Nervous System Terminology (Introduction)

ANATOMY AIIMS, GROSS ANATOMY, EMBRYOLOGY, NEUROANATOMY, MICROANATOMY, APPLIED/ CLINICAL ANATOMY

                             

                   

Hello Friends,

In this article, I will be dealing about different anatomical terms used in Neuroanatomy. Starting from basic unit neuron, it will go to complex terms. I will try to cover almost all important terms. It will act as a ready reference for you. If, you forgot meaning of any term, you can visit this page and you will get your answer. So, let us start the journey of terminology in Neuroanatomy.

                             

1. Neuron- Cell specialized for sending and receiving chemically mediated electrical signals is termed as Neuron.

2. Cell body- Part of neuron that includes the nucleus is termed as Cell body.

3.   Dendrite- Short branching processes from neuron that receives signals from other neurons are termed as Dendrite.

4. Axon- Process of neuron, which is highly variable in length and typically conducts impulses away from cell body is termed as Axon.

5.  Axoplasm- Cytolasm of axon is known as Axoplasm.

6.  Axolemma- Plasma membrane of axon is known as Axolemma.

7.  Perikaryon- Cytoplasm of cell body is termed as Perikaryon.

8.  Neurite- Any neuronal process axon or dendrites are termed as Neurite.

9.  Neurone doctrine- Each neuron is a structural and functional unit.

10. Neuropil- Delicate neuronal and glial processes in which cell bodies of neurons are embedded within gray matter is collectively termed as Neuropil.

11.  Nucleus- Collection of neurons inside the central nervous system is termed as Necleus.

12. Ganglion- Collection of neurons in peripheral nervous system outside CNS is termed as Ganglion.

13. Sensory Neurons- Those neurons, which convey impulses from receptors to the CNS.

14. Motor Neurons- Those neurons, which convey impulses from CNS or ganglia to effector cells.

15. Somatic Afferent Fibers- Nerve fibers, which covey sensations of pain, temperature, touch and pressure from body surface alon-with pain and proprioception from organs within the body are termed as Somatic afferent fibers.

16. Visceral Afferent Fibers- Nerve fibers, which convey pain and other sensations from internal organs, mucous membrane, glands and blood vessels, are termed as Visceral afferent fibers.

17. Somatic Efferent Fibers- Nerve fibers, which carry impulses to skeletal muscle, are termed as Somatic efferent fibers.

18. Visceral Efferent Fibers- Nerve fibers, which carry involuntary impulses to smooth muscle, cardiac muscle and glands, are termed as Visceral efferent fibers.

19. Intercalated Neurons- Neurons that form a communicating and integrating network between sensory and motor neurons are termed as Intercalated neurons.

20.  Multipolar Neuron- Neuron having one axon and two or more dendrites is known as Multipolar neuron.

21. Bipolar Neuron- Neuron having one axon and one dendrite is known as Bipolar neuron.

22. Pseudounipolar (Unipolar) Neuron- Neuron having single axonal process dividing into two close to cell body, one branch going to periphery and other extending to CNS is known as Pseudounipolar (Unipolar) Neuron.

23.  Nissl Bodies- Collections of rough endoplasmic reticulum in cell body and may be extending to dendrites also in neurons are known as Nissl bodies.

24. Axon Hillock- Area of cell body of neuron near axon lacking large cytoplasmic organelles like Nissls bodies, free ribosomes and golgi apparatus is known as Axon hillock.

25. Initial Segment of Axon- Region of axon between the apex of axon hillock to the starting point of myelin sheath, where action potential is generated is known as Initial segment of Axon.

26. Principal cell- The large neuron of nucleus is known as Principal cell.

27.  Interneuron- Smaller neurons in territory of a principal neuron, contacting with its axon to dendrites of principal cell is known as Interneuron.

28. Golgi Type I Neuron- Neurons of motor nuclei of CNS, whose axons travel a long distance to reach effector targets, skeletal muscle; are known as Golgi type I neurons.

29.   Golgi Type II Neuron- Interneuron of CNS having short axon is known as Golgi type II neuron.

                      

30. Synapse- Specialized junctions between neurons or with neuron and effector cells (like muscle and glands) to facilitate transmission of impulses from one neuron to other neuron or effector cell are known as Synapse.

31.  Axodendritic synapse- Synapse between axons and dendrites is known as Axodendritic synapse.

32.  Axosomatic synapse- Synapse between axons and cell body is known as Axosomatic synapse.

33. Axoaxonic synapse- Synapse between axons and axons is known as Axoaxonic synapse.

34.  Chemical synapse- Synapse in which transmission of impulses takes place by release of chemical substances/ neurotransmitters from presynaptic neurons is known as Chemical synapse.

35. Electrical synapse- Synapse that doesn’t require neurotransmitters for conduction of impulse and transmission occurs by movement of ions across cells through gap junctions creating spread of electrical current known as Electrical synapse.

36. Presynaptic Neuron- Neurons from which end neurotransmitters are released at synapse is known as Presynaptic neuron.

37. Postsynaptic Neuron- Neuron which end as receptor sites for neurotransmitters at synapse is known as Postsynaptic neuron.

38. Synaptic Cleft- 20-30 nm gap between presynaptic and postsynaptic membrane at synapse is known as Synaptic cleft.

39. Synaptic Vesicle- Membrane bound structure which contain neurotransmitters is known as Synaptic vesicle.

40.   Ribbon synapse- Specialized type of chemical synapse present in receptor hair cells of internal ear and photoreceptors of retina is known as Receptor synapse.

41.  Bouton en passant- Several synaptic contacts made by presynaptic neuron along its way on the surface of postsynaptic neuron are known as Bouton en passant.

42. Bouton terminal- presynaptic axon after making several synapses with postsynaptic neuron finally end as enlarged tip is known as Bouton terminal.

43. Excitatory Synapse- Synapse in which presynaptic terminal releases neurotransmitters like acetylcholine, glutamine, serotonine causing opening of cation channels prompting influx of Na or other ions leading to depolarization and generation of action potention finally leading to generation of nerve impulse; is known as Excitatory synapse.

44. Inhibitory synapse- Synapse in which presynaptic terminal releases neurotransmitters like GABA (gamma amino butyric acid) or glycine causing opening of chloride or other anion channels leading to influx of chloride or other anions causing hyperpolarization of postsynaptic membrane and stoppage of action potential and nerve impulse; is known as Inhibitory synapse.

45. Neurotransmitters- Chemical molecules released by presynaptic neurons acting on receptors located on postsynaptic membranes causing either excitation or inhibition of nerve impulse transmission are known as Neurotransmitters.

46. Neuromodulators- Chemical molecules that don’t directly interact with receptors but can modify the interaction of neurotransmitters with receptors are known as Neuromodulators.

47. Neuromelanin- Black pigment present in neurons which as catecholamines like dopamine and noradrenaline as neurotransmitters is known as Neuromelanin.

48. Axonal Transport- Mode of intracellular communication carrying molecules and informations from axon terminals to the nerve cell body and from nerve cell body to axon terminal is known as Axonal transport.

49. Anterograde Transport- Transmission of molecules and informations from nerve cell body to periphery is known as Anterograde transport.

50.   Retrograde Transport- Transmission of molecules and informations from axonal terminals and dendrites to nerve cell body is known as Retrograde transport.

51. Slow Transport System- Transport of molecules and informations from nerve cell body to axon terminal at the rate of 0.2 to 4 mm/day is known as Slow transport system.

52. Fast Transport System- Transport of molecules and informations from nerve cell body to axon terminal or in opposite direction at the rate of 20-400mm/day is known as Fast transport system.

53. Gray Matter- Part of central nervous system containing cell bodies of neurons along-with neuropils (delicate neuronal and glial processes) is termed as Gray Matter.

54. White Matter- Part of central nervous system containing mainly long processes of neurons, lacking cell bodies is termed as White Matter.

55. Central nervous system- Part of nervous system including brain and spinal cord is termed as Central nervous system.

56.  Peripheral nervous system- Part of nervous system consisting of cranial, spinal and peripheral nerves conducting impulses from the central nervous system via efferent or motor nerves and to the central nervous system via the afferent or sensory nerves is termed as Peripheral nervous system. It also includes collections of nerve cell bodies outside CNS, termed as Ganglia and motor and sensory nerve endings.

57. Somatic nervous system- Somatic parts of CNS and PNS providing sensory and motor innervations to all parts of body except viscera, smooth muscles, cardiac muscles and glands is termed as Somatic nervous system. Its functions are under conscious voluntary control except reflex arc.

58.   Autonomic nervous system- Autonomic part of CNS and PNS providing efferent inputs to involuntary motor innervations to smooth muscles, cardiac muscles and glands along-with providing afferent sensory innervations from the viscera is termed as Autonomic nervous system. It is further divided into enteric, sympathetic and parasympathetic nervous system.

                                   

59.    Neuroglia- Supporting cells of nervous system are termed as Neuroglia.

60.  Peripheral Neuroglia- Supporting cells of PNS are termed as Peripheral Neuroglia.

61. Central Neuroglia- Supporting cells of CNS are termed as Central neuroglia.

62. Schwann Cells- Neuroglia forming myelin sheath in PNS, developing from neural crest cells and supporting myelinated and unmyelinated nerve fibers are termed as Schwann cells.

63. Satellite Cells- Neuroglia, small cuboidal in shape surrounding neuronal cell bodies functioning to help in establishing and maintaining controlled microenvironment around neuronal cell body, electrical insulation and also providing pathway for metabolic exchanges are known as Satellite cells.

64. Teloglia (Terminal Neuroglia)- Neuroglia associated with motor end plates are known as Teloglia.

65.    Muller cells- Neuroglia associated with retina are known as Muller cells.

66. Enteric Neuroglia- Neuroglia associated with ganglia in wall of alimentary canal, similar to astrocytes in CNS, acting as structural, metabolic and protective support of neurons are known as Enteric Neuroglia.

67.  Astrocytes- Largest neuroglia present in CNS, functioning to support and modulate the activities of neurons along-with formation of blood-brain barrier are termed as Astrocytes.

68.  Protoplasmic Astrocyes- Astrocytes present in gray matter of brain having numerous short branching processes are termed as Protoplasmic astrocytes.

69. Fibrous Astrocytes- Astrocytes present in white matter of brain having few processes are termed as Fibrous astrocytes.

70. Oligodendrocytes- Neuroglia forming myelin sheath in CNS having few processes as compared to astrocytes are known as Oligodendrocytes.

71.  Microglia- Neuroglia, part of mononuclear phagocytic system, smallest in size and phagocytic in function; known as Microglia.

72. Gitter Cells- Microglia cells filled with lipid rich phagocytosed material are known as Gitter cells.

73. Ependymal cells- Neuroglia of CNS forming epithelial lining of fluid filled cavities having single layer of cuboidal to columner cells known as Ependymal cells.

74. Tanycytes- Specialized ependymal cells most numerous in 3^rd ventricle having no cilia and free surface in direct contact with CSF, involved in transport of substances from CSF to blood in hypothalamic circulation are known as Tanycytes.

75. Myelin Sheath- Compact layer of Schwann cells (in PNS) or Oligodendrocytes (in CNS) around the axon is known as Myelin sheath.

76. Myelination- Formation of Myelin sheath around axon is known as Myelination.

77.  Abaxonal Plasma membrane- Myelinating cell membrane (Schwann cell in PNS/ Oligodendrocytes in CNS), which is exposed to external environment is known as Abaxonal plasma membrane.

78.   Adaxonal Plasma membrane- Myelinating cell membrane (Schwann cell in PNS/ Oligodendrocytes in CNS), which is in contact with axon is known as Adaxonal plasma membrane.

79. Mesaxon- Double layer membrane that connects abaxonal and adaxonal plasma membrane is known as Mesaxon.

80. Neuregulin- Growth factor regulating thickness of myelin sheath formation is known as Neuregulin.

81. Node of Ranvier- Junction or gap between segmented myelin sheath is known as Node of Ranvier.

82. Internodal Segment- Myelin sheath present between adjacent  nodes of Ranvier is known as Internodal segment.

83. Embryonic Glial cells- Neuroglia around the neural tube are known as Embryonic glial cells.

84. Glia Limitans- Almost impermeable barrier formed by foot-plates of astrocytes and piamater is known as Glia limitans.

85. Saltatory Conduction- Nerve impulse jumping from note to node along myelinated axon is known as Saltatory conduction.

                                    

86. Peripheral Nerve- Bundle of nerve fibers present together packed by connective tissues is known as Perpheral nerve.

87. Endoneurium- Loose connective tisue present around individual nerve fiber is known as Endoneurium.

88.  Perineurium- Connective tissue surrounding each nerve fascicle is known as Perineurium.

89. Epineurium- Dense irregular connective surrounding each peripheral nerve is known as Epineurium.

90. Blood-Nerve Barrier- Tight junctions present between cells of perineurium along with their basal lamina forming a selective barrier maintaining ionic environment of peripheral nerve is known as Blood Nerve Barrier.

91. Receptor- Specialized structures located at distal end of peripheral processes of sensory neurons, which initiates nerve impulse in response to a stimulus is known as Receptor.

92.  Exteroceptors- Receptors that react to stimuli from external environment like touch, pressure, temperature, smell, sound and vision are known as Exteroceptors.

93.  Enteroceptors- Receptors that react to stimuli from within the body like stretch of bowel, bladder and blood vessels are known as Enteroceptors.

94.  Proprioceptors- Receptors that react to stimuli from within the body and providing sensations of position, muscle tone and movement are known as Proprioceptors.

95.  Encapsulated Endings- Sensory nerve endings having connective tissue sheath around are known as Encapsulated endings.

96.   Muscle spindles- Encapsulated sensory nerve endings present in skeletal muscles acting as proprioceptors are known as Muscle spindles.

97. Free Nerve Endings- Terminal branches of group C and A fibers, receptive to all types of cutaneous sensations known as Free Nerve Endings.

98.   Peritrichial Nerve Endings- Nerve endings of axons around hair follicle are known as Peritrichial Nerve Endings.

99. Merkel cells- Mechanoreceptors associated with sensory nerve endings, present in stratul basale layer are known as Merkel Cells.

100. Pacinian Corpuscles- Deep pressure receptors for pressure and vibration sensations present in deeper dermis and hypodermis, composed of myelinated nerve endings which loses its sheath after 1-2 nodes surrounded by Schwann cells and endoneural derived concentric lamellae; Known as Pacinian corpuscles.

101. Meissner’s Corpuscles- Touch receptors present in dermal papilla of hairless skin made up of 1-2 unmyelinated nerve endings spiraling around Schwann cells; is known as Meissner’s corpuscles.

102. Ruffini’s Corpuscles- Encapsulated mechanoreceptors fusiform in shape made up of single myelinated fibers forming dense arborization within the capsule responding to mechanical stress is known as Ruffini’s Corpuscles.

103.  Golgi Tendon Organ- Receptor present near the attachment of tendon to muscle having Aβ nerve fibers stimulated by tension in the tendon providing protection against damage from strong muscular contraction is known as Golgi tendon organ.

104. Motor End Plate/ Myoneural Junction- Synapse between endings of motor axon and muscle fiber is known as Motor End Plate.

105.   Aα Fibers- Myelinated nerve fibers, which are motor to skeletal muscles and sensory from muscle spindles carrying propriceptive sensations and having diameter of 12-20µm with conduction velocity 70-120 m/sec are known as Aα Fibers.

106.   Aβ Fibers- Myelinated nerve fibers, which are sensory from tendons and from rufini nerve endings having diameter 10-15µm with conduction velocity of 60-80 m/sec are known as Aβ Fibers.

107.   Aγ Fibers- Myelinated nerve fibers, which are motor to intrafusal fibers of muscle spindles having diameter of 3-8 µm and conduction velocity of 15-40 m/sec are known as Aγ Fibers.

108.   Aδ Fibers- Myelinated nerve fibers, which are sensory from hair follicles and free nerve endings carrying pain and temperature sensations having diameter of 3-8 µm and conduction velocity of 10-30 m/sec are known as Aδ Fibers.

109. B Fibers- Myelinated nerve fibers, which are having preganglionic autonomic fibers of cranial nerves 3, 7, 9 and 10 ; and also white rami communicantes having diameter of 1-3 µm with conduction velocity of 5-15 m/sec are known as B Fibers.

110. C Fibers- Unmyelinated nerve fibers carrying pain and temperature, olfaction and also present in postganglionic autonomic nerve having diameter of 0.2-1.5 µm with conduction velocity of 0.5-2.5 m/sec are known as C Fibers.

111. Blood-brain barrier- The Blood-Brain Barrier is highly selective semipermeable membrane, which prevents non-selective crossing of solutes from circulating blood to the extracellular fluid of central nervous system.

112.  Circumventricular Organ- Certain areas of the adult brain, endothelial cells are not adhered by tight junctions and a free exchange of molecules occurs between blood and adjacent brain mostly located close to the ventricles; are termed as circumventricular organs.

113. Anterograde (Wallerian) Degeneration- Injury to axon causing degeneration distal to site of injury is known as Anterograde or Wallerian degeneration.

114.   Retrograde (Traumatic) Degeneration- Degeneration of axon proximal to site of injury is known as Traumatic degeneration.

115. Transneuronal Degeneration- Degeneration occurying in neuronal cell bodies is known as Transneuronal degeneration.

116. Chromatolysis- During process of degeneration, Nissl’s bodies move towards periphery of cell body is known as Chromatolysis.

117. Neuropraxia- Nerve injury leading to failure of conduction of nerve impulse without complete transaction of nerve is known as Neuropraxia.

118.  Regeneration- Formation of cellular bands around newly formed scar in region of axonal degeneration/injury by Schwann cells directing growth of new nerve processes is known as Regeneration.

119. Band of Bungner- Cellular bands of Schwann cells in longitudinal colums during regeneration process of damaged nerve is known as Band of Bungner.

120. Tinel’s Sign- Tapping of part of nerve which is regenerating causes tingling sensation in the areas of distribution of its fibers is known as Tinel’s Sign.

121. Neuroplasticity- Reorganization of Connections within the brain at structural or at physiological levels is known as Neuroplasticity.

122. Corpora Amylacea- Accumulation of glycoprotiens and lipoproteins within processes of astrocytes present mainly in middele aged to elderly; not interfering with functioning of nervous system known as Corpora Amylacea.

So, I have mentioned almost all the introductory terms of nervous system in this article. Now, the second article will be on terminology in regional part of nervous system including CNS, PNS, ANS, Motor and Sensory Pathways, Coverings of CNS and blood supply of different parts. So, please visit the next article for more extensive terminology in nervous system. Have a good Day. All suggestions and doubts are welcomed.

Dr. Rajesh Kumar