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Neurons: Cellular and Network Properties. 8. About this Chapter. Organization of the nervous system Electrical signals in neurons Cell-to-cell communication in the nervous system Integration of neural information transfer. Nervous System Subdivisions. Organization of the Nervous System.

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about this chapter
About this Chapter
  • Organization of the nervous system
  • Electrical signals in neurons
  • Cell-to-cell communication in the nervous system
  • Integration of neural information transfer
model neuron
Model Neuron

Dendrites receive incoming signals; axons carry outgoing information

Figure 8-2

cells of nervous system ns axons transport
Cells of Nervous System (NS):Axons Transport
  • Slow axonal transport
    • Moves material by axoplasmic flow at 0.2–2.5 mm/day
  • Fast axonal transport
    • Moves organelles at rates of up to 400 mm/day
    • Forward transport: from cell body to axon terminal
    • Backward transport: from axon terminal to cell body
cells of ns glial cells and their function
Cells of NS: Glial Cells and Their Function

Glial cells maintain an environment suitable for proper neuron function

Figure 8-5 (1 of 2)

graded potential
Graded Potential
  • The cell body receives stimulus
  • The strength is determined by how much charge enters the cell
  • The strength of the graded potential diminishes over distance due to current leak and cytoplasmic resistance
  • The amplitude increases as more sodium enters, the higher the amplitude, the further the spread of the signal
electrical signals graded potentials
Electrical Signals: Graded Potentials

Subthreshold and suprathreshold graded potentials in a neuron

If a graded potential does not go beyond the treshold at the trigger zone an action potential will not be generated

Figure 8-8a

electrical signals graded potentials1
Electrical Signals: Graded Potentials

Depolarizing grading potential are excitatory

Hyperpolarizing graded potentials are inhibitory

Graded potential= short distance, lose strength as they travel, can initate an action potential

Figure 8-8b

electrical signals trigger zone
Electrical Signals: Trigger Zone
  • Graded potential enters trigger zone- summation brings it to a level above threshold
  • Voltage-gated Na+ channels open and Na+ enters axon – a segment of the membrane depolarizes
  • Positive charge spreads along adjacent sections of axon by local current flow – as the signal moves away the currently stimulated area returns to its resting potential
  • Local current flow causes new section of the membrane to depolarize – this new section is creating a new set of action potentials that will trigger the next area to be depolarized
  • The refractory period prevents backward conduction; loss of K+ repolarizes the membrane – Once the Na+ close they will not open in response to backward conduction until they have reset to their resting position- ensures only one action potential is initiated at time.
electrical signals voltage gated na channels
Electrical Signals: Voltage-Gated Na+ Channels

Na+ channels have two gates: activation and inactivation gates

Figure 8-10c

changes in membrane potential
Changes in Membrane Potential

Terminology associated with changes in membrane potential (chpt 5 figure)

PLAY

Animation: Nervous I: The Membrane Potential

Figure 5-37

electrical signals action potentials
Electrical Signals: Action Potentials

Cell is more positive outside than inside

Rising phase

Figure 8-9 (1 of 9)

electrical signals action potentials1
Electrical Signals: Action Potentials

As ions move across the membrane the potential increases

Rising phase

Figure 8-9 (2 of 9)

electrical signals action potentials2
Electrical Signals: Action Potentials

Graded potentials have brought the membrane potential up to threshold

Rising phase

Figure 8-9 (3 of 9)

electrical signals action potentials3
Electrical Signals: Action Potentials

Beyond threshold potential the sodium gated channels allow the ion to move in, making the inside of the cell more positive

Rising phase

Figure 8-9 (4 of 9)

electrical signals action potentials4
Electrical Signals: Action Potentials

Na+ continues to move into the cell until it reaches electrical equilibrium. At that point Na+ movement stops

Figure 8-9 (5 of 9)

electrical signals action potentials5
Electrical Signals: Action Potentials

Falling phase

K+ moves out of the cell along its gradient and the inside of the cell becomes more and more negative

Figure 8-9 (6 of 9)

electrical signals action potentials6
Electrical Signals: Action Potentials

Hyperpolarization (undershoot) occurs when the potential drops below resting; caused by the continuing movement of K+ out of the cell

Figure 8-9 (7 of 9)

electrical signals action potentials7
Electrical Signals: Action Potentials

Leaked Na+ & K+ in cell increases potential toward resting voltage

Figure 8-9 (8 of 9)

electrical signals action potentials8
Electrical Signals: Action Potentials

Returns to its original state where the outside is more positive than the inside and the membrane potential is -70mv

Figure 8-9 (9 of 9)

electrical signals refractory period
Electrical Signals: Refractory Period

Action potentials will not fire during an absolute refractory period

Figure 8-12

action potential travel down axon
Action Potential Travel Down Axon

Each region of the axon experiences a different phase of the action potential

electrical signals myelinated axons
Electrical Signals: Myelinated Axons

Saltatory conduction- signal seems to “jump” from node to node moving swiftly- compensates for smaller diameter.

Demyelination slows down signal conduction because the current leaks. Sometimes conduction does not reach the next node and dies out.

electrical signals speed of action potential
Electrical Signals: Speed of action potential
  • Speed of action potential in neurons is influenced by:
    • Diameter of axon
      • Larger axons are faster- less resistance to ion flow due to the larger diameter. Large diameter axons are only found in animals with small less complex nervous systems.
    • Resistance of axon membrane to ion leakage out of the cell
      • Myelinated axons are faster – the myelin sheath insulates the membrane allowing the action potential to pass along myelinated are sustaining conduction without slowing down by ion channels opening.
electrical signals coding for stimulus intensity
Electrical Signals: Coding for Stimulus Intensity

Since all action potentials are identical, the strength of a stimulus is indicated by the defrequency of action potentials. Neurotransmitter amounts released are directly propertional to frequency as long as a sufficient supply is available

Figure 8-13b

electricity review
Electricity Review
  • Law of conservation of electrical charges- the net amount of electrical charge produced in any process is zero.
  • Opposite charges attract; like charges repel each other- happens with protons & electrons
  • Separating positive charges from negative charges requires energy – membrane pumps use active transport so separate ions
  • Conductor versus insulator – a conductor allows the charges to move towards each other and an insulator keeps them separate- does not carry current.
separation of electrical charges
Separation of Electrical Charges

Resting membrane potential is the electrical gradient between ECF and ICF

Inside of the cell is more negative than the outside

Electrical gradient create the ability to do work just like concentration gradients

Figure 5-32b

separation of electrical charges1
Separation of Electrical Charges

Resting membrane potential is the electrical gradient between ECF and ICF. Resting membrane potential is due mostly to potassium- it is the equilibrium potential of K+

A relative scale shifts the charge to a -2

Figure 5-32c

sodium equilibrium potential
Sodium Equilibrium Potential

Can be calculated using the Nernst Equation

Concentration gradient is opposed by membrane potential

Figure 5-35

electrical signals nernst equation
Electrical Signals: Nernst Equation
  • Predicts membrane potential for single ion-membrane potentials result from an uneven distribution of ions across a membrane.
  • Membrane potential is influenced by :
    • Concentration gradient of ions – Na+, Cl-, & Ca2+ have higher [extracellular] and K+ has a higher [intracellular]
    • Membrane permeability to those ions - only K+ is allowed to move in so this ion contributes to the resting potential
electrical signals ghk equation
Electrical Signals: GHK Equation
  • Predicts membrane potential using multiple ions- resting membrane potential= the contribution of all ions that cross the membrane X membrane permeability values. Ion contribution is proportional to membrane permeability for that ion. Potentials will be affected if ion concentrations change.
  • P=permeability value
electrical signals ion movement
Electrical Signals: Ion Movement
  • Resting membrane potential determined by
    • K+ concentration gradient
    • Cell’s resting permeability to K+, Na+, and Cl–
  • Gated channels control ion permeability
    • Mechanically gated – respond to physical forces (pressure)
    • Chemical gated - respond to ligands (neurotransmitter)
    • Voltage gated - respond to membrane potential changes
  • Threshold voltage varies from one channel type to another – the minimum stimulus required and the response speed varies for each type
cell to cell postsynaptic response
Cell-to-Cell: Postsynaptic Response

Presynaptic axon

terminal

Slow synaptic potentials

and long-term effects

Rapid, short-acting

fast synaptic potential

Neurotransmitter

G protein–

coupledreceptor

Chemically

gated ion channel

R

G

Inactive

pathway

Postsynaptic

cell

Alters open

state of

ion channels

Activated second

messenger pathway

Modifies existing

proteins or regulates

synthesis of new

proteins

Ion channels open

Ion channels close

More K+

out or

Cl– in

More

Na+ in

Less K+

out

Less

Na+ in

IPSP =

inhibitory

hyperpolarization

EPSP =

excitatory

depolarization

EPSP =

excitatory

depolarization

Coordinated

intracellular

response

Fast and slow responses in postsynaptic cells involve ion channels and G-protein receptor

Figure 8-23

cell to cell chemical synapse
Cell-to-Cell: Chemical Synapse

Chemical synapses use neurotransmitters; electrical synapses pass electrical signals.

Chemical synapses are most common. Electrical synapses are found in the CNS and other cells that use electrical signals (heart)

Figure 8-20

cell to cell calcium
Cell-to-Cell: Calcium

An action potential depolarizes

the axon terminal.

1

Action

potential

2

The depolarization opens voltage-

gated Ca2+ channels and Ca2+

enters the cell.

Axon

terminal

3

Calcium entry triggers exocytosis

of synaptic vesicle contents.

Synaptic

vesicle

4

Neurotransmitter diffuses across

the synaptic cleft and binds with

receptors on the postsynaptic cell.

1

3

Ca2+

5

Neurotransmitter binding initiates

a response in the postsynaptic

cell.

Voltage-gated

Ca2+ channel

Ca2+

Docking

protein

2

4

Receptor

5

Postsynaptic

cell

Cell

response

  • Events at the synapse
  • Exocytosis: Classic versus kiss-and-run

Figure 8-21

cell to cell acetylcholine
Cell-to-Cell: Acetylcholine

Synthesis and recycling of acetylcholine at a synapse

Figure 8-22

integration long term potentiation
Integration: Long-Term Potentiation

Long-term potentiation- mechanism used in learning and memory using Glutaminergic Receptors.

Figure 8-30

cell to cell neurocrines
Cell-to-Cell: Neurocrines
  • Seven classes by structure -
    • Acetylcholine –(Ach) neurotransmitter composed of choline and coenzyme A (acetyl CoA), binds to cholinergic receptors
    • Amines – neurotransmitter, derived from a single amino acid: Dopamine, Norepinephrine, Epinephrine, Serotonin, Histamine
    • Amino acids – an amino acid that functions as a neurotransmitter: Glutamate, Aspartate, Gamma-aminobutyric, Glycine
    • Purines –made from adenine
    • Gases – act as neurotransmitter, half-life of 2-30 sec.
    • Peptides -neurohoromones, neurotransmitters, and neuromodulator,
    • Lipids – eicosanoids
cell to cell amine
Cell-to-Cell: Amine
  • Derived from single amino acid
  • Tyrosine
    • Dopamine -neurotransmitter/neurhormone
    • Norepinephrine -tyrosine , neurotransmitter/neurhormone, secreted by noradrenogenic neurons,
    • Epinephrine - neurotransmitter/neurhormone, also called adrenaline, secreted by adrenogenic neurons
  • Others
    • Serotonin – neurotransmitter, is made from tryptophan
    • Histamine – neurotransmitter, is made from histadine
cell to cell amino acids
Cell-to-Cell: Amino Acids
  • Glutamate: primary excitatory  CNS
  • Aspartate: primary excitatory  brain (select regions)
  • Gamma-aminobutyric(GABA): Inhibitory  brain
  • Glycine
    • Inhibitory  spinal cord
    • May also be excitatory
cell to cell neurocrines1
Cell-to-Cell: Neurocrines
  • Peptides -involved in pain and pain relieve pathways
    • Substance P and opioid peptides
  • Purines- bind purinergic receptors
    • AMP and ATP
  • Gases- produced inside the body, function and mechanisms not totally understood
    • NO and CO
  • Lipids -bind cannabinoid receptors in brain and immune system cells
    • Eicosanoids
cell to cell receptors
Cell-to-Cell: Receptors
  • Cholinergic receptors
    • Nicotinic on skeletal muscle, in PNS and CNS
      • Monovalent cation channels  Na+ and K+
    • Muscarinic in CNS and PNS
      • Linked to G proteins
  • Adrenergic Receptors
    •  and - two classes
    • Linked to G proteins- initiate second messenger
integration injury to neurons
Integration: Injury to Neurons

If the cell body is not damaged the neuron will most likely survive. Axon healing is similar to growth cone of a developing axon.

Figure 8-32

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