Transcript PowerPoint
Ch 8: Neurons: Cellular and Network Properties
Objectives : Describe the Cells of the NS
Explain the creation and propagation of an electrical signal in a nerve cell Outline the chemical communication and signal transduction at the synapse
Organization of NS Afferent division New 3 rd division: Enteric NS
Compare to Fig 8-1
Efferent division
Cells of NS
• •
Nerve cell = Neuron Support cells =
Fig 8-2
Neuron: functional unit of nervous system
–
excitable
can generate & carry electrical signals
•
Neuron classification either structural or functional (?)
Fig 8-3
Axonal Transport
What is it? Why is it necessary?
Slow axonal transport (.2 - 2.5 mm/day) Carries enzymes etc. that are not quickly consumed – Utilizes axoplasmic flow Fast axonal transport (up to 400 mm/day) Utilizes kinesins , dyneins and microtubules Actively walks vesicles up or down axon
Fig 8-4
Neuroglia cells
In CNS:
1.
Oligodendrocytes 2.
Astrocytes 3. Microglia (modified macrophages)
4. (Ependymal cells) In PNS:
5.
Schwann cells 6. Satellite cells
See Fig 8-5
Electrical Signals in Neurons
Changes in membrane potential are the basis for electrical signaling Only nerve and muscle cells are excitable (= able to propagate electrical signals)
Review “resting membrane potential” (Ch 5)
Factors influencing membrane potential 1.
2.
Membrane Potential Review
Nernst equation
describes equilibrium potential for single ions
Goldman-Hodgkin-Katz (GHK) equation
considers contribution of all permeable ions to membrane potential Resting membrane potential of cell is based on combined contributions of conc. gradients and membrane permeability for Na
+
, K
+ (and Cl
)
Depolarization / Hyperpolarization
• Due to net movement of ions across cell membrane •
Very few ions need to move for big change in membrane potential
“same” membrane potential changes but ion conc. stays the • Gated ion channels control membrane permeability – Mechanically gated channels – – Chemically gated channels Voltage-gated channels
Two Types of Electrical Signals
Graded potentials variable strength travel over short distances only Action potentials constant strength travel rapidly over longer distances initiated by strong graded potential
Start using 10 SYSTEM SUITE
Four Basic Components of Signal Movement Through Neuron 1. Input signal (Graded Potential) 2. Integration of input signal at trigger zone 3. Conduction signal to distal part of neuron (= Action Potential) 4. Output signal (usually neurotransmitter)
Input Signal: Graded Potentials
Location? - How created?
Strength (= amplitude) reflects strength of triggering event Travel over short distances to trigger zone Diminish in strength as they travel May be depolarizing (excitatory) or hyperpolarizing (inhibitory)
Diminished strength due to
Graded Potentials
1.
Current leak 2.
Cytoplasmic resistance Fig 8-7
Subthreshold potential vs. Suprathreshold potential
Fig 8-8
Graded potential starts here Trigger zone AP
Conduction Signals: Action Potentials (AP) Location ?
Travel over long distances Do not loose strength as (all-or-none principle): 100mV amplitude they travel
Compare to Fig 8-9
Are all identical Represent movement of Na + membrane and K + across
Ion Movement across Cell Membrane During AP Sudden increase in Na + permeability Na + enters cell down electrochemical gradient (+ feedback loop for ~ .5 msec) Influx causes depolarization of membrane potential = electrical signal
What stops + feedback loop?
Na + Channels in Axon Have 2 Gates
Activation gate and Inactivation gate Na + entry based on pos. feedback loop
needs intervention to stop Inactivation gates close in delayed response to depolarization
Fig 8-10
stops escalating pos. feedback loop
AP-Graph – –
Rising (Na + Falling (K + has 3 phases permeability
) permeability
)
–
“Undershoot” or ________
Absolute & Relative Refractory Periods
No movement of Na + possible Na + channels reset to resting state; K + channels still open
> normal stimulus necessary
Purpose of Refractory Periods 1. Limit signal transmission rate (no summation!)
2. Assure one way transmission!
Forward current excites, backward current does NOT re-excite !
Fig 8-15
Other Characteristics of APs
• Stimulus intensity encoded by AP frequency • One graded potential triggers burst of APs • Amount of NT released at axon terminal is to AP frequency • One AP does not change ion conc. gradients
Conduction speed
depends on . . . .
Axon diameter
Size constraints on axons become problem with increasing organismal complexity
Fig 8-17
Membrane resistance
High resistance of myelin prevents current flow between axon and ECF saltatory conduction from node to node
Fig 8-18
Alteration of Electrical Activity
• Various chemical factors responsible, e.g.: Procaine blocks Na + channels • Altered potassium levels will change resting membrane potential – Hyperkalemia ?
– Hypokalemia ?
– Gatorade and other sports drinks
Output Signal: Cell to Cell Communication at Synapses Synapse = point where neuron meets target cell (e.g. ?) 2 types
chemical electrical
3 components of chemical synapse
presynaptic cell synaptic cleft postsynaptic cell
Chemical Synapses
= Majority of synapses Neurotransmitters carry info from cell to cell Axon terminals have mitochondria & synaptic vesicles containing neurotransmitter
Fig 8-20
Events at the Synapse
AP reaches axon terminal Voltage-gated Ca 2+ channels open Ca 2+ entry Ca 2+ is signal for neurotransmitter release Exocytosis of neurotransmitter containing vesicles
Fig 8-21
3 Classes of Neurotransmitters (of 7)
1. Acetyl Choline
–
Synthesized in axon terminal from acetyl CoA and choline
–
Quickly degraded by ACh-esterase
–
Cholinergic neurons
–
Different receptor types:
– nicotinc – muscarinic
Fig 8-22
2. Amines
–
Serotonin
(histidine) (tryptophane) and Histamine –
Dopamine and Norepinephrine
(tyrosine) – Widely used in brain, role in emotional behavior (NE used in ANS) – –
(Nor)adrenergic
neurons Different receptor types ( and β)
3. Gases
– NO (nitric oxide) and CO
Postsynaptic Responses
Can lead to either EPSP or IPSP
Any one synapse can only be either excitatory or inhibitory
Fast synaptic potentials
Opening of chemically gated ion channel Rapid & of short duration
Slow synaptic potentials
Involve G-proteins and 2 nd messengers Can open or close channels or change protein composition of neuron
Fig 8-23
Integration of Neural Information Transfer Convergence and divergence
Fig 8-26
Multiple graded potentials are integrated at axon hillock to evaluate necessity of AP Spatial Summation: stimuli from different locations are added up Temporal Summation: up sequential stimuli added
Synapse: Most Vulnerable Step in Signal Propagation
• • • • Many disorders of synaptic transmission,
e.g.:
Myastenia gravis
(PNS)
Parkinson’s
(CNS)
Schizophrenia
(CNS)
Depression
(CNS)
Mysterious paralysis