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Main page | Structure | Departments | Department of Histology and Embryology | Materials for students | Modul 1 | NERVOUS TISSUE.

NERVOUS TISSUE. part 1. A) NEURONS. B) GLIAL CELLS.
NEURONS. (=NRs).
Structure of NR: It is composed of 2 parts 1)PERIKARYON (nucleus-containing cell body), 2) PROCESSES, extending from perikaryon (axons and dendrites). Perikaryon contains abundance of: – NEUROFIBRILS (=NFs) and – RER.
NFs are composed of neurotubules and neurofilaments and fulfill function of CYTOSKELETON.  When RER cisternae are stained by die toluidine blue, they appear clumps of basophilic material, named  NISSL  bodies. Funct. of RER – is synthesis of NEROTRANSMITTERS.
NEURON is: A) actively neurotransmitters secreting cell, B) Nerve impulses generating and conducting cell, C) lives in Go-phase (NEVER divides by mitosis).
If process transmitts nerve impulse to perikaryon – it is dendrite; if process conducts  nerve impulse from perikaryon to following neuron – it is axon.
Morphologic classification of neurons (accordingly AMOUNT of processes):
  1. BIPOLAR NRs. – it have 1 axon and 1 dendrite. Locate in RETINA; organ of olfaction.
  2. PSEUDOUNIPOLAR  NRs.– 1 common process extends from perikaryon and later divides on 2 processes (1 axon +1 dendrite). These NRs locate in DORSAL ROOT GANGLION.
  3. MULTIPOLAR NRs. – when 3 and more processes extend from perikaryon. It locate in BRAIN and SPINAL CORD.
Functional classification of neurons:
  1. SENSORY (afferent) NRs. It generate nerve impulse in dendritic terminal and conduct impulse to CNS (to dorsal horn of spinal cord ). These NRs locate in DORSAL ROOT GANGLION.
  2. MOTOR (efferent) NRs. It locate in VENTRAL HORN of SPINAL cord. It send nerve impulses from CNS to skeletal muscles.
  3. INTERNEURONS (intercalated NRs). It connect sensory NRs with motor NRs, intercalating between them. These NRs locate in DORSAL HORN of Spinal cord.
In organism NRs ALWAYS form REFLEX ARCH (=RA):
     1st NR in RA is SENSORY NR. (in dorsal root ganglion).
     2nd NR in RA is INTERNEURON. (in dorsal horn of sp. cord and/or  lateral horn.).
     3rd NR in RA- is MOTOR NR. ( in ventral horn of sp. cord).
Embryonic origins:A) for NRs of CNS (brain and sp. cord) – are neuroblasts of MANTLE zone of NEURAL TUBE. B) for NRs of PNS (of ganglia) –  are neuroblasts of NEURAL CREST.
Crest also give rise to: Satellite cells of ganglia and – to Schwan cells of nerves.
GLIAL cells (supporting cells) of Central Nervous System (CNS):
  1. PROTOPLASMIC astrocytes. It locate in GRAY matter of CNS.
  2. FIBROUS astrocytes. It locate in  WHITE matter of CNS. Struct.: both types have many cytoplasmic processes. Funct.: a) support high concentration of K+ ions between neurons; b) form blood-brain barrier; c) form cellular SCAR in damaged areas of CNS
  3. OLIGODENDROCYTES. Occur in both gray and white matter. Funct.: participate in FORMATION of nerve FIBREs. In MYELINATED nerve FIBRES these cells produce   MYELIN SHEATH. Injury of those cs is accompanied by DEmyelination disease.
  4. EPENDYMAL cells. It line wall of brain VENTRICLES and CENTRAL CANAL of sp. cord. It are cuboidal cells with apical CILIA. Funct.: it participate in secretion and movement of CEREBROSPINAL fluid.
  5. MICROGLIAL cells. It originate from MONOCYTes and are macrophages of nervous tissue. Funct.: phagocytosis of damaged neurons.
GLIAL cells (supporting cells) of Peripheral Nervous System (PNS).
PNS includes : a) ganglia and b) spinal and cranial nerves.
  1. SCHWAN cells. It locate in peripheral nerves. Funct.: participate in FORMATION of nerve FIBREs. In MYELINATED nerve FIBRES these cells produce MYELIN SHEATH. Injury of those cs is accompanied by DEmyelination disease.
  2. Satellite cells. It locate in all GANGLIA, surrounding  perikaryones of neurons.
 NERVOUS TISSUE. part 2. NERVE FIBRES. NERVE ENDINGS.
NERVE FIBRES(=NFs).
There are 2 types of NFs: 1)MYELINATED NFs (=MNFs) and 2) UNmyelinated NFs.(=UNFs).
BOTH types of NFs form peripheral NERVE and white matter of CNS..
Structure of MNF on the cross section (from centre to periphery):
  1. One AXON of neuron is in the centre;
  2. MYELIN SHEATH (=MS) surrounds axon;
  3. NEURILEMMA  surrounds myelin sheath.
MS – is spirally wrapped plasma membrane of SCHWANN cell (in peripheral NERVE) or of OLIGODENDROCYTE (in brain and spinal cord). Chemically MS is composed of phospholipids. Neurilemma – is cytoplasm with nucleus  of SCHWANN cell (in PNS) or of OLIGODENDROCYTE. It means: nerve fibre is formed by 2 cell types:
  1. NEURON gives 1 AXON  for fibre,
  2. SCHWANN cell or OLIGODENDROCYTE give myelin sheath and neurilemma.
Structure of MNF on the longitudinal section
  1. AXON of neuron is in the centre. Plasma membrane of axon is AXOLEMMA.Funct.:it conducts nerve impulse,
  2. INTERNODAL segments. Segment corresponds ONE SCHWANN cell (in PNS) or OLIGODENDROCYTE(in CNS). Segment contains  MYELIN sheath.,
  3. Nodes of RANVIER.  Node corresponds junction BETWEEN adjacent Schwann cs or olygodendrocytes. Node does NOT contain myelin sheath.
Structure of UNF on the cross section (from centre to periphery):
  1. Several  UNmyelinated  AXONs of several neurons  are in cytoplasm of Schwann cell or olygodendrocyte;
  2. NEURILEMMA surrounds several axons,
  3. Double-membrane mesaxons binds each axon with Schwann cell plasma membrane.
SYNAPSES. (=SPs).
SPs – are specialized junctions between 2 neurons (interneuronal SPs) or between neuron and muscle fibre (myo-neural junction). In SP nerve impulse is presented by chemical messengers named NEUROTRANSMITTERS (=NTs).
SP is composed of: 1) PREsynaptic membrane, 2) Synaptic CLEFT, 3) POSTsynaptic membrane.
Presynaptic membr. is formed by  axonic terminal of previous neuron. It contains synaptic vesicles with NTs. Examples of NTs: a) ACETYLCHOLINE, b) NORADRENALINE (or norepinephrine). Group of SPs, containing acetylcholine - are CHOLINERGIC SPs. Group of SPs,  containing noradrenaline - are ADRENERGIC  SPs. Postsynaptic membr. contains RECEPTORs molecules, recognizing  NTs. This membr. may be formed by 3 components of following neuron: a) by perikaryon. Type of so SP – is axo-somatic SP; b) by dendrite of next neuron. Type of so SP- is axo-dendritic SP; c) by axon of second neuron. Type of so SP – is axo-axonic SP. (a,b,c – are morphologic types of interneuronal SPs).
SP between axon terminal and muscle fibre – is myo-neural junction. It belongs to group of MOTOR nerve endings: a) presynaptic membr. is formed by axonic terminal of neuron, BUT b) postsynaptic membr. is formed by muscle fibre SARCOLEMMA (contains receprors to NTs). Functioning of NTs in synaptic cleft provide CONTRACTION of muscle fibre.
NERVE ENDINGS. (=NEs). Includes 3 groups:
  1. MOTOR NEs - it is myo-neural junction. It provides contraction of muscle fibre.
  2. SECRETORY NEs – it is SP between axon and exo-/endocrine GLAND. It  provides secretion  of secretory product.
  3. SENSORY NEs (or receptors). It are formed by DENDRITE terminal. Group of sensory NEs,   perceiving touch, stretch, pressure – are MECHANORECEPTORS. Such as :a) MERKELs disks : unmyelinated dendrite terminal is surrounded by Merkel cells. Disks locate in skin. Funct.: to perceive discriminatory touch. b) MEISSNERs corpuscles: three unmyelinated dendrite terminals are surrounded by Schwann cells and connective tissue CAPSULE. Corpuscles locate in skin dermis. Funct.: for tactile discriminaton. c) PACINIAN corpuscles: one unmyelinated dendrite terminal is surrounded by Schwann cells+ by fibroblasts +by connective tissue CAPSULE. It locate in skin dermis and joints. Funct.: to perceive pressure, touch and vibration.
Составитель – доцент В.В. Бондаренко.
 
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