Nerve Tissue

Chapter 9

Nerve tissue

1. General Characteristics 

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A. Made up of 2 types of cells: nerve cells or neurons -structural and functional unit; glial cells or neuroglia -supporting, protecting and nourishing neurons. B. Neurons have unique processes and contact with each other via synapses forming neural network and circuit. C. Nervous tissue makes up nervous system. The central nervous system (CNS) consists of the brain and spinal cord; the peripheral nervous system (PNS) comprises ganglia, nerves and nerve endings. D. Function -- integrating and coordinating body activity.

2. Neuron

1)Morphology: Most neurons consist of three parts: ---cell body: spherical, pyramidal, fusiform or stellate in shape, 5µm-150µm ---dendrite: like branches of tree ---axon: long thin cord-liked

2) Structure of neuron ① Cell body (perinuclear cytoplasm, perikaryon) ---Nucleus: large, spherical, and pale-staining with a prominent nucleolus, indicating intense synthetic activity.

---cytoplasm: a. Nissl body: LM: coarse or fine basophilic particles EM: clusters of RER plus free ribosomes Function: synthesis of proteins structural proteins proteins for transport

b. Neurofibrils

•LM: Thin threads with silver staining;

• EM: Neurofilaments and microtubules;

• Function: serve as cytoskeleton, and are involved in transportation of substances.

c. other organelles 

Golgi complex

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Mitochondria

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Lysosome

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Lipofuscin is increased in number with age.

② Dendrites:  

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One or more dendrites / neuron. Usually short and thick, and tapered as they branch and rebranch like a tree. Have numerous dentritic spines on the surfaces, representing sites of synaptic contact. Contain similar organelles to perikarya, especially Nissl bodies. Main function is to receive information from other neurons and conduct it to the parent cell body.

③Axons:     

Only one axon / neuron. Usually long and thin with uniform diameter, does not branch profusely, but may have collaterals. Arises from a conical region called the axon hillock that derives from the perikaryon. The axon and axon hillock are devoid of Nissl bodies. Ends in several terminal branches called axon terminals or buttons, which contain vesicles with neurotransmitters in them. Conduct impulses away from one neuron to other neurons or to effector cells such as muscle or gland cells.

* Axonal transports:   

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slow anterograde: proteins and actin filaments. Intermediate anterograde: mitochondria. fast anterograde: transports the substances contained in synaptic vesicles. A retrograde flow: transports the several molecules, inclding material taken up by endocytosis( including viruses and toxins), to the cell body.

3) Classification ---According to number of processes 

multipolar neuron, which have more than two cell

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processes, one process being the axon and the others dendrites; bipolar neuron, with one dendrite and one axon.

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pseudounipolar neuron, single process, soon divides into two in a T-shape, with one branch extending to a peripheral ending and the other toward the CNS.

---According to the functional roles 

sensory neuron (afferent neuron): are involved in the reception of sensory stimuli from the environment and from within the body.

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motor neuron (efferent neuron): control effector organs.

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interneuron: establish relationships among other neurons, forming complex functional network.

3. Synapse

---Definition: synapses are sites of functional contact

between neurons or between neuron and other effector cells ---Classification: 

Chemical synapse: use a chemical mediator (neurotransmitter) to transmit impulses in one direction;

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Electrical synapse: permit direct flow of electrical current between two neurons (gap junction).

Neurotransmitters and Neuromodulators  Neurotransmitters are chemicals that , when

combined with a receptor protein, either open or close ion channels or initiate second-messenger cascades.

 Neuromodulators are chemical messengers that do not act directly on synapses but modify neuron sensitivity to synaptic stimulation or inhibition.

Types of synapses   

Axosomatic Axodendritic Axoaxonic

Ⅰ.Chemical synapse ---structure LM: in silver preparation, there are many button-liked structures on the surface of dendrites and cell body, called synaptic button

Synaptic buttons

EM:

Presynaptic terminal Synaptic cleft Postsynaptic terminal

1)Presynaptic terminal Axon terminal  presynaptic membrane  synapse vesicle: --round or flattened, --clear or with electron dense core --contain neurotransmitters or neuromodulators 

mito, SER, microtubule and microfilament

2)Synaptic cleft: Extracellular space between pre- and postsynaptic membranes. 3)Postsynaptic terminal:  postsynaptic membrane  receptors

The sequence of events during synapse activity. 

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When the nerve impulse reaches the presynaptic terminal, the synaptic vesicle fused with the presynaptic membrane and discharges the neurotransmitters into the synaptic cleft by exocytosis. The neurotransmitter then diffuses across the cleft and combines with specific receptor molecules in the postsynaptic membrane. The reaction between the transmitter and the receptor molecules induces an increase in the permeability of the postsynaptic membrane and causes a change in the membrane potential of the postsynaptic neurons.

Ⅱ. Electrical synapse

This type of synapse transmit ionic signals through gap junctions that cross the pre- and postsynaptic membranes, thereby conducting neuronal signals directly.

4. Glial cell (neuroglia)

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Glial cells are supporting cells within the nervous system. They are non-excitable and do not conduct nerve impulses.

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The nunber of glial cells is 5 to 10 times the number of neurons.

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Most glial cells possess processes. However, unlike neurons, the processes of glial cells cannot be classified as dendrites and axon.

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In routine HE histological preparation, only the nuclei of glial cells can be seen. Such nuclei are smaller than those of neurons and lack of nucleoli.

Ⅰ. Astrocyte   

star-shaped cells with multiple radiating processes. Have bundles of intermediate filament that reinforce their structure. end feet: to form glia limitans or vascular feet--constitute blood brain barrier

Fibrous astrocytes:  

Located in the white matter with few long, thin processes

asymmetrical Protoplasmic astrocyte: 

Found in the gray matter With many short, thick processes with more branch

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symmetrical

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Function: 

Serve as structural support and insulation to the neurons;

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Repairing: when the CNS is damaged, astrocytes proliferate to form cellular scar tissue.

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The end feet constitute blood brain barrier and the barrier regulates the diffusion of many substances between the blood and brain.

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It is believed that through the end feet, astrocytes transfer molecules and ions from the blood to the neurons.

Ⅱ. Oligodendrocyte: ---structure: smaller, fewer process ---function: their processes form myelin-sheath of NF in CNS

Ⅲ. Microglia: ---structure: small, elongated cells with short irregular processes.

---function: They are phagocytic and derived from mesoderm (monocytes).

Ⅳ. Ependymal cell: ---structure:  

cuboidal or low columnar epithelial cells apical: microvilli and cilia

---function:  

produce cerebrospinal fluid ---distribution: ventricle of brain and central canal of spinal cord

Ⅴ.Schwann cell Produce the myelin sheath that provides the insulation of neurons in the PNS. One Schwann cells forms myelin around a segment of one axon. (the same oligodendrocyte forms myelin sheaths for several nerve fibers.).

5. Nerve fibers

---definition: a nerve fiber is composed of an axon and a surrounding sheath. ---classification: according to myelin-sheath  myelinated nerve fiber (MNF)  unmyelinated nerve fiber

Ⅰ. Myelinated nerve fiber in PNS

1) Basic structure: axon surrounded by myelin sheath and neurolemma.

2) Along its length the nerve fiber is segmented:   

a. The region devoid of myelin sheath and with bare portion of the axon is known as the node of Ranvier. b. Each segment between two consecutive nodes of Ranvier is called an internode. c. The neurolemma is the outermost layer of cytoplasm, cell membrane and basal lamina of Schwann cells.

Longitudinal section and cross section of myelined nerve fiber

3) Each internode consists of one Schwann cell: 

The myelin sheath results from Schwann cell plasmalemma winding around the axon many times and so contains lipids and proteins.( Schwann cell →invagination and envelop the axon →form mesaxon → mesaxon become longer and longer →spiral around the fiber →form myelin sheath.)

In EM myelin sheath shows a series of concentrically arranged lamellae,

4)The functional role 

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a. Enhancing the speed of conduction along them via saltatory conduction, i.e., impulses jumping from node to node, because myelin sheath serves as an insulator. b. The thicker axon has the thicker myelin sheath and longer internode, and in turn has greater conduction velocity.

Ⅱ.Myelinated nerve fiber in CNS ---structure:  similar to in PNS  myelin-sheath is formed by flattened ending of oligodendrocyte’s processes  one oligodendrocyte can envelop many axons

Ⅲ.Unmyelinated nerve fiber in PNS ---structure:  

no myelin-sheath and Ranvier node one Schwann cell envelops more axons

Ⅳ. Unmyelinated nerve fiber in CNS 

* nothing to envelop the axon---naked axon

6.Nerves

Nerves : made up of nerve fibers and connective tissue. 

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1. Most nerves are mixed, i.e., contain both sensory (afferent) and motor (efferent) nerve fibers, and both myelinated and unmyelinated fibers. 2. There are 3 connective tissue sheets: a.Epineurium, a fibrous connective tissue encloses the entire nerve and also fills the space between bundles of nerve fibers. b.Perineurium is a continuous sheet of flattened epithelium-like cells surrounding each nerve bundles. c.Endoneurium envelops each nerve fiber. It is a very thin layer of loose connective tissue.

E: epineurium

P: perineurium

F: fascis

7. Nerve Ending -classified into sensory and motor nerve endings

1)Sensory nerve ending ---including free and encapsulated nerve endings ①free nerve ending-responsible for heat, cold, and pain. ---structure: NF→lose myelin-sheath → branch → distribute in epidermis, cornea, hair follicle epithelial cell and CT ---function: feel cold, hot, pain and slight touch

Chaper 18

P.368

Chaper 18

P.368

② Encapsulated N ending ---have CT capsule a. Tactile (or Meissner) corpuscle ---structure: 

CT capsule Flattened cell--transverse arranged

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NF→lost myelin sheath→spiral flattened

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cells

---distribution: dermal papillae, especially in tip of finger or toe, palms, soles and lips

---function: touch receptors

Chaper 18

P.368

b. Lamellar (or Pacinian) corpuscle  Distributed in subcutaneous tissue, mesentery, ligament

Composed of CT capsule, concentric lamellae of flattened cells and internal cylinder with the naked axon inserted in it. ---Function: feel deep or heavy pressure 

Chaper 10

c. Muscular spindles ---structure:  Distributed in skeletal muscle.  Formed by CT capsule, intrafusal muscle fibers (thin, striated, nuclei arranged in chain or cluster).  Nerve fibers end as annulospiral or flowerspray endings. ---function: detect muscle length and change in muscle length

P.195

2)Motor nerve ending : --in muscular T and gland ＆ Somatic motor nerve ending ---distribute in skeleton muscle Motor end plate (MEP) Myoneural junction

＠ A moter nerve fiber innervates many muscle fibers comprising a motor unit, whereas a muscle fiber is innervated by only one axon branch. ＠ LM: nerve fibers ramify with each terminal dilating as a plate-like mass and touching a muscle fiber.

Ultrastructure of the MEP. The left drawing shows branching of a small nerve with a MEP for each muscle fiber. The axon loses its myelin sheath and dilates, establishing close, irregular contact with the muscle fiber. Muscle contraction begins with the release of acetylcholine from the synaptic vesicles. This neurotransmitter causes a local increase in the permeability of the sarcolemma. The process is transmitted to the T tubules and SR. SR releases calcium ions that trigger the sliding filament mechanism of muscle contraction. Thin filaments slide between the thick filaments and reduce the distance between the Z lines, thereby reducing the size of all bands except the A band.

Motor neuron. The myelin sheath is produced by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. The neuronal cell body has an unusually large, euchromatic nucleus with a welldeveloped nucleolus. The perikaryon contains Nissl bodies, which are also found in large dendrites. An axon from another neuron is shown at upper right. It has 3 end bulbs, one of which forms a synapse with the neuron. Note also the 3 motor endplates, which transmit the nerve impulse to striated skeletal muscle fibers. Arrows show the direction of the nerve impulse.

Key points      

The cell types that make up nerve tissue; General structural features, distribution, classification and function of neurons; The structural components , micro and ultrastructural features, function and classification of synapses; The structural features, classification and function of glia cells; The classification and organization of nerve fibers and nerves; The structural features and classification of nerve endings.