelin-forming cells in the CNS – electrical insulation of axons faster action potential propagation in the CNS (saltatory conduction) . morphological types: large light medium-sized small dark – ¼ of the light cells Prof. Dr. Nikolai Lazarov 4 Central gliocytes . ependymal cells – neural crest line the ventricles of the brain and central canal of the spinal cord absorption and secretion of cerebrospinal fluid (liquor) tanycytes (ependymal astrocytes) . microglia – 15% of the total cells of CNS non-dividing cells derived from monocytes role of macrophages & antigen-presenting cells – secrete cytokines – immune defense in the CNS (mononuclear phagocyte system in nervous tissue) Prof. Dr. Nikolai Lazarov 5 Peripheral gliocytes . Schwann cells (neurolemmocytes) neural crest origin myelin-forming cells in the PNS maintenance of the axon integrity phagocytotic activity and cellular debris that allows for regrowth of PNS neuron . satellite cells (amphicytes) in sensory and autonomic ganglia help regulate the external chemical environment Prof. Dr. Nikolai Lazarov 6 Myelination . myelination in humans: begin – fetal period end – 7 years regulation – neuroregulin NRG1 Prof. Dr. Nikolai Lazarov 7 Myelination . Myelin-forming cells: oligodendrocytes – CNS Schwann cells – PNS . myelin: lipids – 70% proteins – 30% in CNS MBP - cytosol (myelin basic protein) P0 – peripheral axons PMP-22 – peripheral axons . myelin sheath: major dense lines – fused cytoplasmic surfaces of Schwann cell membrane intraperiod lines – apposed outer bilayers of the Schwann cell membrane Prof. Dr. Nikolai Lazarov 8 Sensory receptors – classification 3 main groups – Sherrington, 1906: exteroceptors proprioceptors interoceptors C.S. Sherrington by sensory modality: 1857–1952 baroreceptors – respond to pressure chemoreceptors – chemical stimuli mechanoreceptors – mechanical stress nociceptors – pain perception thermoreceptors – temperature (heat, cold or both) by location: cutaneous receptors – in the skin muscle spindles – in the muscles by morphology: free nerve endings encapsulated receptors Prof. Dr. Nikolai Lazarov 9 Sense of touch four kinds of touch sensations: light touch (contact) cold heat pain Prof. Dr. Nikolai Lazarov 10 Unencapsulated receptors free nerve endings: . unencapsulated receptors . unspecialized, detect pain . most widely distributed . most numerous in the skin, mucous&serous membranes, muscle, deep fascia, viscera walls peritrichial (palisade) endings of hair follicles . nerve plexus around hair follicle . very light touch Prof. Dr. Nikolai Lazarov 11 Unencapsulated receptors tactile discs of Merkel: . slowly adapting type I mechanoreceptors – pressure and vibration at low frequencies, around 5 to 15 Hz . found in the basal layers of: • glabrous skin • hairy skin ("touch domes“, "hair disks") . in oral and anal mucosa Friedrich Merkel . in the mammary gland (1845-1919) . specialized epithelial cells of Merkel, “Tastzellen” • embryonic origin – nerve crest, epidermal • APUD cells – neuroendocrine function • specific granules . Merkel nerve endings (tactile disks) . Merkel cell–neurite complex Prof. Dr. Nikolai Lazarov 12 Encapsulated receptors Receptors without a lamellar inner core: . Ruffini corpuscles (endings) . Grandry corpuscles Receptors with an asymmetrical lamellated inner core: . tactile corpuscles of Meissner . end bulbs (of Krause) – ‘genital’ corpuscles . Golgi-Mazzoni corpuscles Receptors with a symmetrical lamellated inner core: . Pacinian (Vater-Pacini) corpuscles . Herbst corpuscles Muscle receptors: . neurotendinous organ (Golgi tendon organ) . neuromuscular spindles: • intrafusal fibers • extrafusal fibers Prof. Dr. Nikolai Lazarov 13 Cutaneous receptors Pacinian (Vater-Pacini) corpuscles: . oval-shaped and lamellated . the largest cutaneous receptors – approximately 1 mm in length (2x4 mm) . bulb-like nerve endings in the subcutaneous tissue of the skin: • palms and soles • joints and genitals . capsule of 20 to 60 concentric lamellae . inner core of modified Schwann cells . afferent nerve fiber – parts: • myelinated • unmyelinated preterminal • nerve ending . rapidly adapting (phasic) mechanoreceptors – respond to vibration and pressure with optimal sensitivity 150-300 Hz Prof. Dr. Nikolai Lazarov 14 Cutaneous receptors tactile corpuscles of Meissner: . unmyelinated nerve endings in the dermis of glabrous skin • palm, fingers and lips . responsible for sensitivity to light touch . rapidly adapting receptors sensing vibrations lower than 50 (20-40) Hz end bulbs of Krause: . in the mucous membrane of the lips and tongue . also found in the penis and the clitoris – ‘genital’ corpuscles . respond to pressure – pressoreceptors . formerly – cold receptors (thermoreceptors) Prof. Dr. Nikolai Lazarov 15 Cutaneous receptors Golgi-Mazzoni corpuscles: . in the subcutaneous tissue of the human fingertips . similar in morphology to Pacinian corpuscles . sense vibratory pressure and touch Ruffini endings – 0.5-2 mm: . end organs in the subcutaneous connective tissues of the fingers . ovoid capsule within which the sensory fiber ends with numerous collateral knobs . respond to superficial pressure – slowly adapting mechanoreceptors . early thought to be thermoreceptors (mediate the sense of warmth) Prof. Dr. Nikolai Lazarov 16 Muscle receptors neurotendinous organs (Golgi tendon organs): . located at the insertion of skeletal muscle fibers into the tendons . proprioceptive – reflex regulation of muscle tension . located at the insertion of skeletal muscle fibers into the tendons . proprioceptive – reflex regulation of muscle tension neuromuscular spindles: . intrafusal fibers • nuclear-bag fibers • nuclear-chain fibers . extrafusal fibers . proprioceptors – detect changes in the length of the muscle Prof. Dr. Nikolai Lazarov 17 Effector nerve endings . myoneural junction – motor end plate: structure: myelinated axon collaterals ~50 axon terminals (boutons) • synaptic vesicles – ACh • presynaptic membrane sarcolemma junctional folds postsynaptic membrane • nicotinic ACh receptors . autonomic effector endings: sympathetic – adrenergic (NA) parasympathetic – cholinergic (ACh) purinergic – ATP and adenosine do not make specialized synaptic contacts Prof. Dr. Nikolai Lazarov 18 Thank you… Prof. Dr. Nikolai Lazarov 19 .