Chapter 12: Nerve Tissue Flashcards
CNS
Brain and Spinal Cord
PNS includes
12 Pairs of Cranial Nerves
31 Pairs Spinal Nerves
Somatic
ANS
ENS
Enteric NS: NS of the gut
Plexus
Network/whole bunch of nerves controlling a region
In which NS is axon regeneration more easily accomplished?
PNS
Sensory Neurons purpose?
Afferent: Out of periphery into spinal cord or brain
Motor Neuron
Efferent: Out from CNS to PNS (Effector)
Integrative neuron
Association neuron: Within CNS
- Small neuron b/w sensory and motor, typically in the spinal cord
- Processing takes place here
Two main nerve cells
Neurons and neuroglia
Difference bw Neurons and Neuroglia
Neurons: Electrically excitable (Produce nerve impulse (AP)
Neuroglial (Glue: Cells located around neruons providing structure and protection
3 Parts of most neurons
Cell body
Dendrites
Axon
Cell body
Location of nucleus
Dendrite
Small nervous appendages - bring info into cell body from other neurons, receptors etc.
Axon
Long tail conducting/carring AP
Nissl Bodies
Important organelles in cell body - Clusters of rough ER (Critical for repair and growth of damaged nerve)
Axon Hillock
Location where axon joins cell body
Initial segment
Trigger zone, portion of axon where nerve impulse begins
Axolemma
Membrane around axon on outermost portion (structure is a phospholipid)
Axoplasm
Cytoplasm of axon
Trophic factos
Any chemical hormone etc. that will stimiulate chemical growth
Lipofuscin
An example of a trophic factor
Synaptic Vesicle
Stores various NT’s before releasing them into the Synaptic cleft
What factors slow axonal transport
Non-myelinated
Smaller diameter
What is a slow speed for an axon to transport at?
1-5mm per day
what is a fast speed for axonal trasport
200-400mm per day
What factors speed up axonal transport?
Myelation
Larger axon diameter
Multipolar Neuron
Multiple dentridtes, one main axon
Typically found in brain and spinal cord
i.e Motor neurons & Interneurons
Bipolar Neurons
One main dendrite and one axone
(Typically found in special senses ex taste, smell, sight etc.)
Unipolar (Pseudounipolar)
Dendrite and axon is fused with the cell body off to the side
(Typically seen in general senses ex, touch, pressure etc.)
What can the maturation of Bipolar neurons result into
Unipolar
Purkinjie
Larger nerves typically found in heart branch off of main nerves and end up in tissue (very sensitive – can produce coordinated pumping action)
multipolar
Pyrmidal Neurons
Mainly found in cortex, labelled by there shape
Astrocytes
Many processes and most abundant neuroglia cell
Maintain environment around neurons and form BBB
Protoplasmic
Short branching processes in gray matter
Fibrous
Long unbranched process in white (Myelinated) matter
Astrocytes role
Regulate body growth in fetus, aid in learning and memory
Form BBB
Oligodentrocytes
Myelination in CNS axons
Myelinate several axons at a time
Influence repair rate (Slowing it down)
Microglia
Phagocytes of CNS
In dendrites
Prune unwanted synapses and routes (Remaining becomes more functional and stronger)
Ependymal cells
Cuboidal/columnar neuroglia cells, line ventricles in brain and central canal in spinal cord produce and direct CSF
Schwann (Neurolemmocytes)
Single axon myelination in PNS to help with regeneration
Satellite Cells
Supply nutrients to neurons in PNS and structure
Which Nervous system has the greatest capacity toregrowth?
PNS
What is MS
Myelin sheath begins to break down causing loss of movement and control
What myelinates in PNS
Schwan cells
What myelinates in CNS
Oligodendrocytes
myelin is composed of
Lipd and protein and is multilayered
- Nodes of Ranvier (Neurofibril node) – gaps in myelination
- Only occur in Myelinated neurons where there is NO myelination – extreme concentration of pumps at these locations, allows electricity to jump from NODE to NODE
Ganglia/on
Collection of Cell Bodies in the PNS
Nucleus
Collection of cell bodies in CNS
Nerve
Bundlw od axons in PNS
Tract
Bundle of axons in CNS
Gray Matter
Mainly made up of high conc. of cell bodies, dendrites, unmyelinated axons, axon terminals and neuroglia.
White matter
Primarily made up of myelinated neurons and axons
How does an AP for movement get to a muscle
Initiated in motor cortex, signal travels down tracts in spinal cord, leaves spinal cord level and then travels to muscle
How does a nerve impulse relate to the strength of a movement?
Larger number of muscle fibers required for movement; larger number of neurons required for movement
Action Potentials
Typically starting at intial segment
Travel great distances
Only in one direction
ALWAYS same strength
Graded Potentials
Electrical impusle travels in both direcitons
Short distance communication
- Can be of different strengths.
- If graded potential is strong enough it becomes action potential.
Upper Motor Neuron
- Motor Neurons in CNS that leave the brain and connect to the Lower Motor Neurons
Lower Motor Neuron
- Motor Neurons in CNS that leave the brain and connect to the Lower Motor Neurons
RMP
Resting MEmbrane Potential
(Electrical voltage difference across cell membrane)
Ion Channels
allow ions to move across membrane down concentration gradient
Allow electrolytes to move back and forth to create current/signal
What produces electrical impulses?
AP and GP are produced when ion channels open and close to let - / + charged ions flow back and forth like electrical current down their electrochemical gradient.
How do Ion channels work?
- Ions move: H to L concentration
- Negative to Positive areas
- Positive to Negative areas
- Channels open and close due to the presence of gates.
Allows electrolytes to flow back and forth
4 Main types of ion channels
Leakage
Voltage-Gated
Ligand-Gated
Mechanically Gated
Leakage Channels
Always open (Small amount of electrolytes to flow through continuously)
- Mainly responsible for establishing RMP
Voltage-gated Channels
Opens and closes in response to changes in voltage on either side of axon
Ligand Gated Channels
Implies channel has receptor site and ligand (NT) must attach to that channel which then opens it and allows flow.
Mechanically Gated
Gate opens in response to mechanical forces: touch, itch, tickle etc.
Potential means
Difference in electrical charge
At resting potential the cell is…
More negative inside and more positive outside
Typical RMP is
-70mV
(Negative indicates inside cell is more negative)
3 major factors that create RMP
Unequal distribution of ions (Electrolytes)
Inability of anions to leave cell (Negativity is established inside cell)
Electogenic natur of Na+-K+ ATPases (enzyme that runs the sodium potassium pump)
Graded Potential Commonalities
- Most in dendrites and cell bodies
MOST common in dendrites
LEAST common in axons
OFTEN dies out, won’t always produce AP
- Small deviation from RMP
- Permits variations in amplitude of signal (Signal strgth based on number of gatese open)
- Local Current (Can move in either direction)
- Decremental conduction – GP dies out as they move along
Generation of Action Potential
- Two main phases:
- Depolarization – the negative membrane potential becomes less negative, reaches zero, and then becomes positive
- Inside of cell becomes more positive
- Repolarization – MP restored to -70mV IMPORTANT diagram
- Threshold of AP
-55 mV
Result of Sub threshold stimuli
- Causes some depolarization but doesn’t cause AP bc it doesn’t reach the threshold (Therefore no sensitivity to that stimulus
Result of supra threshold stimulus
Produces several action potentials
Describe the four steps of an AP
Resting state: Voltage gated channels are closed, axon membrane is at resting potential
- Small build up of negative charges along inside of membrane and positive along outside
Depolarizing Phase: Membrane potential of axon reaches threshold, Na+ voltage gated channels open (Activation channels)
- Na+ ions move into neuron through these channels
- Positive ions buildup along inside of axon membrane causing it to become depolarized
Repolarization begins: Na+ channel inactivation gates close and K+ channels open. Membrane starts to repolarize as K+ Ions leave neurone (Build up of negative charges along inside membrane.
Repolarization continues: K+ outflow continues, as K+ leaves more negative ions build up along inside membrane. K+ outflow eventually restores RMP. Na+ inactivation gates open .
Returns to rest when K+ channesl close
Refractory Period
- Time after the FIRST action potential when a second one is NOT POSSIBLE
- Absolute Refractory Period
- Even with STRONG stimuli another AP cannot be produced (Sodium channels unable to open any more)
- Relative Refractory Period
no response (AP) to normal Stimuli - AP can be produced if it is STRONG stimulus (voltage gates are open longer)
Neurotoxins
– “poisons” that effect nervous system (eg. Puffer fish)
- Stopping the voltage gated channels from working (no depolar/repolarization)
How do Local Anesthetics work?
- Stopping production of Aps (perception of pain)
- Block the gates in the axons in that local area
- Sicknesss can affect effectiveness
How does icing work?
- Slowing velocity of action potential
How do nerve impulses propagate on a non-myelinated axon
- Continuous (step by step depolarization and repolarization)
How do nerve impulses propagate on myelinated axons
- Saltatory (myelinated axons) – leaping AP due to uneven distribution of voltage-gated channels
- Typically in myelinated axon
- Fast
- Energy efficient (less gates involved)
Classifications of nerve fibres
A - Biggest axon diameter (Fastest)
B
C - Smallest Axon Diameter (Slowest)
How does temperature affect AP?
Cooler temperature = slower velocity
Different types of synaptic signal transmission?
Axondendritic (Most common)
Axosomatic (Axon to cell body)
- Somatic neuron
Axoaxonic
What is an electrical synapse?
AP travels between two plasma membranes through gap junctions
- Tunnels that electrical current can pass through (connexons)
* Faster
* Synchronize group of neurons (or fibers)
* Common in cardiac muscle, viscera, and embryo
What does tropic mean?
Growth
I = Ionotropic Receptors
NT binds directly to the channel
M=Metabotropic Receptros
Second messenger system
- First binding to G protein which causes channel to close
-Slightly slower than I
EPSP
ExcitatoryPost Synaptic Potential
Type of GRADED POTENTIAL
Gets closer to AP threshold (Depolarizing signal)
IPSP
Inhibitory Post Synaptic Potential
- Postsynaptic membrane is polarized farther away from the AP threshold therefore less likely for the neuron to generate an AP
What are the 3 ways NTs are removed
- Diffusion
- NT floats away in synaptic cleft - Enzymatic Degradation
- Enzyme breaks down NT and it becomes inactive - Uptake by cells
- Cell that release NT or neightboring cells reuptake the cell to be used later
Spatial Summation
: Many Presynaptic neurons fire at once summating towards an AP on a single postsynaptic neuron.
-1000 to 10000 synapse at atypical neuron in CNS
Summation of post synaptic potential
Temporal Summation
One presynaptic neuron continuing to release NTs which add together on the postsynaptic neuron.
What is the result of Summation?
EPSP, IPSP or AP
How many types of NTs are there?
100+
Modification of NT Effects examples
Increase/decrease release of NT (I.e Parkinsons inadequate release of dopamine)
- Enhance (agonist) or blocked (antagonist)
- Some drug or chemical that mimics the effects of the receptor
- OR Blocks receptor - Stimulated or inhibited the removal of NT
- Illicit drugs
Simplest Neural Circuit
One pre and one post synaptic neuron
Diverging circuit
Begins with one neuron and recruits more and more
(The spine uses this)
Converging Circuit
Many neurons converge into one
(i.e. Many receptors in nose converge into olfactory nerve)
Reverberating circuit
Similar to a positive feedback loop (Memory, breathing, HR etc.)
Parallell after discharge Circuit
Likely used for complex thinking and analysis skills
How is a Myelin Sheath Formed?
Oligodendrocytes (CNS) or Schwann Cells (PNS) wrap around small section of the axon forming myelin sheath.
The cytoplasm and nucleus are pushed to the outside forming the neurolemma.
Saltatory
Saltatory (myelinated axons) – leaping AP due to uneven distribution of voltage-gated channels
3 main functions of NS to maintain homeostasis
Sensory
Motor
Integrative
CNS vs PNS
CNS is brain and Spinal Cord
PNS is Motor and sensory division
of the motor: their is the somatic and autonomic division
Of ANS: Sympathetic and Parasymathetic
