Nerve Tissue Flashcards
Nerve Tissue function
-provides rapid and specific communication between organs and body
Neurons
Neuroglia
- highly speacialized cells. Similar to muscle cells they are electrically excitable
- support cells, also called glial cells
Number of Neurons in the body
100,000,000 neurons
Sensory neurons
-gather information from receptors
Interneurons
-form a communicating network between neurons
Motor neurons
-convey impulses from the nervous system to the effector cells
Neuron nucleus
-large, rounded usually euchromatic nucleus with prominent nucleolus(i)
Perikaryon
- cell body
- varies in size between 5-135 micrometers
Rough ER of neurons
- extremely well developed
- forms dense structure, visible in the light microscope called Nissl bodies.
Nissl bodies
-parallel arrays of RER cisternae
Golgi complex of Neurons
Well-developed
-well developed RER and Golgi reflect the need for the neuron to produce membrane and neurotransmitter in large quantities.
Mitochondria
Neurons have many mitochondria
Lysosomes
-usually present in the cytoplasm of neurons
Neurons and centrioles
- neurons usually lack the centrioles
- mature cell is not capable of cellular division
- some neurons retain the centrosome, which may play a role in nucleation of microtubules
Neuroblast
- few neuroblasts that are able to divide and reproduce in small numbers.
- only neurons that are replaced in an adult body on a regular basis are the olfactory neurons
Neuron cytoskeleton
-very well developed and consists of neurofilaments (type of intermediate filaments), microfilaments (composed of actin), and microtubules
Two processes of neurons
Axon and dendrite
-neurons form synapses, which are used for communication with other neurons, muscle cells, and glandular cells
Axon
- one axon per cell
- convey signals from the perikaryon to the next neuron or to the effector cell.
- end with an axon terminal
Size of axon
-usually fairly long (up to 1 meter), and have more or less constant diameter throughout their length
Axon Hillock
- begin from an elevated platform on the perikaryon.
- Nissl bodies are absent from the axon hillock
Myelin sheath
- insulation sheath that covers axons
- allows electric impulse to travel rapidly through the axons.
- abnormalities in the formation of the myelin sheath result in severe disorders
Axolemma
-continuation of the plasma membrane that encloses the axon
Axoplasm
- cytoplasm of the axon
- does not contain Nissl bodies or ribosomes, but has well developed SER.
Axon cytoskeleton
-formed by numerous microtubules and neurofilaments
Axonal transport
-presence of microtubules indicates intense transport of material through the axon.
Anterograde flow
- perikaryon to the periphery of the axon
- transport of actin filaments, proteins, organelles (Such as mitochondria), and vesicles.
- motor protein is kinesin
Slow axonal transport (anterograde flow type)
~1-6 mm/day
-move tubulin molecules, actin molecules, and proteins that form neurofilaments, from the perikaryon to the end of the axon
Fast axonal transport (type of anterograde flow)
~100-400 mm/day
-used to move membrane-bound organelles, such as RER compartments, synaptic vesicles, and mitochondria
Retrograde flow
Fast retrograde flow
Retrograde- transport from the distal part of the axon towards the perikaryon
Fast- transport of material taken up by endocytosis at axon terminal back to perikaryon
- used by some viruses (herpes, rabies) to travel through NS
- Toxins (tetanus) can be taken up to the perikaryon by the retrograde flow as well
- protein motor is dynein
Dendrites
- most neurons have several dendrites per cell.
- deliver the signal from the cell periphery to the perikaryon
Dendrite characteristics
- typically numerous thick, short, and tapered processes of nerve cells
- not myelinated
- cytoplasmic composition is similar to that of the perikaryon
- contain ribosomes and RER, but not Golgi apparatus
Dendritic Tree
- branch profusely which increases the area for synaptic contacts
- up to 200,000 synapses in one dendritic tree
Dendritic spines
- on the surface of dendrites
- synapses with axonal processes of other neurons
- have a “mushroom” shape and the “head” is where most postsynaptic receptors are located
Pseudounipolar
- primarily sensory neurons that have a single large process that begins from the perikaryon
- single cellular process branches into the peripheral and central processes.
Peripheral process and Central process of pseudounipolar
- PP-reaches the sensory area and collects the information, which is delivered to the central nervous system through the central process (CP)
- conduct like one axon
Location of pseudounipolar neurons
Dorsal root ganglia and some cranial nerve ganglia
Why is it called pseudounipolar?
-beginning of the development of the neuron two processes are formed, a dendrite and an axon, but they fuse later on to form one larger process that begins from the perikaryon
Bipolar neurons
- sensory neurons that are rather limited in their distribution
- found primarily within the major sense organs, such as eye retina, olfactory mucosa, and cochlea, and semicircular canals of the inner ear.
2 processes of bipolar neurons
- axon and dendrite
- dendrite branches in the sensory area and acts as a receptor
- axon delivers the impulse to the CNS
Multipolar neurons
- most common type of neurons
- both motor and interneurons belong to this type.
- have one axon and many dendrites
Golgi type I cells
- type of multipolar neuron
- have a long axon
- large motor neurons found in the motor nuclei of the CNS
Golgi type II cells
- type of multipolar
- short axon
- these are smaller interneurons found in the CNS
Electrophysiology of the nerve
-similar to muscle cells, plasma membrane of a nerve cell is an electric capacitor, like the sarcolemma of a muscle cell
Negative membrane potential in resting cell
- voltage on the inner side of the plasma membrane is negative (-70mV) relative to the outer side
- -possible b/c the Na+ ions are actively pumped outside of the cell, so the concentration of Na+ is ten times greater outside of the cell than inside
Action potentials
-brief positive going changes in the membrane potential that ate propagated along the length of the membrane at speed up to 120m/sec.
Depolarization
-as action potential travel along the membrane they open the voltage-sensitive channels and let the NA+ diffuse into the cell, which decreases the membrane potential
Hyperpolarized
-the membrane potential becomes even more negative, which makes the membrane more difficult to depolarize
Synapses
How nerve cells communicate with each other and with other cells (muscles)
-electrical and chemical
Electrical synapses
-represented by gap junctions, which allow direct passage of ions from one cell to another to transmit the wave of depolarization
Chemical synapses
- principal type of synapses found in mammals
- no protoplasmic continuity between the two cells and the signal is transmitted by release of a chemical (neurotransmitter) by one cell.
- binding of the neurotransmitter to the receptors of the other cell results in either depolarization or hyperpolarization of the membrane
Excitatory synapses
-depolarize the membrane of the postsynaptic cell making the generation of an action potential more likely