Nervous Tissue I Flashcards
Central Nervous System (CNS)
Brain and Spinal Chord-Derived from the neutral tube
Peripheral Nervous System (PNS)
All nervous tissue outside CNS-derived from neutral crest cells
Nervous System main divisions
CNS, PNS
Functions of Nerve Cells
- Irritability
- Conductivity
- Secretion
Irritability
Neurons rapidly respond to some stimuli
Conductivity
Transmission of a response spartially and temporarily to another part of the body
Secretion
Cell secrete neurotransmitters and other substanes that excite or inhibit neighboring cells
Neurons
Nerve cells of the CNS and PNS
Glial Cells
Support and protect Neurons
Connective tissue coverings
mostly everything is covered by connetive tissues
Bipolar Neurons
2 dendrites
- Retina
- Cochlear
- Offactory
Multipolar
MOST COMMON
-multiple dendrites
Pseudounipolar
Found is dorsal root ganglia
- Found in peripheral nervous system-tend to be part of long neurons
- body of cell has been moved to the side
Afferent Neurons
- Sensory Neurons
- Recieve Stimuli from environment and body
- impulse travels towards CNS
- Cell bodies are near CNS
- Consist primarily of pseudounipolar neurons
Efferent Neurons
- Motor Neurons
- Control effector organs such as muscle fibers
- impulse travels away from CNS
- Cell bodies are within CNS (Exception in ANS)
a. further classified
1. Somatic- Voluntary Muscles
2. Autonomic- Involuntary smooth muscles-both sympathetic and parasympathetic systems
Interneurons
Form functional networks or circuits (retina)
- Impulse travels within CNS
- Major Function-Coordinate all neural activities
- Consists mostly of multipolarNeurons
Classification of Neurons
-Myelinated
Myelin: formed by 2 different cells
WHAT ARE THEY??
-Non-Myelinated
Significant differences in transmission of the nerve impulses
Another name for Cell Body of a Neuron
Perikaryon- The space around the nucleus
Oligodendrocytes found in
CNS
Schwann Cells found in
Peripheral Nervous System
Perikaryon Structure
Perikaryon is like the cutoplasm of any cell
Organelles:
RER, Golgi, Mitochondria, Protein Synthesis Machinery
-Nucleus is centrally located in the cell
–When theyres a problem with the neuron, the nucleus is pulled away from the cell.
Nissl Bodies:
- are basophilic and they stain in a different way
- Area of extensive protein synthesis
Cytoskeleton:
- required because neurons are much longer than any other cell.
- Neurofilaments-provide cellular support
- Microtubules-provide intracellular transport/support
- Actin- Provides secondary transport
Lipfuscin-residual bodies left over from lysosomal digestion
Dendrites
Function: Receive Stimuli
- Procesess that extend from perikaryon
- Lack Golgi but contain most other components of perikaryon
Axon
Initial Segment of Axon is NON-Myelated
- Axon Hillock
- Initial Segment
- Axoplasm
- Collateral Branches (axon branches)
- Axon Terminal
- Axonal Transport
- Anterograde: kinesin
- Retrograde: Dyesin
Axon Hillock
- Short segment emanating from perikaryon
- No Nissl Bodies
- Initial site of microtubule bundling
Initial Segment:
- Region between axon hillock and initial myelination point
- Site of Neuron where stimulus is integrated-initiates/suppresses signal down the axon
Axonal transport
Movement of proteins and vesicles within axon
-Vesicles attached via motor proteins move along microtubules
Two Types:
Anterograde (from perikaryon to axon terminal)= Kinesin
Retrograde (from axon terminal to perikaryon)=Dynein
Anterograde Transport
From Perikaryon to Synapses
a) slow stream 1-4 mm/day
i. Necessary for axon growth
b) fast stream 50-400 mm/day
i. Vesicles carrying neurotransmitters and other membrane components needed for synaptic transmission (calcium channels)
Primary protein responsible for anterograde motion
Kinesin
Retrograde Transport
From synapses and axon to perikaryon
a) intermediate stream 10-100 mm/day
i. considered a salvage pathway
b) transports cytoskeletal components
Primary protein responsible for retrograde motion
Dynein
Axon terminal- Synapses
Types of Synapses:
a. Structure behind a Chemical Synapse (has a bigger gap which allows for diffusion)
b. Structure of an electrical synapse- composed of transmembrane proteins called CONNEXINS
c. combining both results in a mixed synapse
Chemical Synapses
Vesicles:
- Neurotransmitter vessicles
- Adrenaline Secreting
- Acetocholine secreting
- Hormone Vesicles
- Depolarization
Arrival of a nerve impulse causes an influx of Ca2+ into the axon terminal
Ca2+ influx triggers exocytosis of neurotransmitter vesicles into the synaptic cleft -Sympatheric, Parasympathetic
Neurotransmitter open or close ion channels to regulate depolarization at the postsynaptic membrane
Types of Synapse
Axo-dendritic
Axo-somatic
Dendro-dendritic
Axo-axonic
Gemmules
Show on pic
Synapses-Membrane Structure
- Presynaptic Membrane
- Postsynapic membrane
- Synaptic Cleft
Presynaptic Membrane
Terminal ending membrane of axon-
Postsynapic membrane
Membrane of cell juxtaposed to presynaptic membrane
Synaptic Cleft
Space between the 2 membranes
Synapses Histological View
PIC
Presynapic Cells are seen by staining of vesicles
Juxtaposed postsynaptic cell dendrites are vesicle-free
Plasma Membrane Pumps
- Pumps shuttles ions from one side of the membrane to another
- Na+/K+ ATPase pump shuttles 3Na+ out for every 2k+ pumped in
- ATP is necessary fo rthe action of this pump
- Sets up an uneven electrochemical gradient
How is a plasma membrane pump dipole?
Dipole: polarized with negative charge on the inside of cell and positively charged on the outside
Ion Channels
PIC
Nerve Impulse- Resting Potential
More K+ Leak channels are open compared to Na+ channels
- Leakage creates a net negative charge inside
- Electrochemical difference between the membrane results in an inactive (resting) neuron
- Chloride Ions usually dont contribute to membrane potential because eiquilibrium potential for Cl-ions equals the resting membrane potential
How do you measure Resting Potential?
Using 2 microelectrodes
-Inserted into axon
-Contact with external media
(both connected to voltmeter)
**before insertion, the voltmeter reads 0. Upon Insertion -70 mV are read
Nerve Impulse- Action Potential
- Resting State
- Depolarization
- Repolarization
- Hyperpolarization
- Back to resting State
*together the membrane is said to be excitable- phenomenon only resides in membranes of a few types of cells
Propagaion of action potential is in ____ Direction
ONE direction- this is assured by the inactivared form of the Na+ Channel
-Nothing can get through the channel and the channel cannot be repopened for 2-3 msec
Transition States of voltage-gated Na+ channels
Closed->open->inactivated->Closed
Closed=membrane polarized
Inactivated, Open=Membrane Depolarized
PIC
Action Potential
a. In response to a signal, the soma end of the axon becomes depolarized
b. the depolarization spreads down the axon while the the first part of the membrane repolarizes because Na+ Channels are inactivated and additional K+ channels have opened, the membrane cannot depolarize again]
c. the action potential continues to travel down the axon
Nerve Impulse-Saltatory Conduction
A. cross section of myelin-forming cell which wraps around the axon, the cell forms successive layers of membrane
B. Nodes of Ranvier form as spaces bw cells are left unmyelinated
Shwann Cell purpose
Schwann cell is wrapping around axon- insulating it from any electrical changes happening outside- changing at different intervals
The only point along the axon where Na+ channels are exposed to intersitial fluids. Only point where an action potential can occur
Nodes of Ranvier
Speed of each action potential is greatly enhanced because…
The nodes are spaced much further apart than each Na+ Channel
Synapses Saltatory Conduction: Conduction Velocity
Effects of myelination:
- Myelin sheaths insulate and prevent leakage of charge
- Saltatory conduction in myelinated axons is about 30x faster
- Voltage- gated Na+ channels located at the nodes
- AP’s appear to jump rapidly from node to node
- Voltage- gated Na+ channels located at the nodes
Synapses Chemical Transmission
- Action potentials arrive at axon terminal
- Voltage-gated Ca2+ channels open
- Ca2+ enters the cell
- Ca2+ signals to vesicles
- Vesicles move to the membrane
- Docked vesicles release neurotransmitter by exocytosis
- Neurotransmitter diffuses across the synaptic cleft and binds to to receptors
Binding of neurotransmitter to receptor allows the ligan gated Na+ ion channel to open in the post synaptic membrane allowing….
Na+ ions to diffuse (facillitated diffusion) from the sunaptic cleft into the post-synaptic bulb.
*
Synapses-Chemical Transmission
- Cholinergic junctions are those that release acetlycholine
- Most Efferent nerves fall into this catagory (parasympathetic)
- Adrenergic Juntions are those that release norepinephrine
- Postganglionic sympathetic neurons fall into this category
