Chapter 12 Flashcards
Nervous System
Definition of nervous Tissue:
- Tissue specialized for rapid transmission of signals from cell to
cell. - Works along with the Endocrine System to coordinate homeostasis
and body activities.
Organization of the Nervous System:
1.Central Nervous System (CNS)
2Peripheral Nervous System
3. Enteric Nervous System
Central Nervous System (CNS) is composed of what two things
- Brain
- Spinal Cord
Peripheral Nervous System is composed of what three things
- Somatic Nervous System
- Autonomic Nervous System
a. Sympathetic Nervous System
b. Parasympathetic Nervous System - Enteric Nervous System
Neurogenesis:
- the creation of new neurons from undifferentiated stem cells
Axon Repair in the Peripheral Nervous System
- occurs in myelinated axons where the neurolemma in intact
Early effects of regeneration of nervous tissue
- The cell body swells
- Wallerian Degeneration occurs - the portion to the axon distal to
the injury degenerates - the proximal portion of the axon degenerates to the nearest Node
of Ranvier - Nissl bodies become granular in appearance
Later effects of regeneration
- Schwann cells undergo mitosis to form a Regeneration tube across
the injury site - the axon is gradually reconstructed with in the sheath that is
left behind at a rate of 1 to 5 mm/day - scar tissue in the “tube” of to wide of a gap will stop
regeneration - function is restored_
Neuropeptides
- neurotransmitter 3 to 40 bonded amino acids
- can be excitatory or inhibitory
- often associated with controlling pain and modifying
neuronal activity
ex. Endorphins, Enkephalins, Substance P
The role of Nitric Oxide (NO)
- thought to play a role in memory and learning
- is not produce in advance
Biogenic Amines
- modified amino acids
a. Dopamine
b. Serotonin
c. Epinephrine, Norepinephrine
Dopamine
- regulates skeletal muscle tone, movement, emotional
responses
ex. Parkinson Disease - associated with the progressive degeneration of
Dopamine releasing neurons in the Brain - patient develops tremors, slow and uncoordinated
movements
Serotonin
Serotonin
- associated with sensory perception, mood control,
temperature regulation, appetite, sleep induction
ex. Selective Serotonin Reuptake Inhibitor (SSRI)
- Prozac - allows serotonin to remain in the synaptic
cleft longer
Amino Acids
- Glutamate and Aspartate are excitatory
- Gamma Aminobutyric Acid (GABA) and Glycine are inhibitory
What is a Neurotransmitter and the four types?
chemicals essential for transmission of across the synaptic cleft
1.Acetylcholine
2. Amino Acids
3. Biogenic Amines
4. 4. Nitric Oxide (NO)
Small Molecule Neurotransmitter
Acetylcholine
- can be excitatory or inhibitory
- deactivated by Acetylcholinesterase
ex. Alzheimer Disease
- progressive loss of the ability to reason and care for one-
self
- associated with high mortality
- caused by degeneration of ACh releasing neurons in the
Brain
Summation
- occurs at the axon hillock
- integration of the EPSP’s & IPSP’s until a threshold stimulus reached (this will rapidly dissipate)
Spatial Summation
- the build-up of graded potentials caused by several synaptic end
bulbs releasing neurotransmitter
Temporal Summation
- the build-up of graded potentials caused by only one synaptic end
bulb releasing neurotransmitter in rapid succession
Deactivation/Removal of Neurotransmitter from the Synaptic Cleft
this prevents continuous stimulation of Post-synaptic Neurons
(End Organs)
1. Diffusion from the synaptic cleft
2. Enzymatic degradation
- ex. Acetylcholinesterase deactivating Acetylcholine
3. Uptake by cells
- reuptake by the secreting neuron or a glial cell
D. Summation:
- occurs at the axon hillock
- integration of the EPSP’s & IPSP’s until a threshold stimulus is
reached (this will rapidly dissipate)
1. Spatial Summation
- the build-up of graded potentials caused by several synaptic end
bulbs releasing neurotransmitter
2. Temporal Summation
- the build-up of graded potentials caused by only one synaptic end
bulb releasing neurotransmitter in rapid succession
V. Neurotransmitter:
- chemicals essential for transmission of across the synaptic cleft
A. Small Molecule Neurotransmitter
1. Acetylcholine
- can be excitatory or inhibitory
- deactivated by Acetylcholinesterase
ex. Alzheimer Disease
- progressive loss of the ability to reason and care for one-
self
- associated with high mortality
- caused by degeneration of ACh releasing neurons in the
Brain
2. Amino Acids
- Glutamate and Aspartate are excitatory
- Gamma Aminobutyric Acid (GABA) and Glycine are inhibitory
The Postsynaptic Neuron generates a graded potential that is
transmitted to the Axon Hillock-
2 primary effects on the Postsynaptic Neuron:
a. Excitatory Postsynaptic Potential (EPSP)
= depolarization (positive ions move into the Dendrite)
- this adds to the threshold stimulus (i.e. makes it more
positive)
b. Inhibitory Postsynaptic Potential (IPSP)
= hyperpolarization (negative ions flow in or positive ions
flow out of the Dendrite)
- a stimulus that takes away from the threshold stimulus
(i.e. makes it more negative)
The neurotransmitter diffuses across the synaptic cleft and
interacts with a specific receptor site on the Post-synaptic Neuron
- one way transfer of the signal
- the neurotransmitter must be deactivated shortly after
Chemical Synapse
A nerve impulse stimulates a vesicle (located in the axon terminus)
to release neurotransmitter into the synaptic cleft (i.e. the space
between the Presynaptic and Postsynaptic Neurons)
Electrical Synapse
- ionic current spreads through Gap Junctions in adjacent cells
- are faster than chemical synapses
- ex. Cardiac muscle
Signal Transmission at Synapses the two types
A. Electrical Synapse
- ionic current spreads through Gap Junctions in adjacent cells
- are faster than chemical synapses
- ex. Cardiac muscle
B. Chemical Synapse
1. A nerve impulse stimulates a vesicle (located in the axon terminus)
to release neurotransmitter into the synaptic cleft (i.e. the space
between the Presynaptic and Postsynaptic Neurons)
The effect of axon diameter.
- The rate of nerve impulse transmission is determined by the
diameter of the axon and the presence (or absence) of myelin
Fiber Types:
- Fiber Types:
a. A Fiber: - myelinated
- largest diameter axon (5 - 20 micrometers)
- short absolute refractory period
- fastest nerve impulse transmission (27 - 280 mph)
b. B Fiber: - myelinated
- axon diameter is smaller than A fibers (2 - 3micrometers)
- absolute refractory period is longer than A fiber
- slower impulse transmission than A fiber (32 mph)
c. C Fiber: - unmyelinated
- smallest axon diameter (0.5 - 1.5 micrometers)
- longest absolute refractory period
- slowest impulse transmission (1 - 4 mph
Saltatory Conduction
the Action Potential is moved along the myelinated portion of the
axolemma, passing from one Node of Ranvier to the next until the
impulse reaches the synapse
(the signal is passed along the myelinated regions via anionic
current)
Continuous Conduction
- Propagation of a nerve impulse in a step by step depolarization
and repolarization of adjacent areas of the axolemma - associated with non-myelinated neurons
Propagation of Nerve Impulses
- Propagation = Conduction
- as sodium rushes inside the axolemma, adjacent Voltage Gated
Channels are activated (achieve Threshold Stimulus); the process
of activating adjacent channels continues down the axon towards the
synapse
Relative Refractory Period
During hyperpolarization of the axon membrane, a larger than
threshold stimulus is needed to activate an action potential
Absolute Refractory Period
The Voltage Gated Sodium Channels need to reach a resting state
before they can be reactivated.
= The period of time from activation of Voltage Gated Sodium
Channels until they reach a resting state.
Refractory Period
The period of time in which an excitatory cell cannot generate
another Action Potential
Events Associated With An Action Potential: 1
- Threshold Stimulus is reached (-55mV)
Events Associated With An Action Potential: 2
- Voltage Gated Sodium Channels open initiating Depolarization
- sodium rushes inside the axolemma
- the membrane voltage goes to 0 mV, then to approx. +30 mV(more
positive inside the membrane)
- Voltage Gated Sodium Channels close at +30 mV
Events Associated With An Action Potential: 3
- Repolarization begins
- with the Voltage Gated Sodium Channels closed, the action of the
Sodium-Potassium ATPase Pumps begin to restore the proper ion
concentrations along the axolemma
- the Voltage Gated Potassium Channels are now fully opened (they
respond slowly to the Threshold Stimulus)
- the voltage across the axolemma becomes more negative (back to 0
mV, then to -90 mV
Events Associated With An Action Potential: 4
- Hyperpolarization occurs
- Hyperpolarization = membrane voltage is below RMP (-70 mV)
- this occurs because the Voltage Gated Potassium Channels are slow
to close (positive charges are still leaking out of the axon)
All-or-none principle
- the initial stimulus must reach threshold before an action
potential is generated (there is no partial activation of an action
potential) - this initial electrical stimulus will quickly dissipate if the
threshold stimulus is not achieved
Threshold Stimulus
any electrical stimulus that is strong enough to initiate an
action potential
Action Potential
= Depolarization + Repolarization
- the electrical signal (impulse) that is propagated down the axon
Repolarization
- recovery of the RMP
Depolarization
- voltage gated Na+ Channels open in response to a stimulus, the
membrane voltage becomes more positive as the positive ions rush
inside the membrane - depolarization originates at the Axon Hillock
Sodium/Potassium ATPase Pumps
- located in axolemma
- takes care of ion (i.e. Na+ and K+) leakage across the axon
membrane
Voltage Gated Channels
- located in the axolemma
- open and close in response to changes in voltage
- ex. Voltage Gated Na+ Channels
Voltage Gated K+ Channels
Resting Membrane Potential (RMP) -
the electrical potential across the
axon membrane, this is produced by the different ions located on each
side of the membrane
- outside is positive, inside is negative
- extracellular area has more Sodium and Chloride ions
- intracellular area has more potassium ions, phosphates, and amino
acids
- RMP is approximately equal to -70 mV (i.e. more negative
intracellularly)
Neuroglia = Glia = Glial cells
- cells that support the function of the Nervous System
- types:
1. Astrocytes - located in the CNS
- provide nutritional support and the proper chemical environment
to Neurons
2. Oligodendrocytes - in CNS
- produce myelin sheaths
3. Microglia - in CNS
- mobile macrophage
4. Ependymal cells - in CNS
- line the ventricles of the Brain
- produce Cerebral Spinal Fluid (CSF)
5. Schwann cell - in PNS
- produce myelin sheaths
- Neurolemma = the outermost layer of the cell, contains the
nucleus
6. Satellite cells - in PNS
- nutritional support to Neurons
