HISTOLOGY 1 13 NERVOUS TISSUE Nervous tissue is

HISTOLOGY 1. 13. : NERVOUS TISSUE Nervous tissue is specialized to generate and conduct impulses. Origin: neuroectoderm Tissue components: nerve cells, glial cells and their processes Blood supply: densely capillarized

The neuron: Generalized schematic drawing of a multipolar neuron Nissl staining Silver impregnation

Classification of neurons: on the basis of their processes Unipolar neurons Pseudo-unipolar neurons Bipolar neurons

Classification of neurons: on the basis of their processes Motoneuron in the spinal cord (Nissl staining) MULTIPOLAR NEURON-TYPES Cortical pyramidal cell (silver impregnation) Cerebellar Purkinje cell

Classification of neurons: on the basis of their activity Excitatory cell type: it has spiny dendrites, its axon makes asymmetrical synapses using excitatory neurotransmitters Inhibitory cell type: it has non-spiny beaded dendrites, its axon makes symmetrical synapses using inhibitory neurotransmitters

Model of a multipolar neuron within the nervous tissue: 1. 2. 3. 4. 5. 6. 7. 8. Dendrite Axon (myelinated) Nucleus Nucleolus Golgi apparatus r. ER Axon hillock and initial segment Synaptic boutons terminating on the membrane of the neuron 9. Glial cell endfeet 10. Capillary with erythrocytes 11. Compact neural tissue (neuropil)

Cell body (perikaryon) of the neurons (electron micrograph) EM G M L N=nucleus n=nucleolus Asterisks label stacks of r. ER LM (Nissl bodies) M=mitochondrion L=lipofuchsin G=Golgi apparatus Non-visible on the picture: microtubules neurofilaments s. ER

Processes of the neurons: dendrites Highly branched processes. Dendrites may contain microtubules, neurofilaments, s. ER, free ribosomes and mitochondria. Their membranes exhibit postsynaptic densities (arrows), the sites of synaptic transmission, thus: dendrites are the „receiving” processes, accepting impulses from other neurons. Cross-section Transverse section

Processes of neurons: axons Axon hillock Initial segment Long, cylindrical process with few branches along its course and multiple terminal branches (telodendrion). Axons originate from axon hillock. Initial segment: free of myelin sheath, receive synapses from other neurons. LM Axon Terminal bulb, or synaptic bouton Telodendrion EM

Characteristics of dendrites and axons: a summary Axon: 1. Extends from cell body or dendrite 2. Begins with initial segment 3. May be absent (amacrine cells) 4. Unique in most cells 5. May be myelinated or no 6. Never contains ribosomes 7. Smooth contours, cylindrical shape 8. The thinnest process at the origin 9. Ramifies by branching at obtuse angles 10. Gives rise to branches of same diameter 11. May extend long distances from soma 12. Neurofilaments predominate in axons 13. Capable of generating action potentials propagating them and synaptic transmission 14. Primarily engaged with conduction and transmission Dendrite: extends from cell body in proximal portion continues cytoplasm May be absent (dorsal root ganglion) Usually multiple Rarely myelinated Contain r. ER, or ribosomes Irregular contours, appendages (spines) Originates as thick, tapering process Ramifies by branching at acute angles Subdivides into smaller branches Confined to the vicinitiy of cell body Microtubules predominate in dendrites Conduct in a decremental fashion but may be capable of generating action potentials Primarily engaged with receiving synapses

Synapses A synapse between neurons is a site of morphological specialization where one neuron is able to influence the excitability of another neuron. Types of synapses: 1. / Electrical synapse: is a gap junction between the membranes of two adjacent neurons 2. / Chemical synapse: changes the membrane potential of the postsynaptic neuron by releasing neurotransmitter molecules. Electrical synapse (gap junction) Chemical synapse

Types of chemical synapses: A. / On the basis of the postsynaptic site: Axodendritic Axosomatic Axospinous (Less frequent types /not shown here/: Axo-axonic synapse, Dendro-dendritic synapse, Dendro-axonic type Reciprocal synapse)

Types of chemical synapses: A. / On the basis of the function: 1. / Excitatory (arrow): Usually asymmetrical 2. / Inhibitory („hands”): Usually symetrical

Neurotransmitters of the chemical synapses: Acetyl choline (ACh) Amino acids: glutamate (Glu) Amino acid derivatives: serotonin (5 -HT) aspartate (Asp) dopamine (DA) glycine (Gly) g-amino butyric acid (GABA) histamine (His) Peptides: opioids (enkephalins, endorphins, dynorphins, etc. ) neurohypophyseal (vasopressin, oxytocin, neurophysin) tachikinins (substance P and K, neurokinin, etc. ) gastrins ( gastrin, cholecystokinin-CCK) Somatostatin (SOM) Vasoactive intestinal polypeptide (VIP) Neuropeptide Y (NPY) etc. Purins: adenosin