Ch. 7 The Nervous System: Neurons and Synapses Flashcards
What is included in the Central Nervous System (CNS)
Brain and spinal cord
What is special about the CNS?
It is encased in the spine and skull
What is included in the Peripheral Nervous System (PNS)
Cranial and spinal nerves
Define Neurons
Conduct impulses but generally cannot divide
–but can repair if severed
Define Glial Cells (neuroglia)
Support the neurons and cannot conduct impulses, but can divide
–work together
Neurons
Respond to chemical and physical stimuli
–pain, pressure, heat…
Conduct electrochemical impulses (action potential)
Release chemical regulators (at synapse)
Neurons enable perception of?
- Sensory stimuli
- Learning
- Memory
- Control of muscles (voluntary and involuntary) and glands
Can neurons divide? Can neurons repair?
Most neurons cannot divide, but can repair
What is included in the structure of a neuron?
Cell body
Dendrites
Axons
Neuron Structure: Cell Body
Contains the nucleus and other organelles; cluster in groups
Clustered groups of cell bodies in the CNS are called ____.
Nuclei
Clustered groups of cell bodies in the PNS are called ____.
Ganglia
Neuron Structure: Dendrites
Receive impulses and conducts a graded impulse toward the cell body
Shorter projections than axon; project off cell body
Neuron Structure: Axon
Conducts action potentials away from the cell body
Axon is a long projection (can be up to 1 meter long)
Neuron Structure: Axon Hillock
Area where axon meets cell body; action potentials generate here then propagate out
What is Axonal Transport?
An active process needed to move organelles and proteins from the cell body to axon terminals
Require energy (b/c it’s an active process)
Axonal Transport: Fast
Component moves vesicles (neurotransmitters)
In vesicles, so it’s exocytosis bulk transport
Axonal Transport: Slow
Components move microfilaments, microtubules, and proteins
aka Cytoskeleton
Axonal Transport: Anterograde Transport
From cell body to dendrites and axon
Cell body –> away
Axonal Transport: Retrograde Transport
From dendrites and axons to cell body
The functional classification of neurons is based on?
The direction impulses are conducted
Sensory Neurons
Conduct impulses from sensory receptors to CNS
Motor Neurons
Conduct impulses from CNS to target organs (muscles or glands)
2 branches:
Somatic motor neurons
Autonomic motor neurons
Somatic Motor Neurons
Responsible for reflexes (which are involuntary) and voluntary control of skeletal muscles
Autonomic Motor Neurons
Innervate voluntary targets such as smooth muscle, cardiac muscle, and glands
Subdivided into Sympathetic and Parasympathetic
Autonomic Motor Neurons: Sympathetic
Emergency situations; “fight or flight”
Autonomic Motor Neurons: Parasympathetic
Normal functions; “rest and digest”
What are nerves?
Bundles of axons located outside the CNS
Nerves that are compose of sensory and motor neurons are known as?
Mixed nerves
Are most nerves mixed nerves?
Yes
Some of the cranial nerves have ____ fibers only.
Sensory
A bundle of axons in the CNS is called a ____.
Tract
What are neuroglia (glial cells)?
Cells that are non-conducting but support neurons
What are the 2 types of neuroglia in the PNS?
- Schwann cells (neurolemmocytes)
2. Satellite cells (ganglionic gliocytes)
What are the 4 types of neuroglia in the CNS?
- Oligodendrocytes
- Microglia
- Astrocytes
- Ependymal cells
Neuroglia: Schwann Cells
aka Neurolemmocytes
in PNS
Form myelin sheaths around peripheral axons
Neuroglia: Satellite Cells
aka Ganglionic Gliocytes
in PNS
Support cell bodies w/in the ganglia of the PNS
–secrete growth factors, etc.
Neuroglia: Oligodendrocytes
in CNS
Form myelin sheaths around the axons of CNS neurons
analogous to Schwann cells
Neuroglia: Microglia
in CNS
Migrate around CNS tissue and phagocytize foreign and degenerated material
Mobile, move around
Neuroglia: Astrocytes
in CNS
Regulate external environment of neurons
Play important role in BBB, regulate tight junction w/in endothelial cells, associate w/ endothelia cells too
Neuroglia: Ependymal Cells
in CNS
Line ventricles and secrete cerebrospinal fluid (CSF)
Blood Brain Barrier (BBB)
Capillaries in brain do not have pores between adjacent cells but are joined by tight junctions
Substances can only be moved by very selective processes of diffusion through endothelial cells, active transport, and bulk transport
Movement is transcellular not paracellular
Astrocytes regulate formation of BBB and support cell bodies
Neurons have a resting potential of ____.
-70mV
Resting Membrane Potential
Established by large negative molecules in side cell
Na+/K+ pumps
Permeability of membrane to positively charged, inorganic ions
At rest, there is a ____ concentration of K+ inside the cell and a ____ concentration of Na+ outside the cell.
High; High
The resting membrane potential is close to the equilibrium of ____.
K+
Membrane Potential: Depolarization
Occurs when positive ions enter the cell (usually Na+)
Membrane potential becomes less negative
Membrane Potential: Hyperpolarization
Occurs when positive ions leave the cell (usually K+) or negative ions (Cl-) enter the cell
Membrane potential becomes more negative
Depolarization of the cell is ____.
Excitatory
Hyperpolarization of the cell is ____.
Inhibitory
K+ Leakage Channels
Not gated (always open)
Increase permeability to K+
Voltage-Gated K+ Channels
Open when a particular membrane potential is reached; closed at resting potential
Na+ voltage-gated channels are closed at rest; the membrane is ____ permeable to Na+ at rest.
Less
Voltage-Gated Na+ Channels
Open if membrane potential depolarizes to -55mV (threshold)
Sodium rushes in due to electrochemical gradient
Membrane potential climbs toward sodium equilibrium potential
These channels are deactivated at +30mV.
If Voltage-Gated Na+ channels open, Na+ moves in…which is what direction on the concentration gradient?
Down the concentration gradient
Refractory Period
Channel is inactivated, but not closed all the way
Voltage-Gated K+ Channels
At around +30mV, voltage-gated K+ channels open, and K+ rushes out of the cell following the electrochemical gradient.
Makes the cell depolarize back toward the potassium equilibrium potential
What is the threshold membrane potential?
-55mV
Action Potentials: Postive Feedback Loop
At threshold membrane potential (-55mV), voltage-gated Na+ channels open, and Na+ rushes in. As cell depolarizes, more Na+ channels are open, and the cell becomes more and more permeable to Na+
Known as a positive feedback loop
Causes an overshoot of membrane potential
–membrane potential reaches +30mV
Action Potentials: Negative Feedback Loop
At +30mV, Na+ channels close, and K+ channels open
Results in repolarization of membrane potential
Known as negative feedback loop
Once the threshold has been reached, will an action potential happen?
Yes
Will the size of the stimulus affect the size of the action potential?
No, it will always reach +30mV
It may recruit more neurons though
Will the size of the stimulus affect action potential duration?
No, but it will make action potentials occur more frequently
Refractory Period
Action potentials can only increase in frequency to a certain point. There is a Refractory Period after an action potential when the neuron cannot become excited again.
Absolute Refractory Period
Occurs during the action potential. Na+ channels are inactive (not just closed)
Relative Refractory Period
When K+ channels are still open. Only a very strong stimulus can overcome this.
Still in hyperpolarization stage.
T/F: Each action potential remains a separate, all-or-none event.
True
Why is the refractory period important?
B/c we have to reset the resting membrane potential in order to fire another action potential
Conduction of Nerve Impulses
When an action potential occurs at a given point on a neuron membrane, voltage-gated Na+ channels open as a wave down the length of the axon.
The action potential at one location serves as the depolarization stimulus for the next region of the axon.
Refractory period is good b/c it keeps depolarization moving down the axon.
What are the 2 types of conduction?
- Unmyelinated
2. Myelinated
Conduction: Unmyelinated
Axon potentials are produced down the entire length of the axon at every patch of membrane.
Conduction rate is slow b/c so many action potentials are generated and each one is an individual event.
Amplitude of each action potential is the same - conducted w/o decrement.
Is Unmyelinated Conduction organized or unorganized?
Unorganized and chaotic
Conduction: Myelinated
Myelin provides insulation, improving the speed of cable properties.
Nodes of Ranvier allow Na+ and K+ to cross the membrane every 1-2mm.
Na+ ion channels are concentrated at the nodes.
Action potentials “leap” from node to node.
This is called Saltatory Conduction.
What are the Nodes of Ranvier?
Spaces between myelination.
Is Myelinated Conduction organized or unorganized?
Organized
How is action potential conduction speed increased (2 things)?
- Increasing diameter of neuron
- -reduces resistance to spread of charges via cable properties (ability to conduct through cytoplasm, which is poor due to high resistance and leak of ions ‘out’ through membrane) - Myelination b/c of saltatory conduction
Discuss the neuron speed of a thin, unmyelinated cell vs. a thick, myelinated cell.
Thin, unmyelinated neuron speed is 1.0m/sec
Thick, myelinated neuron speed is 100m/sec
Action potential can be describe as?
All or none
What is a synapse?
Functional connection between a neuron and the cell it is signaling
Synapse in the CNS
In the CNS, the second cell is another NEURON
Synapse in the PNS
In the PNS, the second cell will be in a MUSCLE or GLAND; often called myoneural or neuromuscular junctions.
Presynaptic Neuron
The first neuron when one neuron is signaling another
Can signal the dendrite, axon, or cell body of a second neuron
Postsynaptic Neuron
The second neuron when one neuron is signaling another
Synapses can be ____ or ____.
Electrical; Chemical
Electrical Synapses
Occur in smooth muscle and cardiac muscle, between some neurons of the brain, and between glial cells
Electrical Synapses: Glial Cells
Remember, glial cells do NOT conduct impulses (and glial cells are support cells)!
Yet the neuron will send information to the glial cell (via electrical synapses) to tell the glial cell about the support needed so it can do its job.
In electrical synapses, the cells are joined by ____ ____.
Gap junctions
Stimulation of electrical synapses causes phosphorylation or dephosphorylation of ____ ____ to open or close the channels.
Connexin proteins
Chemical Synapses
Most synapses involve the release of a chemical called a neurotransmitter from axon terminal.
The synaptic cleft is very small, released neurotransmitter can readily diffuse across this space
Is the space w/in a chemical synapse tightly joined?
No, it is not tightly joined (this allows for neurotransmitters to easily diffuse)
Where is the neurotransmitter enclosed?
In synaptic vesicles in the axon terminal
How are neurotransmitters released?
When the action potential reaches the end of the axon, voltage-gated Ca2+ channels open.
Ca2+ stimulates the fusing of synaptic vesicles to the plasma membrane and exocytosis of the neurotransmitter.
Discuss Ca2+ concentrations inside and outside the cell.
Low Ca2+ inside cell
High Ca2+ outside cell
____ is the signal for neurotransmitter release.
Calcium (Ca2+)
Actions of the Neurotransmitter
Neurotransmitter diffuses across synapse, where it binds to a specific receptor protein
- -neurotransmitter is referred to as the ligand
- -results in the opening of chemically regulated ion channels (aka ligand-gated ion channels)
Acetylcholine (ACh)
ACh is a neurotransmitter that directly opens ion channel when it binds to its receptor.
Excitatory ACh
In some areas of CNS, in some autonomic motor neurons, and in all somatic motor neurons
Inhibitory ACh
In some autonomic motor neurons
Nicotinic ACh receptors
Can be stimulated by nicotine
Found on the motor end plate of skeletal muscle cells (neuromuscular junction), in autonomic ganglia, and in some parts of CNS
Muscarinic ACh receptors
Can be stimulated by muscarine (from poisonous mushrooms)
Found in CNS and plasma membrane of smooth and cardiac muscles and glands innervated by autonomic motor neurons
The ANS (autonomic nervous system) uses which 2 receptors?
Nicotinic ACh receptors and Muscarinic ACh receptors
Agonists
Drugs that stimulate a receptor
–ex: ACh
Mimics natural ligand for that receptor
Antagonists
Drugs that inhibit a receptor
–ex: Beta-blockers
Acetylcholinesterase (AChE)
AChE is an enzyme that inactivates ACh activity shortly after it binds to the receptor
–terminates ACh signal
Hydrolyzes ACh into acetate and choline, which are taken back into the presynaptic cell for reuse (will recycle it and make more ACh)
What are Monoamines?
Regulatory molecules derived from AAs
Monoamines: Catecholamines
Derived from tyrosine; include dopamine, norepinephrine, and epinephrine
Monoamines: Serotonin
Derived from L-Tryptophan
Monoamines: Histamine
Derived from histidine
Monoamine Action and Inactivation
Like ACh, monoamines are made in the presynaptic axon, released via exocytosis, diffuse across the synapse, and bind to specific receptors.
They are quickly taken back into presynaptic cell (called reuptake) and degraded by monoamine oxidase (MAO).
