1. Chapter 11 The Nervous System (Part B)

2. Electrochemical Gradient
Ions flow along their chemical gradient:
From high concentration to low concentration
Ions flow along their electrical gradient:
Toward an area of opposite charge
In a cell the charge imbalance occurs only in the immediate membrane region.
Electrochemical gradient:
Electrical & chemical gradients together

3. Resting Membrane Potential (Vr)
A potential difference of (–70 mV)
Across the membrane of a resting neuron
Generated by the different concentrations (intra & extracellular) of:
Na+ & Cl (extracellular)
K+ & protein anions (A) (intracellular)
Ionic concentration differences are the result of:
Differential membrane permeability to Na+ & K+
Operation of the sodium-potassium pump

4. Resting Membrane Potential (Vr)
Figure 11.8

5. Membrane Potentials: Signals
Used for information:
Receiving
Integration, and
Sending
Types of signals
Graded potentials (travel short distances)
Action potentials (travel long distances)

6. Changes in Membrane Potential
Changes are manifested by three events
Depolarization:
The inside of the membrane becomes less negative
Repolarization:
The membrane returns to its resting potential
Hyperpolarization:
The inside of the membrane becomes more negative than the resting potential

7. Changes in Membrane Potential
Figure 11.9

8. Graded Potentials Short-lived, local changes in membrane potential
Decrease in intensity with distance
Their magnitude varies directly with the strength of the stimulus
Can initiate action potentials if sufficiently strong

9. Graded Potentials
Figure 11.10

10. Graded Potentials Voltage changes in graded potentials are decremental
Current is quickly dissipated due to the leaky plasma membrane
Can only travel over short distances

11. Graded Potentials
Figure 11.11

12. Action Potentials (APs)
An AP is a brief reversal of membrane potential with a total amplitude of 100 mV
An AP in the axon of a neuron is a nerve impulse
APs are only generated by:
muscle cells and
neurons (nerve cells)
They do not decrease in strength over distance
Are the principal means of neural communication

13. Action Potential: Resting State
Na+ and K+ gated channels are closed
Leakage accounts for small movements of Na+ and K+
K+ move out faster than Na+ moving in
This creates membr-ane resting potential
Figure

14. Action Potential: Depolarization Phase
Na+ permeability increases; membrane potential reverses
Na+ gates are opened; K+ gates are closed
Threshold – a critical level of depolarization (-55 to -50 mV)
At threshold, depolarization becomes self-generating
Figure

15. Action Potential: Repolarization Phase
Sodium gates close
Membrane permeability to Na+ declines to resting levels
As sodium gates close, voltage-sensitive K+ gates open
K+ exits the cell and internal negativity of the resting neuron is restored
Figure

16. Action Potential: Hyperpolarization
Potassium gates remain open, causing an excessive efflux of K+
This efflux causes
hyperpolarization of the membrane (undershoot)
Figure

17. Action Potential: Role of the Sodium-Potassium Pump
Repolarization:
Restores only the resting electrical conditions of the neuron
Does not restore the resting ionic conditions
Ionic restoration to resting conditions:
Is achieved by the sodium-potassium pump
Three Na+ are pumped outside the cell and two K+ are pumped inside

18. Phases of the Action Potential
1 – resting state
2 – depolarization phase
3 – repolarization phase
4 – hyperpolarization

19. Threshold and Action Potentials
membrane is depolarized by 15 to 20 mV (-70 to -55 mV)
Weak (subthreshold) stimuli:
are not relayed into action potentials
Strong (threshold) stimuli:
are relayed into action potentials
All-or-none phenomenon:
action potentials either happen completely, or not at all

20. Absolute Refractory Period
Is the time from the opening of Na+ gates until they begin to close
The absolute refractory period:
Prevents the neuron from generating a simultaneous action potential
Ensures that each action potential is separate
Enforces one-way transmission of nerve impulses

21. Absolute Refractory Period
Figure 11.15

22. Relative Refractory Period
The interval following the absolute refractory period when:
Sodium gates are closed
Potassium gates are open
Repolarization is occurring
The threshold level is elevated, allowing:
Strong stimuli to increase the frequency of action potential events

23. Saltatory Conduction
Current passes through a myelinated axon only at the nodes of Ranvier
Voltage-gated Na+ channels are oncentrated at these nodes
Action potentials are triggered only at the nodes
They jump from one node to the next
Saltatory conduction is much faster than continuous conduction occurring along unmyelinated axons

24. Saltatory Conduction
Figure 11.16

25. Multiple Sclerosis (MS)
An autoimmune disease
Mainly affects young adults
Symptoms include, among others:
Weakness
Loss of muscular control
Myelin sheaths in the CNS are:
Destroyed gradually

26. Synapses Junctions that mediate information transfer
Information transfer is from:
One neurone to nother neurone
A neurone to an effector cell (muscle or gland)
Presynaptic neuron:
Conducts impulses toward the synapse
Postsynaptic neuron:
Transmits impulses away from the synapse

27. Types of Synapses Axodendritic synapses: Axosomatic synapses:
Between a neuron axon and a dendrite of another neuron
Axosomatic synapses:
Between a neuron axon and the soma (cell body) of another neuron
Other less common types:
Axoaxonic (axon to axon)
Dendrodendritic (dendrite to dendrite)
Dendrosomatic (dendrites to soma)

28. Figure 11.17

29. Electrical Synapses Electrical synapses:
Are less common than chemical synapses
Correspond to gap junctions found in other cells
Direct cytoplasmic connection between neurons
Direct flow of ions
Are important in the CNS in:
Mental attention
Emotions
Memory

30. Chemical Synapses
Specialized for the release and reception of neurotransmitters
Typically composed of two parts:
Axonal terminal of the presynaptic neuron that contains synaptic vesicles
Receptor region on the dendrite(s) or soma of the postsynaptic neuron

31. Synaptic Cleft A fluid-filled space
Separates pre and postsynaptic neurons
Prevents direct passage of nerve impulses between neurons
Transmission across the synaptic cleft:
Is a chemical event
Ensures unidirectional communication between neurons

32. Synaptic Cleft: Information Transfer
Nerve impulses reach presynaptic axonal terminal
These impulses open Ca2+ channels causing
Neurotransmitter release into synaptic cleft (via ..?)
Neurotransmitter crosses the synaptic cleft
It binds to receptors on the postsynaptic neuron
Postsynaptic membrane permeability changes
Permeability changes lead to:
An excitatory effect or
An inhibitory effect

33. Synaptic Cleft: Information Transfer
Figure 11.19

34. Termination of Neurotransmitter Effects
Removal of neurotransmitters occurs by:
Degradation by:
Enzymes
Reabsorption by:
Astrocytes, or
Presynaptic terminals
Diffusing away from the synaptic cleft

35. Synaptic Delay Neurotransmitter must be: Synaptic delay is: Released
Diffuse across the synapse
Bind to receptors
Synaptic delay is:
The time needed for all phases ( ms)

36. Postsynaptic Potentials
Neurotransmitter receptors mediate local changes in membrane potential.
Mediation is accomplished according to:
The amount of neurotransmitter released
The neurotransmitter-receptors binding time
The two types of postsynaptic potentials are:
EPSP:
Excitatory Postsynaptic Potentials
IPSP:
Inhibitory Postsynaptic Potentials

37. Excitatory Postsynaptic Potentials
Are graded potentials
Can initiate an action potential in an axon
Use only chemically gated channels (no voltage gated channels)
Simultaneous Na+ and K+ flow in opposite directions
Postsynaptic membranes do not generate action potentials (at dendrites/soma)

38. Excitatory Postsynaptic Potentials
Figure 11.20a

39. Inhibitory Synapses & IPSPs
A neurotransmitter-receptor binding at inhibitory synapses
It causes:
Increased membrane permeability to K+ (out) and Cl- (in) ions
Increased negative charge on the inner surface of the membrane
Reduced postsynaptic neuron’s ability to produce an action potential

40. Inhibitory Synapses & IPSPs
Figure 11.20b

41. Neurotransmitters Chemicals used for neuronal communication
Fifty different types have been identified
Are classified as:
Chemical neurotransmitters
Functional neurotransmitters

42. Chemical Neurotransmitters
Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Novel messengers:
ATP
Dissolved gases (NO and CO)

43. Neurotransmitters: Acetylcholine
First neurotransmitter identified
Best understood
Synthesized & enclosed in synaptic vesicles
Released at the neuromuscular junction
Degraded enzymatically by:
Cetylcholinesterase (AChE)
Released by the following neurons:
All neurons stimulating skeletal muscle
Some neurons in the autonomic nervous system
Some neurons found in the CNS

44. Neurotransmitters: Biogenic Amines
Biogenic amines include:
Catecholamines:
Dopamine
Norepinephrine (NE)
Epinephrine (E)
Indolamines:
Serotonin
Histamine
They are broadly distributed in the brain
They play roles in:
Emotional behaviors
Our biological clock

45. Neurotransmitters: Amino Acids
Amino acid neurotransmitters include:
Glutamate
GABA – Gamma ()-aminobutyric acid
Glycine
Aspartate
They are found only in the CNS

46. Functional Neurotransmitters
Functional neurotransmitters are classified into:
Excitatory
Inhibitory
Excitatory neurotransmitters:
Cause depolarizations
Ex. Glutamate
Inhibitory neurotransmitters:
Cause hyperpolarizations
Ex. GABA & glycine

47. Functional Neurotransmitters
Some neurotransmitters have both:
Excitatory effects
Inhibitory effects
The neurotransmitter effect is determined by:
The receptor type of the postsynaptic neuron
Example: Acetylcholine:
Excitatory effect:
At the neuromuscular junctions (skeletal muscle)
Inhibitory effect:
In the cardiac muscle

48. Break Slide Biol2401.