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126 Cards in this Set

  • Front
  • Back
How many neurons and synapses?
100B neurons, 100T synapses
100K miles of nerves
85K neurons lost every day
Major anatomical subdivisions of nervous system
Central nervous system (CNS) - brain & spinal cord

Peripheral nervous system (PNS) other nerves & ganglia
Parts of PNS
Sensory Division: Visceral & Somatic Sensory divisions
Motor Division: Visceral & Somatic Motor Divisions
Visceral Motor: Sympathetic & Parasympathetic divisions
(Autonomic Nervous System)
Functional Divisions of PNS: Sensory (afferent) divisions
Incoming Information:

Visceral & Somatic Receptors signal to CNS
Functional Divisions of PNS: Motor (efferent) division - Visceral motor subdivision (aka autonomic nervous system)
Info Output: cardiac muscle, smooth muscle, glands
Sympathetic system: fight or flight
Parasympathetic system: rest and relax
Functional Divisions of PNS: Motor (efferent) division - Somatic Motor subdivision
Info Output: Effectors, Skeletal muscle, Skin
Cells of the Nervous System
Neurons: carry impulses
Neuroglia: (aka - supporting cells, glia, or glial cells)
Neuroglia of CNS
Astrocytes
Oligodendrocytes
Microglia
Ependymal cells
Neuroglia of the PNS
Schwann cells
Fundamental Properties of Neurons
Excitability (responsiveness) - can respond to stimuli (changes in body and external environment
Conductivity - can produce traveling electrical signals
Secretion-release chem. neurotransmitter at synapse
Parts of a Neuron: List
and Information Flow
- Dendrites, Cell Body (soma), Axon Hillock, Axon, Synaptic knob
- Transition from local to action to chemical (repeat)
Parts of a Neuron: Dendrites
Receive signal
transfer signal toward soma
Parts of a Neuron: Axon hillock
- first part of the axon plus the part of the cell body where the axon exits
- lowest threshold for the action potential
Parts of a Neuron: Axon
- aka nerve fiber
- signals away from soma
- site of rapid conduction of action potential
Parts of a Neuron: Synaptic Knob
- Swelling at end of axon
- Contains synaptic vesicles
-- neurotransmitter
Chemical Synapse
- Functional junction between synaptic knob of one neuron and dendrites or soma of next neuron
- Cells do not touch
Information Flow in Neurons: Dendrites 1
Local Potential
Information Flow in Neurons: Cell Body (Soma)
Local Potential
Information Flow in Neurons: Axon Hillock
Local Action Potential
Information Flow in Neurons: Axon
Local Potential
Information Flow in Neurons: Synaptic Knob
Transition Action to Chemical Signal
Information Flow in Neurons: Synapse
Chemical signal crosses synaptic cleft
Information Flow in Neurons: Dendrites 2
Open ion channels initiate next local potential
Information Flow in Neurons
Transition from local
Multipolar Neuron
Most common
Many dendrites
One axon
Unipolar Neuron
Sensory from skin and organs to spinal cord
Types of Neurons by Function
Sensory neurons
Interneurons
Motor neurons
Functional Types of Neurons - Sensory Neurons
Sensory (Afferent) Neurons conduct signals from receptors to the CNS
Functional Types of Neurons - Interneurons
Interneurons (Association Neurons) are confined to the CNS
Functional Types of Neurons - Motor Neurons
- All are multipolar neurons in the PNS
- Send signals out to effectors - organs that carry out responses called effectors; muscles and gland cells
- aka Afferent Neurons
Neuronal Signalling Sequence
1. Sensation - Sensory Neurons
2. Integration - Interneurons
3. Response - Motor Neurons
Axonal Transport - Where are Proteins Made?
Proteins are made in the Soma and must be transported to axon and axon terminal
Axonal Transport - What happens to waste products in Axon Terminal?
Waste products in the axon terminal must be transported (recycled) to the soma
Anterograde Axonal Transport
Movement from soma toward axon terminal
- Repair axolemma, gated ion channel proteins, enzymes, neurotransmiters
Retrograde Axonal Transport
Movement from axon terminal toward soma
- recycled materials, waste products
What is Kinesin?
A protein that "walks" along microtubules (as in the axon)
Neuroglia
aka glial cells
- 90% of CNS cells
- 50% of volume
- 5 main types
Five Main Types of Neuroglia
- Astrocytes -Oligodendrocytes
- Schwann cells (PNS only)
- Microglia
- Ependymal cells
Astrocytes (Most abundant glial cell) - Functions
- Maintain neural spatial relationships
- Induce formation of blood brain barrier
- Control interstitial environment
Microglia
- Type of white blood cell
- Migrating immune defense
Oligodendrocytes
- Form myelin in CNS
- Each wraps processes around many nerve fibers
Schwann Cells
- Form myelin in PNS
- Each wraps processes around single spot on one nerve fiber
Ependymal Cells
- Line cavities within CNS ventricles and spinal canal
-- Move fluid via cilia
- Make cerebrospinal fluid (choroid plexus)
Myelin Sheath 1
- Insulating layer around a nerve fiber -- Prevents ions from passing through plasma membrane
- Formed from wrappings of plasma membrane
-- 20% protein & 80% lipid (looks white)
Where is Myelin Sheath Made?
- Made by oligodendrocytes in CNS
- Made by Schwann cells in PNS
- Axolemma is axon outermost layer
- Neurilemma is myelin outermost layer
How is Myelin Sheath Made?
- In PNS, hundreds of layers wrap axon
- Outer coil mostly schwann cell cytoplasm (neurilemma)
- External to neurilemma is a thin sleeve of fibrous connective tissue (endoneurium) (then basal lamina)
Nodes of Ranvier
- Gaps between myelin segments (between Schwann cells)
-- Found in both CNS & PNS
PNS Myelin Sheath Formation
- Myelination begins during fetal development
-- Proceeds most rapidly during infancy
Unmyelinated Axons
- Neurilemma (maybe endoneurium/basal lamina) without myelin sheath
Factors in Speed of Conduction of Action Potentials
> Diameter = < Resistance = > Speed
> Myelin = > Speed of conduction
A Fibers are larger, myelinated fibers, faster conduction
C Fibers are thinner, unmyelinated, slower conduction
Speed of Conduction - A Fibers
- Large diameter, Myelinated
- Fastest fibers, up to 140 meters per second (300mph)
Speed of Conduction - C Fibers
- Small diameter, unmyelinated
- Slowest fibers, 0.5 mps (1mph)
- Located in visceral efferent nerves
Electrical States of Neurons - Polarization
- Any state, other than 0mV
-- positive or negative
- Most cells are polarized: -70mV
Electrical States of Neurons - Depolarization
- Membrane potential becomes less negative than the resting potential
-- membrane potential moves toward 0mV
Electrical States of Neurons - Repolarization
- Membrane returns to resting potential after depolarization
Electrical States of Neurons - Hyperpolarization
- Membrane becomes more negative inside
-- more negative than resting potential
Electrolyte Balance
Na+
K+
Ca++
Na+ More outside the cell (ECF)
Ca++ More outside the cell (ECF)
K+ More Inside the cell (ICF)
Na+ K+ ATPase Pump
- Primary Active Transport (made of proteins)
Na+ Movement
3 Na+ ions out of cell
- 3 Na+ ions from ICF bind to carrier
- Carrier phosphorylated - ATP -- ADP
- Carrier changes conformation - release Na+ to ECF
Na+ K+ ATPase Pump
- Primary Active Transport (made of proteins)
K+ Movement
2 K+ ions into cell
- Carrier binds 2 K+ ions from ECF
- Carrier loses phosphate - reverts to original shape
- Releases K+ ions to ICF
Why is Na+ K+ ATPase Pump Important?
- Important for establishing membrane potential & Na/K gradients
- For maintaining cell volume
Channel Proteins - Integral proteins that form pores (channels) for diffusion of water or solutes
1. Leak Channels - Constantly open - Specific
2. Gated-channels - open and close in response to stimuli - important in nerve signal and muscle contraction
Channel Proteins - Gated Channels - 3 Kinds
1. Ligand-regulated gates at synapses bind to chemical messengers
2. Voltage-regulated gates action potential changes across plasma membrane
3. Mechaniclaly regulated gates (receptors) - physical stress such as stretch and pressure
Membrane Transport - Diffusion
- Net molecular diffusion down concentration gradient
- Tends toward a steady state (0 net diffusion = equilib.)
- Through lipid bilayer or protein channels
-- Passive, no ATP
Passive Ion Movement - Electrical Gradient
- Ions move along electrical gradient
- Charge diff. between adjacent areas produces gradient
Passive Ion Movement - Electrochemical gradient
- Net effect of electrical + concentration gradients
Passive Ion Movement - Equilibrium Potential
- Voltage at which the electrical gradient balances the concentration gradient
Membrane Potential - Separation of charges across plasma membrane
- Creates ability to do work
- Present in all cells (not just neurons)
Membrane Potential - Where are the Charges Found?
- Negative charges along inside plasma membrane, positive charges just outside
- Most of ICF and ECF is electrically neutral
Membrane Potential - How does it Develop?
- Develops due to differences in the concentration and permeability of key ions -- K+ and Na+
- Distribution of Cl- ions is passively driven by the established membrane potential (high in ECF)
Membrane Potential - in Excitable Tissues
- Produce rapid, transient changes in membrane potential
- Change their resting potentials into electrical signals
-- muscle & nerve cells
Normal Resting Potential vs Hyper- & Depolarization
Polarized: -70mV
Hyperpolarized: < -70mV
Depolarized: > -70mV
If Only K+ Decided Membrane Potential:
- K+ would leak out of cell along chemical gradient until diffusion counterbalanced by electrical gradient
- Eq. (membrane) potential of potassium = -90mV
-- 90mV of potential work, negative inside cell
If Only Na+ Decided Membrane Potential:
- Na+ would leak into cell along chemical gradient until diffusion counterbalanced by its electrical gradient
- Equilibrium (membrane) Potential of sodium = +60mV
-- 60mV of potential work, positive inside cell
Resting Membrane Potential - How is Potential maintained when cell is not sending signals?
- Na+-K+ pumps have a small direct effect on membrane potential (20%)
- Transports 3 Ns+ ions to ECF & 2 K+ ions to ICF
- Pumping of Na/K = leaking of Na/K
What is the Effect of K+ on the Membrane Potential?
- K+ brings the resting membrane potential down to -70mV from about -10mV
Resting Membrane Potential - Why is MP mostly a result of concentration gradient and diffusion of K+?
- At rest, membrane is more permiable to K+ than to Na+
- Many K leak channels - Few Na leak channels
- So, more K+ diffuses into the ECF than Na+ into the ICF
- Resting Potential = -70mV
Electrical Signals
- Changes in membrane potential in response to a triggering event
-- Local change in electrical field: transmission of AP
- Chemical messenger: e.g. neurotransmitter
What are 2 Types of Membrane Changes?
1. Graded/Local potential
2. Action Potential
- Caused by a change in ion flow across plasma membrane through gated channels
Electrical Signals: Graded/Local Potentials (1)
- Local disturbances in membrane potential (1mm max)
-- can depolarize or hyperpolarize membrane
- Occur when cell is stimulated
- Caused by local changes in ion flow - gates opening
Depolarization due to Opening of Gated Na+ channels
- Na+ rushes in down concentration and electrical gradients
- Na+ diffuses for short distance inside membrane producing a change in voltage called a local potential
Electrical Signals: Graded/Local Potential (2)
- Graded: Stronger triggering event=greater elec. change
- Local: spread only 1mm
- Decremental: get weaker as the spread (with distance)
- Excitatory (depolarization) or Inhibitory (hyperpolar.)
Electrical Signals: Graded/Local Potential - Duration
- Duration of electrical change is proportional to duration of triggering event
-- Longer triggering event = Longer electrical change
Spread of Local Potentials: Localized Current Flow
- Current
- Current: movement of electrical charges (ions)
Spread of Local Potentials: Localized Current Flow
- Ion Movement
- Ions move from depolarized (active) region to adjacent resting membrane regions
- bi-/multidirectional - passive current flow via Elec Grad
- Contiguous: touches every part of membrane
Spread of Local Potentials: Decremental Conduction
- Magnitude decreases further form the initial active area
- depolarization spreads only about one millimeter
Electrical Signals: Action Potential
- Large brief changes in membrane potential ~100mV
- Identical propagation long distance along membrane
- Plasma Membrane must reach threshold =~-50mV
-- All or None
Electrical Signals: Action Potential - 3 Phases
- Depolarization to ~ +30mV
- Repolarization to resting potential (~-70mV)
- Hyperpolarization to ~ -90mV
4 Characteristics of an Action Potential
- All or None
- Irreversible
- Nondecremental
- Unidirectional
Stimulation, Threshold, and AP Relationship
- Subthreshold stimulation = No AP
- Stronger stimulation = more APs, not more depolarized
- Speed of propagation depends on properties of nerve carrying signal, not on stimulation strength.
Ions and Ion Channels in an AP
- Na+ & K+ voltage gated channels
- Depolarizaiton by influx of Na+ ions
- Re- / hyperpolarization by end of Na+ influx & K+ efflux
- Pump maintains huge gradient even after many APs
Voltage-Gated K+ Channel
- One Gate: Activation Gate
-- slow response to threshold ~ -55mV
- Two states: Open & Closed ready to open
- ~300x > permeability to K+ Slow Close = Hyperpolarized
Voltage-Gated Na+ Channel
- Two Gates: Activation and Inactivation (thresh ~ -55mV)
-- Activation gate: fast, gate-like
-- Inactivation gate: slow ball & chain
- Close ready to open, - Open, - Closed unable to open
Action Potentials: Voltage Gate Sequence
1. Na+ gate opens and Na+ rushes in (Depolarization)
2. Na+ gate closes, K+ gate opens K+ out (Repolarization)
3. K+ gate closes slowly as K+ moves out (Hyperpolarizes)
4. K+ closed and Na+ diffuses to restore RMP
Types of AP propagation - Contiguous Conduction
- AP spreads along every bit of membrane by local current flow. Moves adjacent membrane to threshold.
- Activates next point of axonal membrane
- Self perpetuating cycle of ion and electrical changes
Types of AP propagation - Saltatory Conduction
- Describes spread of APs in myelinated fibers
- AP jumps from node to node
- up to 300x faster than contiguous conduction
Saltatory Conduction
Voltage-gated Na+ channels needed for APs
- Few in myelin-covered regions
- Many in nodes of Ranvier
- Fast diffusion occurs between nodes
Refractory Periods: Time when another AP either cannot be generated or is harder to generate - Two Types
- Absolute Refractory Period
- Impossible to fire new AP
- Maintain one-way flow
- Prevent oscillations of APs
Refractory Periods: Time when another AP either cannot be generated or is harder to generate - Two Types
- Relative Refractory Period
- Harder to fire new AP
- Limits frequency of APs
-- Longer refractory period = lower AP frequency
-- Different refractory periods for different neurons
Absolute Refractory Period
- No stimulus can initiate another AP
- Na+ voltage-gated channels are closed and unable to open
- Must fall below threshold before they can open again
Relative Refractory Period
- Time when another AP can be produced only by a stronger than normal triggering event
- Many K+ channels still open: Cell is hyperpolarized
- Some Na+ channels still active: fewer able to react
Neural Coding: Qualitative Information
1. Codes what type of info - pain, touch, sound, etc.
-- Depends on which neurons fired
Neural Coding: Quantitative Information
2. Codes how intense stimulus is - Stronger = more rapid fire rate -- Able to overcome hyperpolarization faster -- CNS judges stimulus strength from frequency
- Strong stimuli excite more neurons (recruitment)
Neurotransmitters
- Chemical messengers
- Small rapid acting molecules (100+ known)
- Released by one neuron and bind to receptors of next
Current View of the Chemical synapse
- First neuron releases neurotransmitter from synaptic knob
- Chemical crosses gap between neurons
- Second neuron changes chem signal to electric signal
Chemical Synapse
- Junction between two neurons
- Presynaptic neuron
- Postsyaptic neuron
Anatomy of a Chemical Synapse
- Synaptic Knob contains syaptic vessicles
- Synaptic Cleft between cells
- Subsynaptic membrane contains Ligand (chemically)-gated channels
Synapse Locations: Synapse may be -
- Axodendritic
- Axosomatic
- Axoaxonic
- Some neurons receive over 100,000 synaptic inputs
Excitatory Synapse
- Neurotransmitter binding Na+ & Ca+ channel
- Many Na+ flow into cell
- Depolarization brings cell closer to threshold
- Excitatory Postsynaptic Potential (EPSP)
Inhibitory Synapse
- Binding opens K+ or Cl- channels
- Outflow of K+ or influx of Cl- hyperpolarizes cell
- Larger stimulus required to reach threshold
- Inhibitory Postsynaptic Potential (IPSP)
Cleaning up Chemical Synapses
- NT Re-uptake by pre-synaptic cell and reused
- Degredation - NT broken down by enzymes
- Diffusion NT diffuses away into the ECF
- Absorbed by glial cells
Changing Chemical Signal into Electrical Signal
- Ion channel changes alter MP of post-synaptic neuron
-- Local potential in dendrites and cell body
- Excitatory causes depolarization
- Inhibitory causes hyperpolarization
Neural Pathways
- Convergence: one cell is influenced by many others
- Divergence: one cell influences many others
- 100B neurons
- 100T synapses
Neurotransmitter Actions
- Always excitatory
- Always inhibitory
- Depends on at which synapse
Neurotransmitter Speed
- Fast at chemically-gated synapse
- Slower and longer lasting at 2nd messenger pathways
Neuromodulators
- Long-term depression or enhancement of synapse
-- Don't usually produce IPSPs or EPSPs
- Act via 2nd messenger - often cosecreted with NTs
- e.g. neuropeptides, nitrous oxide
Types of Neurotransmitters
- 100 types in 5 categories:
- Acetylcholine - Amino Acid NTs - Monoamines
- Neuropeptides - Dissolved Gasses - Others
Synaptic Transmission
1. Excitatory cholinergic synapse (ACh)
2. Inhibitory GABA-ergic synapse (GABA)
3. Excitatory Adrenergic synapse (NE)
Excitatory Cholinergic Synapse
- Nerve Signal Opens V-gated Ca+ channels
- Triggers release of ACh which crosses synapse
- ACh Triggers opening of Na+ channels to produce LP
- Depolarization may reach threshold and trigger AP
Inhibitory GABA-ergic Synapse
- Nerve signal triggers GABA release which crosses gap
- GABA receptors trigger opening of Cl- channels
- Cl- flow leads to post-synaptic hyperpolarization
Cessation of Synaptic Signal - Mechanisms to turn off stimulation
- Diffusion of NT away from synapse (astrocytes)
- Synaptic knob reabsorbs amino acids and monoamines by endocytosis - breaks down with MAO
- Acetylcholinesterase degrades ACh in SCleft for recycle
Ecitatory Adrenergic Synapse
- NT is Norepinephrine (NE)
- Acts through 2nd messenger system
- G protein activates adenylate cyclase to convert ATP to cAMP
cAMP has multiple effects. What are they?
- synthesis of new enzymes
- activating enzymes
- opening ligand gates
- produce a postsynaptic potential
Neuronal Interactions
- Summation (temporal and spatial)
- Presynaptic modulation (inhibition and facilitation)
- Neuronal pools: reverbrating & after-discharge circuits
PreSynaptic Inhibition
One presynaptic neuron suppresses another
Summation
Combination of temporal or spatial inputs that may reach threshold
Presynaptic modulation
Axon is innervated by another axon terminal to inhibit or enhance NT release