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What is the sequence of development of CNS?
neuroepithelium -->neural plate-->neural groove--> neural tube
What are the derivatives of neural crest?
- Neural crest cells stream off the edges of the neural groove before formation of the neural tube.
- Sensory neurons of cranial and spinal sensory ganglia
- Most sensory neurons and Schwann cells of the PNS
- Enteric and autonomic ganglia and their postganglionic neurons and associated glia
- Most of the mesenchymal (ectomesenchymal) cells of the head and anterior portion of the neck
- Melanocytes of the skin and oral mucosa
- Odontoblasts (cells responsible for the production of dentin)
- Cells of the arachnoid and pia mater (rostral to the mesencephalon)
- Chromaffin cells of the adrenal medulla
What are the types of neurons?
- Unipolar neurons possess a single process but are rare in vertebrates
- Bipolar neurons possess a single axon and a single dendrite. These neurons are present in some sense organs (e.g., the vestibular–cochlear mechanism).
- Multipolar neurons possess a single axon and more than one dendrite. These neurons are the most common type of neuron in vertebrates.
- Pseudounipolar neurons possess a single process that extends from the cell body and subsequently branches into an axon and dendrite. They are present in spinal and cranial ganglia.These neurons originate embryologically as bipolar cells whose axon and dendrite later fuse into a single process functionally categorized as an axon.
What are the features of neuronal cell body?
- The nucleus is large, centrally located with abundant euchromatin and a large nucleolus (owl-eye nucleus).
- Nissl bodies are composed of polysomes and rough endoplasmic reticulum (RER). They appear as clumps under light microscopy and are most abundant in large motor neurons.
- The Golgi complex is near the nucleus
- Melanin-containing granules in some neurons in the CNS and in the dorsal root and sympathetic ganglia.
What are the cytoskeletal component of neurons?
- Neurofilament (intermediate)-->three intertwining polypeptide chains
- Microfilament (actin)--> associated with the plasma membrane
Dendrites receive stimuli that are transmitted..................soma
What are the features of dendrites?
- Possess arborized terminals (except in bipolar neurons), which permit a neuron to receive stimuli simultaneously from many other neurons.
- Cytoplasm is similar to that of the soma except that it lacks a Golgi complex.
- Organelles are reduced in number or absent near the terminals except for mitochondria, which are abundant.
- Spines on the surface of dendrites increase the area available for synapse formation with other neurons. These diminish with age and poor nutrition and exhibit altered configurations in individuals with trisomy 21 or trisomy 13.
Axons conduct impulses ............... the soma
What are the features of axon?
- Originate from the axon hillock, a specialized region of the soma that lacks RER, ribosomes, Golgi cisternae, and Nissl bodies but contains many microtubules and neurofilaments; abundance of the latter may regulate neuron diameter. Further, it permits passage of mitochondria and vesicles into the axon.
- Axons may have collaterals, branching at right angles from the main trunk.
- Axon cytoplasm (axoplasm) lacks a Golgi complex but contains smooth endoplasmic reticulum (SER), RER, and elongated mitochondria.
- A plasma membrane surrounding the axon is called the axolemma.Axons terminate in many small branches (axon terminals) from which impulses are passed to another neuron or other types of cells
Axon hillock lack........
RER, ribosomes, Golgi, Nissl bodies
Axon Hillock has abundant
Microtubules and neurofilaments
Cytoplasm of axon and dendrite lack....................
What are the general characteristics of glia?
- Neuroglial cells comprise several cell types and outnumber neurons by approximately 10 to 1.
- These cells are embedded in a web of tissue composed of modified ectodermal elements; the entire supporting structure is termed the neuroglia.
- They function to support and protect neurons, but they do not conduct impulses or form synapses with other cells.
- Neuroglial cells possess the capacity to undergo cell division.
What are the functions of astrocyte?
- 1) Astrocytes are the largest of the neuroglial cells. They have many processes, some of which possess expanded pedicles (vascular feet) that surround blood vessels, whereas others exhibit processes that contact the pia mater.
- 2) Astrocytes scavenge ions and debris from neuron metabolism and supply energy for metabolism3) form a protective sealed barrier between the pia mater and the nervous tissue of the brain and the spinal cord.
- 4) They provide structural support for nervous tissue.They proliferate to form scar tissue after injury to the CNS.
What are the types of astrocytes?
- Protoplasmic astrocytes reside mostly in gray matter and have branched processes that envelop blood vessels, neurons, and synaptic areas. They contain some GFAP . These astrocytes help establish the blood–brain barrier .
- Fibrous astrocytes reside mostly in white matter and have long, slender processes with few branches. They contain many intermediate filaments composed of GFAP.
What are the features of oligodendroglia?
- Live symbiotically with neurons (i.e., each cell type is affected by the metabolic activities of the other).
- They are necessary for the survival of neurons in the CNS.
- They possess a small, round, condensed nucleus and only a few short processes.
- Their electron-dense cytoplasm contains abundant organelles.
- Produce myelin, a lipoprotein material organized into a sheath that insulates and protects axons in the CNS.
- Each oligodendrocyte produces myelin for several axons.
What are the features of Schwann cells?
- Schwann cells are flat cells with only a few mitochondria and a small Golgi region.
- Derived from neural crest cells
- They protect and insulate neurons in PNS. Schwann cells form either unmyelinated or myelinated coverings over neurons. However, a single Schwann cell can only insulate a single axon, whereas a single oligodendrocyte may insulate several axons.
- A myelin sheath consists of several Schwann cell plasmalemmae wrapped around asingle axon
What are the features of microglia?
Microglia are small, phagocytic neuroglial cells that are derived from the mononuclear phagocytic cell population in the bone marrow. They have a condensed, elongated nucleus and many short, branching processes. Activated microglial cells become antigen-presenting cells and secrete cytokines.
What are the features of ependymal cells?
- Ependymal cells, derived from the neuroepithelium, are the epithelial cells that line the neural tube and ventricles of the brain. In certain regions of the brain, they possess cilia, which aid in moving the cerebrospinal fluid (CSF).
- Modified ependymal cells contribute to the formation of the choroid plexus
What are the features of chemical synapses?
- Involve the release of a chemical substance (neurotransmitter or neuromodulator) by the presynaptic cell, which acts on the postsynaptic cell to generate an action potential.
- Most common neuron–neuron synapse and the only neuron–muscle synapse.
- Signal transmission across these synapses is delayed by about 0.5 ms, the time required for secretion and diffusion of neurotransmitter from the presynaptic membrane of the first cell into the synaptic cleft and then to the postsynaptic membrane of the receiving cell.
- Neurotransmitters do not effect the change, they only activate a response in the receiving cell.
What are the features of electrical synapses?
- These synapses involve movement of ions from one neuron to another via gap junctions, which transmit the action potential of the presynaptic cell directly to the postsynaptic cell.
- Electrical synapses are much less numerous than chemical synapses.
- Signal transmission across these synapses is nearly instantaneous.
How is the morphology of synapsis?
- Axon terminals are two types: Boutons terminaux are bulbous expansions that occur singly at the end of axon terminals. Boutons en passage are swellings along the axon terminal; synapses may occur at each swelling.
- The presynaptic membrane (thickened axolemma ) contains voltage-gated Ca2+channels, which regulate the entry of calcium ions into the axon terminal. The postsynaptic membraneThe synaptic cleft
- Synaptic vesicles discharge neurotransmitters into the synaptic cleft by exocytosis.
What are the two excitatory neurotransmitters?
What are the two inhibitory transmitter?
What are the roles of 5HT?
- Pain inhibitor
- mood control
............ are individual axons enveloped by a myelin sheath, Schwann cells in the PNS, or oligodendrocytes in the CNS
What is synaptic fatigue?
- When excitatory synapses are repetitively stimulated at a rapid rate, the
- number of discharges by the postsynaptic neuron is at first very great, but the
- firing rate becomes progressively less in succeeding milliseconds or seconds.
- This is called fatigue of synaptic transmission.
.......is the most important mechanism for termination of epilepsy
The mechanism of fatigue is mainly.....................
exhaustion or partial exhaustion of the stores of transmitter substance in the presynaptic terminals
Part of the fatigue process probably results from two other factors as well:
(1) progressive inactivation of many of the postsynaptic membrane receptors and (2) slow development of abnormal concentrations of ions inside the postsynaptic neuronal cell.
What is the effect of acidosis and alkalosis on synpase?
Alkalosis greatly increases neuronal excitability while acidosis greatly depresses neuronal activity
Cessation of oxygen for only a few seconds can cause ................
complete inexcitability of some neurons
caffeine, theophylline, and theobromine ...........neuronal excitability by.................
all increase neuronal excitability / reducing the threshold for excitation of neurons.
Strychnine ...... neuronal excitability by.......................
- increases/inhibits the action of some normally inhibitory transmitter substances,
- especially the inhibitory effect of glycine in the spinal cord
What is the effect of anesthetics on synaptic transmission?
- Most anesthetics increase the neuronal membrane threshold for excitation and thereby decrease synaptic transmission at many points in the nervous system
- Because many of the anesthetics are especially lipid soluble, it has been reasoned that some of them might change the physical characteristics of the neuronal membranes, making them less responsive to excitatory agents.
What are the features of myelin sheath?
- Produced by oligodendrocytes in the CNS and by Schwann cells in the PNS.
- It consists of several spiral layers of the plasma membrane of an oligodendrocyte or Schwann cell wrapping around the axon.
- Interrupted by nodes of Ranvier.
- Its thickness is constant along the length of an axon; however, thickness usually increases as axonal diameter increases.
- Under electron microscopy, the myelin sheath displays the following features: Major dense lines represent fusions between the cytoplasmic surfaces of the plasma membranes of Schwann cells (or oligodendrocytes), Intraperiod lines represent close contact, but not fusion, of the extracellular surfaces of adjacent Schwann cell (or oligodendrocyte) plasma membranes, Clefts (incisures) of Schmidt–Lanterman (observed in both electron and light microscopy) are cone-shaped oblique discontinuities of the myelin sheath due to the presence of Schwann cell (or oligodendrocyte) cytoplasm within the myelin
What are the features of nodes of Ranvier?
- Nodes of Ranvier are regions along the axon that lack myelin and represent discontinuities between adjacent Schwann cells or oligodendrocytes.
- In the PNS, the axon at the nodes of Ranvier is covered by interdigitated cytoplasmic processes of adjacent Schwann cells that protect the myelin-free surface of the axon.
- In the CNS, however, the axon is not covered by cytoplasmic processes of oligodendrocytes. Instead, the myelin-free surface of the axon at the node of Ranvier is covered by a foot plate of an astrocyte.
- The axolemma at the nodes contains many Na+ pumps, and in electron micrographs, it displays a characteristic electron density
What are the Connective tissue investments of the nerves?
- Epineurium is the layer of fibrous dense connective tissue (fascia) that forms the external coat of the nerves.
- Perineurium surrounds each bundle of nerve fibers (fascicle). Its inner surface consists of layers of flattened cells joined by tight junctions(zonulae occludentes) that prohibit passage of most macromolecules.
- Endoneurium is a thin layer of reticular fibers, produced mainly by Schwann cells, that surrounds individual nerve fibers.
What is the difference between craniospinal and autonomic ganglia?
- Craniospinal no synpasis! (psuedounipolar)
- ANS--> synapsis
What are the characteristics of resting potential?
- The resting membrane potential exists across the plasma membrane of all cells.
- The resting potential of most neuron plasmalemmae is negative, -70 mV inside the cell compared to outside the cell.
- It is established and maintained mostly by K+ leak channels and to a lesser extent by the Na+–K+ pump, which actively transports three Na+ ions out of the cell in exchange for two K+ ions. The resting potential exists when there is no net movement of K+ (i.e., when outward diffusion of K+is just balanced by the external positive charge acting against further diffusion)
What are the features of AP?
An action potential is the electrical activity that occurs in a neuron as an impulse is propagated along the axon and is observed as a movement of negative charges along the outside of an axon. It is an all-or-nothing event with a constant amplitude and duration
How is AP generated?
- An excitatory stimulus on a postsynaptic neuron partially depolarizes a portion of the plasma membrane (the potential difference is less negative).
- Once the membrane potential reaches a critical threshold, voltage-gated Na1 channels in the membrane open, permitting Na+ to enter the cell
- The influx of Na+ leads to reversal of the resting potential in the immediate area (i.e., the external side becomes negative).
- The Na+ channels close spontaneously and are inactivated for 1 to 2 ms (refractory period).
- Opening of voltage-gated K+ channels is also triggered by depolarization. Because these channels remain open longer than the Na+ channels, exit of K+ during the refractory period repolarizes the membrane to its resting potential.
- The ion channels then return to their normal states. The cell is now ready to respond to another stimulus
How is AP propagated?
- Propagation results from longitudinal diffusion of Na+ (which enters the cell at the initial site of excitation) toward the axon terminals (orthodromic spread).
- The longitudinal diffusion of Na+ depolarizes the adjacent region of membrane, giving rise to a new action potential at this site.
- Propagation does not result from diffusion of Na+ toward the soma (antidromic spread), because the Na+ channels are inactivated in this region.
- Action potentials are propagated most rapidly in myelinated fibers, which exhibit saltatory conduction. In this type of conduction, the action potential jumps from one node of Ranvier to the next.
postganglionic ANS neurons are...........
Anterograde transport is done by....... while retrograde transport is done by............
Bipolar neurons are found in.........
Retina, Olfactory mucosa, vestibular and cochlear ganglia
What are the changes following transection of axon in PNS?
chromatolysis (disruption of Nissl bodies with a concomitant loss of cytoplasmic basophilia), increase in soma volume, and movement of the nucleus to a peripheral position
What are the anterograde changes following axon transection?
- The axon and its myelin sheath, which are separated from the soma, degenerate completely (wallerian degeneration), and the remnants are removed by macrophages.
- Schwann cells proliferate, forming a solid cellular column that is distal to the injury and that remains attached to the effector cell.
What are the retrograde changes following axonal injury?
- The distal end, closest to the wound, initially degenerates, and the remnants are removed by macrophages.
- Growth at the distal end then begins (0.5–3 mm/day) and progresses toward the columns of Schwann cells.
- Regeneration is successful if the sprouting axon penetrates a Schwann cell column and reestablishes contact with the effector cell
Which nerve sheath exclude macromolecules?
What is the main constituent of Axon Hillock?