(Lectures 4-6, Chapters 7 & 10) Nervous System, Neuron, CNS, ANS Flashcards
Synapse
Site of communication between a neuron and another part of the body (neuron, muscle, gland, etc.)
Leak channels
Allow Na+ and K+ to flow into/out of (respectively) the neuron
Dendrites
Extensions from cell body that receive info from other neurons
Axon hillock
Area where action potentials are initiated, right below the cell body
Axon
Transmits action potentials
Cell body
Contains nucleus and most organelles
Anterograde vs retrograde transport
Anterograde = movement away from cell body (e.g. mitochondria, enzymes)
Retrograde = towards cell body (e.g. degrading organelles, viruses, bacterial toxins)
Central nervous system
Brain + spinal cord
Peripheral nervous system
Cranial/spinal nerves & sensory receptors in the skin
3 functions of the nervous system
- Sensory (sensing stimuli)
- Integrative (CNS decides on response based on sensory info)
- Motor (movement is conveyed from CNS to effectors via PNS)
2 types of cells in the nervous system
- Neurons (specialized, excitable)
- Neuroglia (support neurons)
Types of neuroglia in the CNS
- Ependymal cells (barrier between central spinal fluid and tissue fluid w/CNS cells)
- Oligodendrocytes (form/maintain CNS myelin)
- Astrocytes (support, control surrounding chemical environment)
- Microglia (protect neurons from pathogens/debris, stabilize injured neurons)
Types of neuroglia in the PNS
- Satellite cells (support neurons)
- Schwann cells (generate myelin)
Synaptic end bulb
Contains synaptic vesicles that release neurotransmitters
Presynaptic vs postsynaptic neuron
- Presynaptic neuron fires synapses, releases neurotransmitters
- Postsynaptic neuron receives signal (i.e. neurotransmitters)
How are synapses named? + examples
Named by the cells/tissues that they connect (e.g. neuronal = neuron-neuron, neuromuscular = neuron-muscle)
Function of myelin
Protect/insulate axon
What cells produce myelin?
Schwann cells (PNS), oligodendrocytes (CNS)
Is myelination continuous along an axon?
No; gaps are nodes of Ranvier
T/F: all neurons have myelinated axons
False
Plasticity vs repair
Plasticity = ability to adapt/repair over a lifetime Repair = regeneration after being damaged
Regeneration in the PNS
- Occurs if cell body is intact & Schwann cell is active
- Damaged myelin is removed
- Schwann cells use substances from the cell body to repair the axon
T/F: damage to the CNS is permanent
True
T/F: neuron repair takes a long time to finish
True - starts quickly, but takes weeks/months for the neuron to function again
How do neurons communicate?
Changes in membrane potential
Concentrations of ions (neurons)
- High [Na+] and [Cl-] in the ECF
- High [K+] and [Pr-] in the cytosol
What is the resting membrane potential of a neuron?
-70mV
What would happen if Na+ and K+ only moved into/out of the neuron via leak channels (at rest)? Why?
Resting membrane potential would increase because there are more leak channels for K+ in the membrane
How is the resting membrane potential maintained?
Sodium-potassium pump; ejects 3 Na+, intakes 2 K+
T/F: no ions move across a neuron’s membrane at rest
False
Membrane potentials of Na+ and K+
Na+: 60mV
K+: -90mV
Why is the electrochemical force for Na+ greater than that of K+?
Membrane potential of Na+ is farther from resting membrane potential than that of K+
Factors in determining resting membrane potential (3):
- Unequal ion distribution in ECF + cytosol
- Differences in permeability of membrane for different ions
- Action of Na+/K+ ATPase
All but two ions have non-negligible effects on the membrane potential of neurons:
Na+, K+
What are gated channels?
Channels that open/close in response to stimuli, changing the rate of ion movement across the membrane by changing the membrane’s permeability
3 types of gated channels
- Voltage-gated (respond to voltage)
- Chemically-gated (respond to chemicals/ligands)
- Mechanically-gated (respond to mechanical forces)
How do voltage-gated potassium channels work?
- Channel closes = increase in membrane potential (from -70 to +30), K+ can’t leave
- Channel opens = decrease in membrane potential (from +30 to -90), K+ leaves cell
How do voltage-gated sodium channels work?
- At rest, the activation gate is closed and the inactivation gate is open
- At depolarization, both gates are open, so Na+ can enter the cell
- Right after depolarization, the inactivation gate closes
Where are chemically-gated channels found?
Dendrites, cell body
Where are mechanically-gated channels found?
Sensory receptors
Signals are either _____or ______.
Excitatory, inhibitory
Graded potential vs action potential
- GP: small, used to communicate over short distances (in the cell body)
- AP: large, used to communicate over long distances (between neurons)
- A significant enough change in the membrane potential will initiate an action potential at the axon hillock
Temporal summation + spatial summation
Graded potentials can sum together over time (same stimulus repeated over small time intervals) and area (different stimuli at the same time)
What is the threshold membrane potential (i.e. required to start an action potential)
-55mV
Is an action potential triggered for every stimulus we receive?
No
All-or-None Principle
Once a graded potential reaches threshold, an action potential occurs. The size of the stimulus doesn’t have an impact beyond what’s needed for an action potential (i.e. a threshold stimulus and a superthreshold stimulus result in the same change in membrane potential)
3 steps in generation of action potential
1) Depolarization (occurs from rest until +30mV)
2) Repolarization (membrane potential drops after action potential, towards K+ membrane potential)
3) After Hyperpolarization (membrane potential returns to -70mV)
Ion flow in action potentials
1) Rest: Na+ and K+ move across the membrane via leak channels, Na+/K+ ATPase pump
2) Depolarization: Na+ flows in
3) Repolarization: K+ flows out
4) After Hyperpolarization: resting membrane potential is restored by K+ outflow
Absolute Refractory Period
Membrane can’t respond to any stimulus, regardless of strength
Relative Refractory Period
Membrane can only respond to (and initiate an action potential due to) a stronger-than-normal stimulus
Continuous Conduction
Action potential propagates along the entire length of an unmyelinated axon
Saltatory Conduction
Action potential travels along a myelinated axon by “jumping” between nodes of Ranvier
Between continuous and saltatory conduction, which is faster?
Saltatory (by a lot; 0.5-10m/s vs 150m/s)
Electrical Synapses
- Action potentials are conducted between adjacent cells via gap junctions
- Connexons in gap junctions function as tunnels connecting the cells’ cytosol
- Allow for fast communication and synchronization of a group of neurons or muscle fibers
On a myelinated axon, where do ions interact with open channels?
Nodes of Ranvier
Chemical Synapses
- Involve release of a neurotransmitter into the synaptic cleft, which separates two neurons
- Postsynaptic potential occurs in response to the chemical signal - action potentials can’t actually cross a synaptic cleft
Where are synaptic vesicles located before releasing neurotransmitters?
In the cellular matrix; cytoskeleton of synaptic bulb
How is the release of neurotransmitters triggered?
- Voltage-gated Ca2+ channels open when the action potential reaches the synaptic bulb
- Flow of Ca2+ into the bulb triggers exocytosis of synaptic vesicles
- ## Neurotransmitters are released into the synaptic cleft + bind to receptors on the postsynaptic neuron (e.g. chemically-gated channels)
Synaptic cleft
Space between neurons
Inhibitory Post-Synaptic Potential (IPSP)
Neurotransmitter release causes a hyperpolarization of the post-synaptic membrane
Excitatory Post-Synaptic Potential (EPSP)
Neurotransmitter release causes a depolarization of the post-synaptic membrane
How does an action potential occur at the trigger zone of a postsynaptic neuron? (hint: IPSP and EPSP)
The net summation of IPSPs + EPSPs is a depolarization that reaches threshold
Characteristics of neurotransmitters (4)
- Synthesized in neurons
- Released following depolarization
- Bind to postsynaptic receptor
- Inactivated after binding
3 ways in which a signal is terminated, by removing the neurotransmitter
- Diffusion away from the synaptic cleft
- Enzymatic degradation
- Uptake by other cells (either neuron that released them, or adjacent neuroglia)
Autonomic Nervous System
Regulates the activity of visceral effectors (smooth/ cardiac muscle, glands)
3 branches of the ANS
- Sympathetic (fight/flight)
- Parasympathetic (rest/digest)
- Enteric (tissue in GI tract)
Characteristics of autonomic motor pathway
- Preganglionic neuron emerging from spinal cord
- Postganglionic neuron emerging towards tissues
- Ganglia (cell bodies around synapse)
- ACh as the primary neurotransmitter
T/F: In the sympathetic nervous system, the preganglionic neurons are longer than the postganglionic neurons
False
Innervation of organs (by nerves of ANS)
- Multiple sympathetic postganglionic nerves innervate one organ
- One parasympathetic postganglionic neuron innervates one organ
Adrenal medulla
- Sympathetic pre-neurons extend to chromaffin cells in the adrenal medulla, which release NE and epinepherine into the bloodstream
Where does an autonomic postganglionic neuron communicate with visceral effectors? Where are the neurotransmitters released?
Neuroeffector Junction; varicosities
Cholinergic neurotransmitters/receptors
- Receptors bind ACh
- Neurotransmitters are first released from pre-neuron in both the sympathetic + parasympathetic nervous systems
- Neurotransmitters are then released by post-neurons in the parasympathetic nervous system
Adrenergic neurotransmitters/receptors
- Norepinepherine/epinepherine
- First released by post-neurons in the sympathetic nervous system, then by chromaffin cells in the adrenal medulla
Autonomic tone
Balance between sympathetic and parasympathetic activity
What part of the brain regulates autonomic tone?
Hypothalamus
Why is autonomic tone important?
Do visceral tissues always receive signals from both branches?
- Ensures that visceral tissue function is stable at rest
- Allows for quick responses from visceral tissue when needed
- Visceral tissues always receive signals from both branches; one intensifies signals, the other attenuates them
Functions of the parasympathetic branch
- Salivation
- Lacrimation (tearing up)
- Urination
- Digestion
- Defecation
Functions of the sympathetic branch
- Support vigorous physical activity + rapid ATP production
- Diffuse effects (spread out over several organs)
- Excitement, emergency, exercise, embarrassment
Which branch of the ANS has longer-lasting effects?
Sympathetic
Somatic nervous system
Regulates activity of skeletal muscle - allows for voluntary contractions
Neuromuscular junction
Site where a somatic nerve interacts with a skeletal muscle fiber
How does signal transmission differ at the neuromuscular junction (compared to a neuronal synapse)?
- Depolarization of motor end plate causes an end plate potential, which is larger in amplitude than an EPSP (so it can depolarize a muscle fiber to threshold)
- Muscle action potential initiated by Na+ inflow via voltage-gated channels
- Muscle action potential propagates in 2 directions away from the NMJ, triggering events to cause muscle contraction
- Breakdown of ACh into acetyl and choline by AChE; neither can activate the ACh receptor
Motor end plate
- Region of muscle fiber opposite to the synaptic end bulb
- Contains 30-40M ACh receptors
T/F: tissues innervated by the ANS will no longer function if their nerve supply is cut
False
For the sympathetic and parasympathetic nervous systems, where are the cell bodies of their preganglionic neurons found?
Sympathetic: thoracic/upper lumbar regions of the spinal cord
Parasympathetic: brain stem/sacral region of the spinal cord
Ionotropic receptors
Postsynaptic receptor that allows ion flux to change cell voltage; type of ligand-gated channel. Response is rapid + short-lasting
Metabotropic receptors
Postsynaptic receptor that acts as a secondary messenger to produce effects in the cell; coupled to a G protein that causes the opening/closing of ion channels. Response is slow and long-lasting
Cholinergic neurons in the ANS
- Release ACh
- All preganglionic neurons
- Most parasympathetic postganglionic neurons
- Sympathetic postganglionic neurons that innervate sweat glands
Adrenergic neurons in the ANS
- Release NE, epinepherine
- Sympathetic postganglionic neurons (except those that innervate sweat glands)
T/F: there are no nodes of Ranvier in the CNS
False
