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 (in CNS)  peripheral – in PNS . central gliocytes – neural tube:   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  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  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  -forming cells in the CNS – electrical insulation of  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  absorption and secretion of cerebrospinal fluid (liquor)  (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 )

Prof. Dr. Nikolai Lazarov 5 Peripheral gliocytes . Schwann cells (neurolemmocytes)

 neural crest origin  myelin-forming cells in the PNS  maintenance of the integrity  phagocytotic activity and cellular debris that allows for regrowth of PNS

. 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 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  – mechanical stress  – perception  – temperature (heat, cold or both)  by location:  cutaneous receptors – in the  muscle spindles – in the muscles  by morphology:  free nerve endings  encapsulated receptors Prof. Dr. Nikolai Lazarov 9 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 . 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) . –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 () . 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 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