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Flashcards in Histology Deck (23):

Discuss the characteristics of nerve tissue

  • provides rapid and specific communication between organs in the body
  • 2 major components of nerve tissue are high specialized cells called neurons and support cells called neuroglia/glial cells
  • neurons are electrically excitable
  • functional categories of neurons
    • sensory
    • interneurons
    • motor


Describe the structure of a neuron.

  • large, rounded, euchromatic nucleus with prominetn nucleolus
  • perikaryon varies in size (5-135um)
  • Nissal bodies formed from well developed RER (dense structures that are parallel arrays of RER)
  • well-developed Golgi complex
  • good RER and Golgi reflect the need for the neuron to produce membrane and neurotransmitter in large quantities
  • many mitochondria
  • lysosomes in cytoplasm
  • lack centrioles (can't divide)
  • cytoskeleton made of neurofilaments, microfilaments, and microtubules


Describe the axon


  • each neuron has only one axon per cell
    • axons convey signals from perikaryon to the next neuron or effector cell, ending with axon terminal 
    • usually fairly long with constant diameter
    • begin at axon hillock
    • Nissal bodies are not present in axonal cytoplasm, nor are ribosomes. does have smooth ER
    • axons insulated with myelin sheath
    • enclosed by the continuation of the plasma membrane called axolemma
    • cytoskeleton of axon formed by numerous microtubules and neurofilaments


Describe axonal transport

  • anterograde flow
    • from perikaryon to periphery
    • allows transport of actin filaments, proteins, organelles, such as mitochondria, and vesicles 
    • kinesin is the motor used
      • slow axonal transport - 1-6mm/day moves tubulin molecules, actin molecules, and proteins that form neurofilaments from the perikaryon to the end of the axon
      • fast axonal transport - 100-400mm/day moves membrane bound organelles, such as SET compartments, synaptic vesicles, and mitochondria
  • retrograde flow
    • distal part of axon towards perikaryon
    • transport materials taken up by endocytosis at axon terminal 
    • some viruses and toxins use this pathway, eg herpes simplex, rabies, and tetanus
    • motor for this type of transport is dynein


Discuss dendrites

  • most neurons have several dendrites per cell
  • designed to deliver signal from periphery to the perikaryon
  • numerous, thick, short, and tapered 
  • branch profusely to form dendritic tree, to increase area for synaptic contacts (up to 200,000 contacts per tree)
  • surface is covered with dendritic spines where synapses with axonal processes of other neurons are formed.
    • mushroom shaped head where most post-synaptic receptors are located
  • not myelinated
  • cytoplasmic composition similar to that of perikaryon
  • have ribosomes and RER, but no Golgi apparatus


Classify neurons based on shape and number of cellular processes 

  • psuedounipolar
    • primarily sensory neurons
    • single large process from perikaryon that branches into peripheral and central processes
    • PP reaches sensory area and collects info
    • info delivered to the CNS via CP
    • conduct as one axon
    • typical location: dorsal root ganglia and some cranial nerve ganglia
    • psuedo bc initially created as 2, but fuse into 1 process
  • bipolar neurons
    • sensory neurons
    • limited distribution
    • found in major sense organs: eye retina, olfactory mucosa, cochlea and semicircular canals of the inner ear
    • 2 processes from cell body, axon and dendrite
      • dendrite acts as sensory receptor
      • axon delivers to CNS
  • multipolar neurons
    • most common type of neuron
    • both motor and interneuron belong in this type
    • have one axon and many dendrites
    • depending on length, get classified as
      • Golgi type I cells - long axon - large motor neurons in motor nuclei of CNS
      • Golgi type II cells - short axon - smaller interneurons found in CNS


Describe the basic electrophysiology of the nerve tissue

  • plasma membrane like electric capacitor
    • similar to muscle cells' sarcolemma
  • voltage on inner side of plasma membrane is negative ( ~- 70mV) relative to outer side so there is negative membrane potential in resting cell
    • possible because Na+ ions are actively pumped out of cell
  • action potentials are brief positive going changes in the membrane potential that are propagated along the length of the membrane at a speed up to 120m/sec
    • considered waves of depolarization
    • as they travel, they open the voltage sensitive channels and let Na+ diffuse into the cell, decreasing the membrane potential
  • membrane is hyperpolarized when the membrane potential becomes even more negative, making the membrane more difficult to depolarize


Describe the structure of a synapse

  • presynaptic terminal
    • contains synaptic vesicles with neurotransmitter capable of binding to receptor to generate wave of depolarization
    • have mitochondria
    • no RER, but could have smooth ER
  • synaptic cleft - between plasma membranes
  • postsynaptic region
    • contains receptors for neurotransmitters


Classify synapse as either electrical or chemical

  • electrical 
    • represented by gap junctions
    • allow direct passage of ions from one cell to another to transmit wave of depolarization
  • chemical 
    • principal type found in mammals
    • no protoplasmic continuity between the two cells 
    • the signal is transmitted by release of a chemical (neurotransmitter) by one cell
    • binding to receptor causes either depolarization (excitatory) or hyperpolarization (inhibitory)


Describe the impulse transmission in a chemical synapse

  • actional potential propagated along the membrane of the presynaptic cell from perikaryon towards axonal terminal
  • reaches presynaptic terminal, opens Ca++ channels briefly
  • influx of Ca++ causes vesicles to migrate to and fuse with the membrane (causes exocytosis)
  • neurotransmitter diffuses across the cleft
  • neurotransmitter bound by receptors on membrane, starting local depolarization of postsynaptic cell
  • extra plasma membrane formed from synaptic vesicles fusing is removed by endocytosis using clathrin coated vesicles
  • neurotransmitter deactived by 2 ways
    • recapture - high affinity reuptake - for most catecholamines; NT is reincorporated by endocytosis into vesicles that are ready for repackaging
    • degradation - enzymes break down remaining NT that is left in the cleft; ACh is broken into acetate and choline in the cleft
    • clinically - inhibition of the enzyme that breaks down NT norepinephrine or inhibition of high-affinity reuptake, has beneficial effect in tx of depression


Discuss morphotypes of synapses

  • differentiate based on connections
  • axodendritic - connect bw axon and dendrite
  • axosomatic - connect bw axon and perikaryon
  • axoaxonic - connect bw axon and another axon (commonly find inhibitor synapses)
  • motor-end plate - represents neuromuscular junction and is specialized type of synapse consisting of
    • axon terminal that contains presynaptic vesicles with NT ACh
    • synaptic cleft bw nerve cell and muscle cell
    • sarcolemma of a muscle cell forms multiple junctional folds in the area of the motor end plate which hold the receptors for ACh
  • neuroglandular junction - connects axon to gland


Discuss support cells in the PNS

  • Schwann cells
    • form lipid layer called myelin sheath that surrounds axons in PNS
    • "envelope" unmyelinated axons
    • myelin sheath isolates axon from the surrounding tissue and provides electrical insulation, necessary for rapid conduction of electrical impulses
    • support both myelinated and unmyelinated nerve fibers
      • myelinated - single axon wrapped in lipoprotein complex - saltatory conduction - fast
      • unmyelinated - several axons enveloped into simple clefts of Schwann cell, in middle of nerve bundle; action potential is wave-like
  • Satellite cells
    • found in ganglia of the PNS
    • surround bodies of individual neurons
    • create microenvironment and provide weak electrical insulation
    • do not have myelin
    • provide a pathway for metabolic exchange necessary for the neurons


Describe the myelin sheath

  • created by Schwann cells
  • wraps around axon, plasma membrane wraps with it, squeezing the cytoplasm out
  • forms gaps where there are interruptions in the myelin sheath - nodes of Ranvier
    • represent spaces in adjacent Schwann cells
  • axolemma of myelinated nerve fibers in the area of nodes of Ranvier have high concentration of Na+ channels
  • action potentials travel via saltatory conduction, meaning membrane is only depolarized at the NoR. 
  • myelin sheath insulates well enough to allow depolarization at one node to elevate the voltage at the next node to the level necessary to generate an action potential 
  • NoR are also axoaxonal synapse sites and branches


What are the support cells of the CNS?


  • astrocytes
  • oligodendrocytes
  • microglial cells
  • ependymal cells


Describe astrocytes and discuss a clinical correlation

  • largest neuroglial cells, support neurons and CNS
  • granular cytoplasm
  • extend b/t neurons and capillaries
  • move metabolic substances b/t blood and nerve cells
  • with endothelial cells of blood capillaries, they form the blood-brain barrier
  • stain positive for glial fibrillary acidic protein (GFAP)
  • 2 main types of astrocytes
    • protoplasmic astrocytes - found in GRAY matter of the brain; numerous, short, branching processes, form structures called perivascular feet along blood capillaries
    • fibrous astrocytes - more prominent cytoskeleton than protoplasmic, found in WHITE matter of the brain; fewer processes with less branching
  • tumors derived from astrocytes are astrocytomas - most common in the brain, representing 20% of brain tumors. astrocytes give rise to 80% of tumors that originate in the brain
  • in case of local damage to the brain, astrocytes are responsible for process called gliosis, resulting in glial scar


Describe oligodendrocytes and discuss a clinical correlation

  • most common neuroglial cells of CNS
  • smaller than astrocytes
  • small nuclei, abundant SER, prominent Golgi
  • have a few tongue-like cellular processes extending from body to wrap around axons and form myelin sheath - similar to Schwann cells, but covers more than 1 axon. also allows space for NoR
  • Multiple Sclerosis - caused by damage to the myelin sheath in CNS done by cells in the immune system 
    • results in partial loss of myelin sheath
    • sx include loss of sensitivity, partial paralysis, etc, depending on area that is damaged


Describe microglial cells and discuss a clinical correlation

  • smallest cells of neurglia supporting CNS
  • derived from blood monocytes (like osteoclasts and macrophages) and are part of the mononuclear phagocytic system
  • dark indented nuclei, limited cytoplasm
  • few, short, twisted processes covered with spikes, possibly equivalent to ruffled border in other phagocytic cells
  • cytoplasm has multiple lysosomes
  • number of microglial cells in the brain increase with injury, so believed to remove debris from the CNS
  • clinically - microglial cells are abdundant in Alzheimer's and PD pts. It is possible the cells are partially responsible for the plaque formation, demyelination and destruction of nerve fibers in the CNS of these pts. 


Describe ependymal cells

  • support cells for CNS, lining the 4 ventricles of the brain and cavities of the spinal cord
  • responsible for production and absoprtion of CSF
  • arranged in simple cuboidal but no basal lamina
  • tightly bound by junctional complexes, and often posess microvilli (absorption CSF), and cilia (mvmt CSF)
  • basal processes of cells interdigitate with astrocyte processes allowing metabolite exchange
  • GFAP is present


Describe the histological structure of the major components of the peripheral nervous system

  • consist of cranial, spinal, and peripheral nerves, ganglia, and special nerve endings
  • nerves - carry sensory and motor info; myelinated and non-myelinated axons; nerve fibers held together by sheets of connective tissue
    • endoneurium surrounds individual fibers
    • perineurium surrounds nerve fascicles
    • epineurium surrounds individual nerves and extends into the spaces between fascicles (thickest)
  • ganglia - clusters of neuron cell bodies outside CNS
    • covered by connective tissue capsule and usually have satellite cells associated with them
    • 2 main types
      • sensory craniospinal ganglia
        • contain pseudounipolar neurons (PP longer and going to receptor/ CP shorter and going to spinal cord or brain)
        • surrounded by satellite cells
      • motor ganglia of autonomic nervous system
        • contain multipolar neurons and satellite cells
  • special endings - either motor or sensory
    • ex: motor-end plates
    • sensory nerve endings have 2 major types
      • special senses nerve endings - smell, sight, hearing, and equilibrium
      • somesthetic receptors - found through the body in epithelial tissues, connective tissues, muscles and joints
        • free nerve endings - branched sensory endings mediating pain
        • encapsulated nerve endings include Meissner's corpuscle, Pacinian corpuscle and several others
    • proprioceptors -designed to collect information about the angulations of joints and muscle tension
      • muscle spindle receptor in skeletal muscle;


Describe the major types of encapsulated nerve endings

  • Meissner's corpuscle 
    • type of somesthetic receptor
    • cylindrical structure formed by the stacks of llamellae that surround one or two sensory nerve endings
    • provide sense of touch and are most common in the skin of fingers and toes
    • go to pseudounipolar in dorsal root ganglia
  • Pacinian corpuscle
    • type of somesthetic receptor
    • largest of encapsulated nerve endings and the most complex
    • spherical in shape and consists of up to 30 concentric sheets of connective tissue with fluid between the layers that surround a single nerve fiber
    • receptors respond to vibrations and deep pressure and are found in teh dermis of the skin, mesenteries, and inside internal organs (e.g. pancreas)


Describe the muscle spindle

  • in proprioceptive receptors
  • specialized stretch receptor
  •  covered with 2 capsules (internal and external) with fluid separating them. I
  • nside spindle are intrafusal fibers (skeletal muscle fibers that are surrounded by the nerve fibers of two types (sensory and motor)
  • sensory nerve fibers wrap around the intrafusal fibers and transmit info about degree of stretching of muscle.
  • motor nerve fibers are thought to regulate the sensitivity of the stretch receptor


Discuss Gray Matter

  • gray matter - neuron bodies and unmyelinated fibers that form dense fibrous network
    • extensive vascular supply through system of capillaries
    • in spinal cord, gray matter is internal to white (opposite of brain)
    • organized into 2 pairs of horns - dorsal receives afferent, ventral transmits efferent
    • gray matter on left and right of spinal cord are connected via the gray commissure
    • in the brain, gray matter is external, has deep folds (gyri); in cerebellum folds are called folia
    • in cerebrum, gray matter has 6 layers with 3 main types of neurons
      • pyramidal cells
      • fusiform cells
      • granule cells
    • in cerebellum, gray matter has 3 layers
      • molecular - external: few cell bodies, called basket cells, many cell processes
      • Purkinje - very large neurons called purkinje cells
      • granular - most internal: adjacent to whtie matter. highly cellular, many small neurons (granule cells)


Discuss white matter

  • consists of myelinated axons and glial cells
  • limited blood supply compared to gray matter
  • tissue is rather dense with limited extracellular space
  • no synaptic contacts within the white matter
  • in spinal cord white is external to gray matter
  • in brain, white is internal to gray matter