Ppt

1. Overview
 Course introduction
 Neural Processing: Basic Issues
 Neural Communication: Basics
 Vision, Motor Control: Models

2. Neural Communication: 1
Communication within and
between cells

3. Transmission of information
Information must be transmitted
 within each neuron
 and between neurons

4. The Membrane
 The membrane surrounds the neuron.
 It is composed of lipid and protein.

5. The Resting Potential
 There is an electrical charge across the membrane.
 This is the membrane potential.
 The resting potential (when the cell is not firing) is a
70mV difference between the inside and the outside.
inside
outside
Resting potential of neuron = -70mV
+
-
+
-
+
-
+
-
+
-

6. Artist’s rendition of a typical cell membrane

7. Ions and the Resting Potential
 Ions are electrically-charged molecules e.g. sodium (Na+),
potassium (K+), chloride (Cl-).
 The resting potential exists because ions are concentrated on
different sides of the membrane.
 Na+ and Cl- outside the cell.
 K+ and organic anions inside the cell.
inside
outside
Na+
Cl-
Na+
K+
Cl-
K+
Organic anions (-)
Na+
Na+
Organic anions (-)
Organic anions (-)

8. Ions and the Resting Potential
 Ions are electrically-charged molecules e.g. sodium (Na+),
potassium (K+), chloride (Cl-).
 The resting potential exists because ions are concentrated on
different sides of the membrane.
 Na+ and Cl- outside the cell.
 K+ and organic anions inside the cell.
inside
outside
Na+
Cl-
Na+
K+
Cl-
K+
Organic anions (-)
Na+
Na+
Organic anions (-)
Organic anions (-)

9. Maintaining the Resting
Potential
 Na+ ions are actively transported (this uses
energy) to maintain the resting potential.
 The sodium-potassium pump (a membrane
protein) exchanges three Na+ ions for two K+
ions.
inside
outside
Na+
Na+
K+
K+
Na+

10. Neuronal firing: the action
potential
 The action potential is a rapid
depolarization of the membrane.
 It starts at the axon hillock and passes
quickly along the axon.
 The membrane is quickly repolarized to
allow subsequent firing.

11. Before Depolarization

12. Action potentials: Rapid
depolarization
 When partial depolarization reaches the activation
threshold, voltage-gated sodium ion channels open.
 Sodium ions rush in.
 The membrane potential changes from -70mV to +40mV.
Na+
Na+
Na+
-
+
+
-

13. Depolarization

14. Action potentials: Repolarization
 Sodium ion channels close and become refractory.
 Depolarization triggers opening of voltage-gated
potassium ion channels.
 K+ ions rush out of the cell, repolarizing and then
hyperpolarizing the membrane.
K+ K+
K+
Na+
Na+
Na+
+
-

15. Repolarization

16. The Action Potential
 The action potential is “all-or-none”.
 It is always the same size.
 Either it is not triggered at all - e.g. too little
depolarization, or the membrane is
“refractory”;
 Or it is triggered completely.

17. Course of the Action Potential
• The action potential begins with a partial depolarization (e.g. from
firing of another neuron ) [A].
• When the excitation threshold is reached there is a sudden large
depolarization [B].
• This is followed rapidly by repolarization [C] and a brief
hyperpolarization [D].
• There is a refractory period immediately after the action potential
where no depolarization can occur [E]
Membrane
potential
(mV)
[A]
[B] [C]
[D] excitation threshold
Time (msec)
-70
+40
0
0 1 2 3
[E]

18. Action Potential
Local Currents depolarize adjacent channels causing
depolarization and opening of adjacent Na channels
Question: Why doesn’t the action potential travel backward?

20. Conduction of the action
potential.
 Passive conduction will ensure that adjacent
membrane depolarizes, so the action potential
“travels” down the axon.
 But transmission by continuous action potentials
is relatively slow and energy-consuming
(Na+/K+ pump).
 A faster, more efficient mechanism has evolved:
saltatory conduction.
 Myelination provides saltatory conduction.

21. Myelination
 Most mammalian axons are myelinated.
 The myelin sheath is provided by oligodendrocytes and
Schwann cells.
 Myelin is insulating, preventing passage of ions over
the membrane.

22. Saltatory Conduction
 Myelinated regions of axon are electrically insulated.
 Electrical charge moves along the axon rather than across the
membrane.
 Action potentials occur only at unmyelinated regions: nodes of
Ranvier.
Node of Ranvier
Myelin sheath

23. Synaptic transmission
 Information is transmitted from the presynaptic
neuron to the postsynaptic cell.
 Chemical neurotransmitters cross the
synapse, from the terminal to the dendrite or
soma.
 The synapse is very narrow, so transmission is
fast.

24. terminal
dendritic spine
synaptic cleft
presynaptic membrane
postsynaptic membrane
extracellular fluid
Structure of the synapse
 An action potential causes neurotransmitter
release from the presynaptic membrane.
 Neurotransmitters diffuse across the
synaptic cleft.
 They bind to receptors within the
postsynaptic membrane, altering the
membrane potential.

25. Neurotransmitter release
 Ca2+ causes vesicle membrane to fuse with
presynaptic membrane.
 Vesicle contents empty into cleft: exocytosis.
 Neurotransmitter diffuses across synaptic
cleft.
Ca2+

27. Ionotropic receptors (ligand gated)
 Synaptic activity at ionotropic receptors
is fast and brief (milliseconds).
 Acetylcholine (Ach) works in this way
at nicotinic receptors.
 Neurotransmitter binding changes the
receptor’s shape to open an ion channel
directly.
ACh ACh

29. Ionotropic Receptors

30. Metabotropic Receptors (G-Protein)

31. Excitatory postsynaptic
potentials (EPSPs)
 Opening of ion channels which leads to
depolarization makes an action potential more likely,
hence “excitatory PSPs”: EPSPs.
 Inside of post-synaptic cell becomes less negative.
 Na+ channels (NB remember the action potential)
 Ca2+ . (Also activates structural intracellular changes ->
learning.)
inside
outside
Na+ Ca2+
+
-

32. Inhibitory postsynaptic
potentials (IPSPs)
 Opening of ion channels which leads to
hyperpolarization makes an action potential less
likely, hence “inhibitory PSPs”: IPSPs.
 Inside of post-synaptic cell becomes more negative.
 K+ (NB remember termination of the action potential)
 Cl- (if already depolarized)
K+
Cl- +
- inside
outside

33. Postsynaptic Ion motion

34. Requirements at the synapse
For the synapse to work properly, six basic events need to happen:
 Production of the Neurotransmitters
 Synaptic vesicles (SV)
 Storage of Neurotransmitters
 SV
 Release of Neurotransmitters
 Binding of Neurotransmitters
 Lock and key
 Generation of a New Action Potential
 Removal of Neurotransmitters from the Synapse
 reuptake

35. Integration of information
 PSPs are small. An individual EPSP will not produce
enough depolarization to trigger an action potential.
 IPSPs will counteract the effect of EPSPs at the
same neuron.
 Summation means the effect of many coincident
IPSPs and EPSPs at one neuron.
 If there is sufficient depolarization at the axon
hillock, an action potential will be triggered.
axon hillock

36. Three Nobel Prize Winners on
Synaptic Transmission
Arvid Carlsson discovered dopamine is a neurotransmitter.
Carlsson also found lack of dopamine in the brain of
Parkinson patients.
Paul Greengard studied in detail how neurotransmitters
carry out their work in the neurons. Dopamine activated a
certain protein (DARPP-32), which could change the function
of many other proteins.
Eric Kandel proved that learning and memory processes
involve a change of form and function of the synapse,
increasing its efficiency. This research was on a certain
kind of snail, the Sea Slug (Aplysia). With its relatively low
number of 20,000 neurons, this snail is suitable for
neuron research.

37. Neuronal firing: the action
potential
 The action potential is a rapid
depolarization of the membrane.
 It starts at the axon hillock and passes
quickly along the axon.
 The membrane is quickly repolarized to
allow subsequent firing.

38. Overview
 Course introduction
 Neural Processing: Basic Issues
 Neural Communication: Basics
 Vision, Motor Control: Models

40. Motor Control: Basics

42. Hierarchical Organization of
Motor System
• Primary Motor Cortex and Premotor Areas

43. Primary motor cortex (M1)
Foot
Hip
Trunk
Arm
Hand
Face
Tongue
Larynx

44. Motor Control Basics
• Reflex Circuits
– Usually Brain-stem, spinal cord based
– Interneurons control reflex behavior
– Central Pattern Generators
• Cortical Control

45. postsynaptic
neuron
science-education.nih.gov

46. Flexor-
Crossed
Extensor
Reflex
(Sheridan
1900)
Painful Stimulus
Reflex
Circuits
With
Inter-neurons

49. Gaits of the cat: an informal computational model