CH 08 - Nervous System Flashcards
What are the two parts of the nervous system?
- Central nervous system (CNS) (brain & spinal cord)
- Peripheral nervous system (PNS)
How can the Efferent neurons further be divided?
- Autonomic division: controls smooth and cardiac muscle, exocrine and some endocrine and adipose (further divided to sympathetic and parasympathetic divisions)
- Somatic division: controls skeletal muscle
What is the enteric nervous system?
network of neurons in the walls of digestive tract, frequently controlled by the autonomic division, but able to function as own integrating center
What is the functional unit of the nervous system?
neurons:
- nerve cell
- uniquely shaped cells with long processes that extend outward from the nerve cell body (either dendrites or axons)
- axons bundled with connective tissue
What is a functional unit?
the smallest structure that can carry out the functions of a system
What is a dendrite?
thin, branched processes that receive and transfer incoming info to an integrating region within the neuron
(recieve signals and have spines)
What is an axon?
- an extension of a neuron
- carries outgoing signal to the target cell
- axon hillock, collaterals, axon terminals, varicocites
How are neurons classified by function?
- sensory neurons
- interneurons of CNS
- Efferent (motor) neurons
Study structural categories of the neuron on figure 8.2
What is axonal transport?
Movement of material between the axon terminal and the cell body
What is anterograde transport?
moves vesicles and mitochondria from the cell body to the axon terminal (type of fast axonal transport)
- aka forward transport
What is retrograde transport?
- returns old cellular components from the axon terminal to the cell body for recycling
- nerve growth factors & some viruses reach cell body by fast retrograde transport as well (either retro or antero grade)
What is the difference between slow and fast axonal transport?
- Slow: moves soluble proteins and cytoskeleton proteins from the cell body to the axon terminal, moves soluble proteins and cytoskeleton proteins (stops ad go — like driving on a street with stoplights)
- Fast: Rapid movement of particles along an axon using microtubules and lines in foot proteins (retrograde of anterograde). It foes in both directions and moves materials at rates up to 400 mm per day (continuous, like driving on an interstate)
What is a synapse?
the region where an axon terminal meets its target cell
What is a chemical synapse?
where the presynaptic cell releases a chemical signal that diffuses across the synaptic cleft and binds to a membrane receptor on the postsynaptic cell
What are electrical synapses?
- in CNS
- allow electrical current and chemical signals to pass between cells through gap junction channels
- communication is bidirectional & faster then at chemical synapses
- allow multiple CNS neurons to coordinate and fire simultaneously
Are synapses fixed for life?
NO! they can be rearranged!
What is a neurotropic factors?
Chemicals secreated by Schwann cells that keep damaged neurons alive
What is a glial cell?
- nonexcitable support cells of the central nervous system
- ## communicate with neurons and provide important biochemical and structural support
What do ependymal cells do?
- create barriers between cavities (line fluid compartments in the CNS) — selectively permeable epithelial layer
- source of neural stem cells
What do astrocytes do?
- source of neural stem cells
- take up K+, water, neurotransmitters
- Secrete neurotrophic factors
- Helps form blood-brain barrier
- Provide substrates for ATP production
What do oligodendrocytes do?
form myelin sheaths in the CNS
What do schwann cells do?
- form myelin sheaths in the PNS
- Neurotropic factors
What do satellite cells do?
Support cell bodies in the PNS
What is a ganglion?
A cluster of nerve cell bodies in the peripheral nervous system (plural = ganglia)
What do microglia do?
- NOT neural tissue
- specialized immune cells in the CNS
- remove damaged cells and foreign invaders
- not always helpful — sometimes release things that cause free radicals
Can stem cells repair damaged neurons?
- if cell body dies, neuron dies
- if axon is severed, then cell body and attached segments survives (severed degenerates)
1. if motor neuron: target muscle results in paralysis
2. if sensory: loss of sensation from innervated area - regeneration more likely to occur in PNS (schwann cells) than CNS
How does the Goldman-Katz equation relate to the membrane potential of a cell?
- Tells us the membrane potential is influenced by concentration gradient of ions and membrane permeability to those ions
- At rest, have pretty steady concentration & permeability — if change either of those can change the membrane potential
What happens If the membrane permeability is changed?
- ion movement —> electrical signal
- Very few ions move to create large changes in membrane potentials (opening/closing channels)
What is resting membrane potential influenced by?
- K+ concentration gradient
- resting membrane permeability to K+, Na+, and Cl-
What do changes in membrane’s permeability result in?
- movement creates an electrical signal
- Very few ions move to create large changes in membrane potentials
What are 2 things that influence membrane potential?
- uneven distribution of ions across the cell membrane (Na+, Cl-, Ca2+, K+ — K+ is only one concentrated in cytosol)
- Differing membrane permeability to those ions: Cell membrane permeability
What does the Nerst equation tell us?
What the membrane potential would be if the membrane was only permeable to 1 ion
What does the GOldman-Hodgkin-Katz (GHK) equation tell us?
calculates RMP using membrane permeability and ion concentration gradients
What is conductance?
the ease at which ions pass through
How does a cell change its ion permeability?
- open/close existing channels in the membrane
- up/down regulation
What do mechanically gated ion channels respond to?
sensory neurons: respond to physical forces like pressure or stretch
what do chemically gated ion channels respond to?
variety of ligands like EC neurotransmitters & neuromodulators, or intracellular signal molecules
What do voltage-gated ion channels respond to?
changes in the cells membrane potential
- play important role in initiation and conduction of electrical signals along axon
- voltage level for opening varies
Look at table 8.3 to compare and contrast graded and action potentials in neurons
What are graded potentials?
depolarizations or hyperpolarization that occur in the dendrites and cell body or (less common) near the axon terminals
- amplitude is directly proportional to the strength of the triggering event
When do graded potentials occur in the neurons of the CNS and efferent division?
occur when chemical signals from other neurons open chemically gated ion channels allowing ions to enter/leave the neuron
- also when an open channel closes (decreasing movement of ions through cell membrane)
Why do graded potentials lose strength as they move through the cytoplasm?
- Current leak: The membrane of the cell body has open leak channels that allow positive charge to leak out into the ECF. Some positive ions leak out of the ell across membrane as depolarization goes through cytoplasm, decreasing strength of signal
- Cytoplasmic resistance: cytoplasm provides resistance to flow of electricity (like water slowing down stone)
What is a trigger zone for graded potentials?
the region of the axon where graded potentials are integrated and an action potential begins if the signal is above threshold
- in efferent & interneurons: axon hillock and initial segment
-
What are subthreshold graded potentials?
graded potential starts about threshold, at its initial point, but decreases in strength as it travels though cell body. at trigger zone, is below threshold and does not initiate action potential
What is a suprathreshold graded potential?
a stronger stimulus at the same point on the cell body creates a graded potential that is still above threshold by the time it reaches the trigger zone, so AP results
What is the magnitude (amplitude) of an action potential?
100 mV (-70mV up to +30 mV
How does an AP release the neurotransmitters stored in vesicles?
Opening voltage gated Ca2+ channels (it causes the vesicles to merge with the membrane
Why does an action potential not loose strength over distance like a graded potential does?
Because the additional Na+ entering the cell reinforced the depolarization
- The action potential at the end of the axon is identical to the one at the trigger zone
Does the strength of the graded potential that initiates an action potential influence the amplitude of the action potential?
nope!
What happens during the rising phase of an Action potential?
- Sudden increase in cell’s permeability to Na+
- Grades potential reaches trigger zone & the membrane depolarizes
- VG Na+ channels open, and Sodium ions flow into the cell (further depolarizes)
What happens in the falling phase of the action potential?
- VG K+ channels open in response to depolarization (much slower to open —peak permeability happens after +30mV reached)
- Cell gets to resting potential
- Once at -70mV, K+ permeability not at resting state…..keeps leaving through VG channels and leak channels
- Membrane hyperpolarizes, brining it to -90mV
- Retention of K+ and leak of Na+ into axon brings the membrane potential back
Does one action potential alter ion concentration gradients?
No!
What causes the refractory period?
Inactivation gates closed
What creates a positive feedback loop in action potentials?
Sodium entry into the cell (look at figure 8.11)
- stops when Na+ gate inactivation gates close
What plays a major role in the refractory period?
The double gating (inactivation and activation) of Na+ channels
** refractory periods are key characteristic that distinguishes AP from graded
What is the absolute refractory period?
The time required for Na+ channel gates to reset to their resting positions
- second AP can not occur before the first has finished
**means APs moving from trigger zone to axon terminal cannot overlap and cannot travel backward
What is a relative refractory period?
- follows the absolute refractory period
- Some, but not all, Na+ channel gates have reset to their original positions
- K+ channels still open
- The Na+ channels that have not unite returned to resting potential can be reopened by stronger-than-normal graded potential
- Any AP that fires here will be smaller then normal
Does depolarization spread in one or multiple directions?
Multiple! look at Figure 8.13
What does conduction mean?
traveling long distances without loosing energy
What is local current flow?
Causes the depolarization of a section of an axon through positive current (spreads in all directions)
What to characteristics affect the speed of the AP?
- Diameter
- Resistance of the axon membrane to ion leakage of out of the cell (the length constant)
(Conduction faster in myelinated axons— leak is minimal)
What is salatory conduction?
The apparent leap-frogging of the action potential down myelinated axons
Why is conduction more rapid in myelinated axons?
- channel slowed slightly
- Only nodes need Na+ channeled becuase of insulating properties of membrane (unmyelinated starts and stops)
- Stops leakage — leakage can cause the threshold to disappear
What does hyperkalemia do?
Brings neuron closer to threshold
What does hypokalemia do?
Moves neuron further from threshold
- now have to have very high stimulus to create AP
What are electrical synapses?
- pass electrical signals through gap junctions
- signal can be bi-directional
- synchronizes the activity of a network of cells
What do chemical synapses do?
Use neurotransmitters that cross synaptic clefts
What are the 7 types of neuroendocrine receptors?
- Acetylcoline (Ach)
- Amines
- Amino acids
- Peptides
- lipids
- Purines
- Gases
What are ionotropic receptors?
- receptor channels
- mediate rapid responses
- alter ion flow across membranes
What are Metabotropic receptors?
- G protein-coupled receptors
- mediate slower responses
- Some open or close ion channels
Do neurotransmitters bind to specific receptors?
YES! except NO (agonist v. antagonist molecules
What does Acetylcholine (Ach) do?
- binds to cholinergic receptors
What are the types of cholinergic receptors and what are they?
- Nicotine receptors:
- on skeletal muscles and in autonomic division of PNS and CNS
- Monovalent cation channels: Na+ and K+ - Muscarinic receptors:
- In CNS and on target cells for autonomic parasympathetic division of PNS
- G-protein coupled receptor
What are amines?
- active in CNS
- derived from single amino acid (tryptophan, histamine, or tyrosine)
-angrenergic/ noreadrenergic neurons secrete norepinephrine - Adrenergic receptros bind norepinephrine and epinephrine (G protein-coupled receptors: a & b classes)
What are amino acids?
- glutamate: excitatory —> CNS
- Aspartame: excitatory —> brain
- Gamma-aminobutyric acid (GABA): inhibitory —> brain (hyperpolarizes target cells by opening Cl- gates)
- Glycine and D-serine
What are peptides?
- substances P and opioid peptides: function as neurotransmitters, neuromodulators, and neurohormones
- Cholystokinin, vasopressin, atrial natriuretic peptide: neurotransmitters and neurohormones
What are purines?
adenosine, AMP and ATP bind to purinergic receptors (GPCRs)
What gasses can be neurotransmitters?
NO, CO, and H2S
- they diffuse into cells
What lipids can be neurotransmitters?
- Eicosanoids, endogenous ligands for cannabinoid receptors
Where are neurotransmitters synthesized?
- polypeptides made in neuron cell body (then eventually taken to axon terminal)
- Small neurotransmitters are made in the axon
- Neurotransmitters stored in synaptic vesicles
How are neurotransmitters released?
- exocytosis
- much faster in neurons than other types of cells
- Neurotoxins such as Tetanus and botulinum toxins can block release
What in the membrane causes neurotransmitters to be released? (Classical model pathway)
- Voltage gated Ca2+ channels open in axon terminal in response to depolarization
- Calcium ions move into cell & bind to regulatory proteins to initiate exocytosis
- membrane of synaptic vesicle fuses with cell membrane, fused area opens & neurotransmitters inside vesicle moves into cleft
- neurotransmitter molecules diffuse across gap to bind with membrane receptors on postsynaptic cell
- each synaptic vesicle has same amount of neurotransmitter
(SLIDE 68 HAS MORE SIMPLIFIED!)
— then vesicle recycled via endocytosis & refilled with neurotransmitter
What is the kiss-and-run pathway for neurotransmitter release?
- vesicles fuse with presynaptic membrane to form fusion pore
- neurotransmitters pass through a channel
How does termination of neurotransmitter activity happen?
- diffusion away from synaptic cleft
- Enzymatic breakdown (acetylchonesterase— AChE)
- Uptake into cells (presynaptic axon terminal, or glial cells)
- some could be in blood
What does synaptic plasticity mean?
- change of activity at the synapses
- occurs primarily in CNS
- could be short or long term
- can enhance or reduce synaptic activity
What is a divergent pathway?
one presynaptic neuron branches to affect a larger number of postsynaptic neurons
What is a convergent pathway?
many presynaptic neurons provide input to influence a smaller number of postsynaptic neurons
How many synapses does each cell body have?
SEVERAL, it’s COVERED in cell bodies
What are puirkinje fibers?
highly branched dendrites of a purkinje cell (neuron) that demonstrate convergence of signals from many synapses onto a cell body
What is spacial summation?
- adding over space
- 2 or more neurons simultaneously fire and have an additive effect
- postsynaptic inhibition (release of neurotransmitter is inhibitory instead of excitatory)
- look at figure 8.25
What is temporal summation?
- adding over time
- Summation occurring from graded potentials occurring close to each other in time and having an additive effect
- look at figure 8.24
Go over slide 79 and 80
How can synaptic activity be modified?
- modular neuron terminates on the presynaptic cell and modulates the release of neurotransmitter
- presynaptic facilitation favors the release of neurotransmitter
- presynaptic inhibition prevents the release of neurotransmitter
What is a modulatory neuron?
Neurons that can be modified
Read last bit of 8.5
Look at slide 85-86
What is glutamate a key element in?
potentiation
- AMPA receptros and NMDA receptors
Long term and short term potentiation
