Chapter 12 - Nervous Tissue Flashcards
What is CNS?
Central Nervous System
Contains:
Brain
Spinal Cord
What is PNS?
Peripheral Nervous System
Contains:
Cranial Nerves
Spinal Nerves
Plexuses (like Enteric Plexus in Small Intestine)
Sensory Receptors (like in Skin)
Where is the Motor and Sensory parts of Cranial and Spinal Nerves?
Motor Nerves are on the anterior part of Cranial and Spinal Nerves
Sensory Nerves are on the posterior of Cranial and Spinal Nerves
What is the organization layout of the Nervous System?
PNS has a Sensory Division and a Motor Division
Sensory Division sends Sensory Input (Somatic/Special Senses) to CNS
CNS sends Motor Output to Motor Division of PNS
Somatic Nervous System in PNS affects Skeletal Muscles
Autonomic Nervous System in PNS is divided into:
Sympathetic Nervous System and Parasympathetic Nervous System, and both affect Smooth Muscle, Cardiac Muscle, and Glands
Enteric Nervous System affects Smooth Muscle and Glands of GI Tract
What are the Functions of the Nervous System?
1- Sensory:
Sense changes through Sensory Receptors
2- Motor:
Respond to Stimuli
3- Integrative (CNS):
Analyze incoming sensory informatiion
Store some aspects
Make decisions regarding appropriate behaviors
What is Neuron?
Neuron is a nervous system cell
Electrically excitable
Has unique cellular structures
What is the Cellular Structure of Neurons?
Cell Body
Cytoplasm
Nucleus
Nissl Bodies
Mitochondria
Neurofibrils
Dendrites (multiple around body)
Dendritic Spines (at ends of Dendrites)
Axon
Axon Hillock
Initial Segment
Axon Collateral (branch of Axon)
Schwann Cell
Nucleus of Schwann Cell
Cytoplasm of Schwann Cell
Myelin Sheath of Schwann Cell
Neurolemma of Schwann Cell
Node of Ranvier (between each Schwann Cell)
Axoplasm
Axolemma
Neurofibril (inside Axon)
Axon Terminal
Synaptic End Bulb
Nerve impulse travels across Axon
Travels in a Saltatory Movement (series of jumps)
Myelin Sheath helps with faster movement
What are Neuron Structural Classifications?
Neurons can be classified based on the number of processes extending from the Cell Body
Multipolar Neuron
Bipolar Neuron
Unipolar Neuron
What is Multipolar Neuron?
One Axon
Many Dendrites extending from Cell Body
Trigger Zone at Axon Hillock
What is Bipolar Neuron?
One Axon
One Dendrite extending from Cell Body
Trigger Zone at Axon Hillock
What is Unipolar Neuron?
Cell Body has one extension
The one extension divides into:
Central Process leading to Axon
Peripheral Process leading to Dendrites
Trigger Zone at start of Dendrites
What are some Examples of Dendritic Branching?
1- Purkinje Cell:
Bipolar
One Axon with many Axon Terminal
One Dendrite divided in 2 large branches and many Dendritic Spines
2- Pyramidal Cell:
Multipolar
One Axon with some Axon Terminal
Many Dendrites with few Dendritic Spines
What are the Functional Classifications of Neurons?
Neurons can be classified based on the direction of nerve impulse propagation
1- Sensory/Afferent Neurons
(SA- Sensory Afferent)
2- Motor/Efferent Neurons
(ME- Motor Efferent)
3- Inter/Association Neurons
Between Neurons
In Gray Matter between Sensory and Motor Nerves in Vertebral Column
What is Sensory Neuron?
Usually Unipolar
Dendrites - act as Sensory Receptors of PNS
Cell Body in PNS
Axon Terminal in CNS - interact with Dendrites of Interneuron
What is Interneuron?
Usually Multipolar
Entirely in CNS
Dendrites - receive stimuli from Sensory Neuron
Axon Terminal - sends stimuli to Dendrites of Motor Neuron
What is Motor Neuron?
Usually Multipolar
Cell Body in CNS
Dendrites in CNS - Receive stimuli from Interneuron
Axon Terminal in PNS - affects Effector like Muscles and Glands
What are Neuroglia?
Neuroglia:
Not electrically excitable
Make up about half the volume of the Nervous System
Can multiply and divide
6 Kinds in total (4 in CNS and 2 in PNS)
What are the 6 Types of Neuroglia?
In CNS:
1- Oligodendrocyte - Produce the Myeline Sheath
2- Ependymal Cells - Barrier between CSF and Interstitial Fluid
3- Microglial Cells - Immune Cells of Nervous System
4- Astrocytes - Barrier that prevent substances from diffusing into Brain
In PNS:
1- Schwann Cells - Produce Myelin Sheath, transport nutrition, Phagocytosis
2- Satellite Cells - Supply nutrients
What is Myelination of Neurons?
The Myelin Sheath is produced by Schwann Cells (in PNS) and Oligodendrocytes (in CNS)
It surrounds the Axons of most Neurons
What is Gray Matter?
Receptive Area
On insides of PNS
On outsides of CNS
What is White Matter?
Area of Tract for information
On outsides of PNS
On insides of CNS
How do Electrical Signals work in Neurons?
Excitable cells communicate with each other via Action Potentials or Graded Potentials
Action Potentials allow communication over short and long distances
Graded Potentials allow communication over short distances only
Production of an Action Potential or a Graded Potential depends upon the existence of a Resting Membrane Potential (RMP) and the existence of certain Ion Channels
AP:
Depends on voltage
RMP -90mV
If RMP changes, Action Potential created
Propagates through short or long distances until action fully done
GP:
Leaking of ions (Na+ and K+) through membrane
Changes and stops
Short distances
Example of Graded Potential and Action Potential when writing?
Sensory Receptors in right hand send an Nerve Action Potential to a Sensory Neuron in PNS
Interneuron in CNS receives Action Potential from Sensory Neuron, and sends a Graded Potential to Interneuron in Thalamus
Interneuron in Thalamus sends an Action Potential left side of Brain in Cerebral Cortex
Upper Motor Neuron receives a Graded Potential from Cerebral Cortex
Upper Motor Neuron sends an Action Potential to Lower Motor Neuron in PNS all the way to right hand to Neuromuscular Junctions
Neuromuscular Junctions send a Muscle Action Potential to skeletal muscles in right hand
What are Ion Channels in Neurons?
Leak Channels
Ligand-Gated Channels
Mechanically-Gated Channels
Voltage-Gated Channels
What is Leak Channel in Neurons?
Alternate between open and closed
K+ channels are more numerous than Na+ channels
Leak Channel randomly opens and closes
One direction - K+ from cytosol to interstitial
Found in nearly all cells
What is Ligand-Gated Channel in Neurons?
Responds to chemical stimuli
Ligand (ACh) binds to receptors
Chemical stimulus opens the channel
2 Directions - K+ cytosol to interstitial or Na+ interstitial to cytosol
Found in Dendrites of some Sensory Neurons like pain receptors
Dendrites and cell Body of Interneurons and Motor Neurons
What is Mechanically-Gated Channel in Neurons?
Respond to mechanical vibration or pressure stimuli
Mechanical stimulus opens the channel
One direction - Ca2+ interstitial to cytosol
Found in Dendrites of some Sensory Neurons like touch receptors, pressure receptors, and some pain receptors
What is Voltage-Gated Channel in Neurons?
Respond to direct changes in RMP
RMP change opens the channel
One direction - K+ cytosol to interstitial
Found in Axons of all types of Neurons
What is Resting Membrane Potential?
The membrane of a non-conducting Neuron is positive outside and negative inside
This is determined by:
1- Unequal distribution of ions across the plasma membrane and the selective permeability of the neuron’s membrane to Na+ and K+
2- Most Anions cannot leave the cell
3- Na+/K+ Pumps (against gradient)
Na+ most outside cell
K+ most inside cell
What is Graded Potential?
Small deviation in RMP
Hyperpolarization:
Becomes slightly more negative
Depolarization:
Becomes less negative
GP occurs in response to the opening of a Mechanically-gated or Ligand-gated Ion Channel
What is Graded Potential Stimulus Strength and Summation?
The amplitude of a GP depends on the stimulus strength
The GP can be added together to become larger in amplitude
What is Action Potential?
Action Potential is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (Depolarization) and eventually restore it to the resting state (Repolarization)
Depolarization:
Na+ goes inside
Repolarization:
K+ goes outside
Hyperpolarization:
Equal charges but K+ still going outside
What does RMP phase do?
RMP:
Voltage-gated Na+ channels are in resting state
Voltage-gated K+ channels are closed
What does Stimulus phase do?
Stimulus causes Depolarization to Threshold
What does Depolarization phase do?
Voltage-gated Na+ channel activation gates are open
Absolute Refractory Period
What does Repolarization phase do?
Voltage-gated K+ channels are open
Na+ channels are inactivating
Absolute Refractory Period
What does Hyperpolarization phase do?
Voltage-gated K+ channels are still open
Na+ channels are in resting state
Relative Refractory Period
What is Action Potential Stimulus Strength?
AP can only occur if the membrane potential reaches Threshold
Suprathreshold Stimulus:
Many AP
What are the Characteristics of Graded Potential?
Origin:
Dendrites and cell body
Types of Channels:
Ligand-gated or Mechanically-gated ion channels
Conduction:
Decremental (not propagated)
Permit communication over short distances
Amplitude (Size):
Depends on strength of stimulus
1mV-50mV
Duration:
Typically longer
Several milliseconds to several minutes
Polarity:
Hyperpolarizing - inhibits AP generation
Depolarizing - helps AP generation
Refractory Period:
Not present
Summation can occur
What are the Characteristics of Action Potential?
Origin:
Trigger Zone and propagates along Axon
Types of Channels:
Voltage-gated Channels for Na+ and K+
Conduction:
Propagate
Permits communication over longer distances
Amplitude (Size):
All or none
About 100mV
Duration:
Shorter
From 0.5 to 2 milliseconds
Polarity:
Always depolarizing phase followed by repolarizing phase and return to RMP
Refractory Period:
Present
Summation cannot occur
What is Propagation of Action Potentials?
In order for communication to occur from one body part to another, AP must travel from where they arise at the Trigger Zone to the Axon Terminals
AP do not die out, they keep their strength as they spread across the membrane of a Neuron
What is Continuous Conduction vs Saltatory Conduction?
Continuous Conduction:
Not myelinated
Saltatory Conduction:
Myelinated
Jumping from node to node (Node of Ranvier)
Faster conduction
What are the Factors that Affect Propagation Speed?
1- Axon Diameter:
Bigger is faster
2- Amount of Myelination:
More is faster
3- Temperature:
More is faster
What is Signal Transmission at Synapses?
A Synapse is the junction between Neurons or between a Neuron and an Effector (muscles)
1- Electrical Synapse:
Gap Junction connect cells and allow the transfer of information to synchronize the activity of a group of cells
2- Chemical Synapse:
One-way transfer of information from a Presynaptic Neuron to a Postsynaptic Neuron
What are Different Synapses between Neurons called?
Axoaxonic:
Presynaptic Neuron connects to Axon of Postsynaptic Neuron
Axodendritic:
Presynaptic Neuron connects to Dendrites of Postsynaptic Neuron
Axosomatic:
Presynaptic Neuron connects to Body of Postsynaptic Neuron
What is Signal Transmission at a Chemical Synapse?
Presynaptic Neuron sends a Nerve Impulse to Postsynaptic Neuron:
1- Nerve Impulse travels along Presynaptic Neuron
2- Ca2+ enters in Cytoplasm of Synaptic End Bulb through Voltage-gated Ca2+ channels
3- Ca2+ lets Synaptic Vessels release their Neurotransmitters from Synaptic End Bulb to Synaptic Cleft by Exocytosis
4- Neurotransmitters binds to Ligand-gated channels and open them
5- Na+ goes from Synaptic Cleft into Postsynaptic Neuron
6- Postsynaptic Potential
7- Nerve Impulse
What is Postsynaptic Potential?
1- Excitatory Postsynaptic Potential:
Depolarizing Postsynaptic Potential
2- Inhibitory Postsynaptic Potential:
Hyperpolarizing Postsynaptic Potential
(ex: sleep drugs)
A Postsynaptic Neuron can receive many signals at once
What is the Structure of Neurotransmitter Receptors?
Neurotransmitters at chemical synapses cause either an excitatory or inhibitory GP
Neurotransmitter Receptors have 2 structures:
1- Ionotropic Receptors
2- Metabotropic Receptors
What is an Example of Ionotropic Receptor?
1- Ionotropic ACh Receptor contains closed cation channel
ACh binds to it
Ionotropic ACh Receptor opens cation channel
Na+ and Cl- go from interstitial fluid to cytosol
K+ go from cytosol to interstitial fluid
Excitatory Postsynaptic Potential (EPSP)
2- Ionotropic GABA Receptor contains closed Cl- channel
GABA binds to it
Ionotropic GABA Receptor opens Cl- channel
Cl- goes from interstitial fluid to cytosol
Inhibitory Postsynaptic Potential (IPSP)
What is an Example of Metabotropic Receptor?
Metabotropic ACh Receptor with G Protein
Next to it K+ channel closed
ACh binds to Metabotropic ACh Receptor with G Protein
K+ channel opens
K+ go from cytosol to interstitial fluid
Inhibitory Postsynaptic Potential (ISPS)
What is Removal of Neurotransmitters?
Neurotransmitter can be removed from the Synaptic Cleft by:
1- Diffusion
2- Enzymatic Degradation
3- Uptake into cells
What is Summation?
If several Presynaptic End Bulbs release their neurotransmitters at bout the same time, the combined effect may generate a nerve impulse due to summation
Summation may be:
1- Spatial:
Simultaneous at same time
2- Temporal:
Single rapid-fire
What is the Function of Dendrites?
Receive stimuli through activation of ligand-gated or mechanically-gated ion channels
In Sensory Neurons:
Produce generator or receptor potentials
In Motor Neurons and Interneurons:
Produce IPSP or EPSP
What is the Function of Cell Body?
Receive stimuli and produce IPSP or EPSP through activation of ligand-gated ion channels
What is the Function of Junction of Axion Hillock and Initial Segment of Axon?
Trigger Zone in many Neurons
Integrates EPSP and IPSP
If sum is Depolarization that reaches threshold, initiates AP (Nerve Impulse)
What is the Function of Axon?
Propagate nerve impulses from Initial Segment (or from Dendrites of Sensory Neurons) to Axon Terminals in self-regenerating manner
Impulse amplitude does not change as it propagates along Axon
What is the Function of Axon Terminals and Synaptic End Bulbs (Varicosities)?
Inflow of Ca2+ caused by Depolarization phase of nerve impulse triggers exocytosis of neurotransmitter from synaptic vesicles
What are Neurotransmitters?
Small molecule neurotransmitters:
1- Acetylcholine (ACh)
2- Amino Acids
(Includes Glutamate, Aspartate, GABA, Glycine)
3- Biogenic Amines
(Includes Norepinephrine, Epinephrine, Dopamine, Serotonin)
4- ATP and other Purines
5- Nitric Oxide
6- Carbon Monoxide (200x more affinity to Hemoglobin than O2)
What are Neuropeptides?
Neuropeptides:
1- Substance P
(Pain sensation)
2- Enkephalins
(Pain inhibitors)
3- Endorphins
(Pain inhibitors)
4- Dynorphins
5- Hypothalamic Releasing and Inhibiting Hormones
5- Angiotensin II
(Restore blood pressure if low, or high blood pressure if normal)
6- Cholecystokinin
(Secreted by cells lining Duodenum and stops gastric phase of digestion)
What is Substance P?
Found in sensory neurons, spinal cord pathways, and parts of brain associated with pain
Enhances perception of pain
What is Enkephalins?
Inhibit pain impulses by suppressing release of Substance P
May have role in memory and learning, control of body temperature, sexual activity, and mental illness
What is Endorphins?
Inhibit pain by blocking release of Substance P
May have role in memory and learning, control of body temperature, sexual activity, and mental illness
What is Dynorphins?
May be related to controlling pain and registering emotions
What is Hypothalamic Releasing and Inhibiting Hormones?
Produced by Hypothalamus
Regulate release of hormones by anterior pituitary
What is Angiotensin II?
Stimulates thirst
May regulate BP in brain
As a hormone, causes Vasoconstriction
Promote release of Aldosterone which increases rate of salt and water reabsorption by kidneys
What is Cholecystokinin (CCK)?
Found in brain and small intestine
May regulate feeding as a “stop eating” signal
As hormone, regulate pancreatic enzyme secretion during digestion and contraction of smooth muscle in GI Tract
What is Neuropeptide Y?
Stimulates food intake
May play role in stress response
What are Neural Circuits?
A neural circuit is a functional group of neurons that process specific types of information
Types of neural circuits:
1- Simple Series
2- Diverging
3- Converging
4- Reverberating
5- Parallel After-Discharge
What is Diverging vs Converging Neural Circuit?
Diverging:
Input starts from 1 neuron, goes to many neurons, then becomes output
Converging:
Input starts from many neurons, goes to one neuron, then becomes output
What is Reverberating vs Parallel After-Discharge Neural Circuit?
Reverberating:
Input starts from neuron
Goes to next neuron
Goes to next neuron and becomes 2 outputs
One output goes to effector
The other output goes to neuron as input and gets back in loop
Parallel After-Discharge:
Input goes from top neurons to next line of neurons in parallel fashion
Becomes output at end when parallel lines converge
What is Regeneration and Repair of Nervous Tissue?
Although the Nervous System exhibits plasticity, neurons have a limited ability to regenerate themselves
Plasticity:
The capability to change based on experience
Regenerate:
The capability to replicate or repair
What is Neurogenesis in the CNS?
In CNS, little to no repair due to:
Inhibitory influences from neuroglia, particularly Oligodendrocytes
Absence of growth-stimulating cues that were present during fetal development
Rapid formation of scar tissue
(No stem cells)
What is Damage and Repair in the CNS?
In PNS, repair is possible if cell body is intact, Schwann Cells are functional, and scar tissue formation does not occur too rapidly
Steps involved in repair process are:
1- Chromatolysis
2- Wallerian Degeneration
3- Formation of a Regenerative Tube
What are some Nervous System Disorders?
Multiple Sclerosis
Epilepsy
Excitotoxicity
Depression
