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

3. Neurons cell body dendrites (input structure)
receive inputs from other neurons
perform spatio-temporal integration of inputs
relay them to the cell body
axon (output structure)
a fiber that carries messages (spikes) from the cell to dendrites of other neurons

4. postsynaptic
neuron
science-education.nih.gov

5. Synapse site of communication between two cells
formed when an axon of a presynaptic cell “connects” with the dendrites of a postsynaptic cell

6. bipolar.about.com/library
Synapse
axon of presynaptic
neuron
dendrite of
postsynaptic
neuron
bipolar.about.com/library

7. Synapse a synapse can be excitatory or inhibitory
arrival of activity at an excitatory synapse depolarizes the local membrane potential of the postsynaptic cell and makes the cell more prone to firing
arrival of activity at an inhibitory synapse hyperpolarizes the local membrane potential of the postsynaptic cell and makes it less prone to firing
the greater the synaptic strength, the greater the depolarization or hyperpolarization

9. Neural Communication: 1 Communication within and between cells

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

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

12. Resting potential of neuron = -70mV
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. +
outside + + + + -
inside - - - -
Resting potential of neuron = -70mV

13. Artist’s rendition of a typical cell membrane

14. 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.
Cl-
Na+
Na+
Cl-
Na+
Na+
outside
inside
Organic anions (-) K+
Organic anions (-) K+
Organic anions (-)

15. 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.
Cl-
Na+
Na+
Cl-
Na+
Na+
outside
inside
Organic anions (-) K+
Organic anions (-) K+
Organic anions (-)

16. 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.
Na+
Na+
Na+
outside
inside K+ K+

17. 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.

18. Before Depolarization

19. 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+ + - - +

20. Depolarization

21. Depolarization

22. 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.
Na+ K+ + -

23. Repolarization

25. Action potentials: Resuming the Resting Potential
Potassium channels close.
Repolarization resets sodium ion channels.
Ions diffuse away from the area.
Sodium-potassium transporter maintains polarization.
The membrane is now ready to “fire” again. K+
Na+ K+ K+
Na+

26. 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.

27. 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 1 2 3
[E]

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

30. 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.

31. 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.

32. 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.
Myelin sheath
Node of Ranvier

33. 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.

34. 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.
terminal
extracellular fluid
synaptic cleft
presynaptic membrane
dendritic spine
postsynaptic membrane

35. Neurotransmitter release
Synaptic vesicles, containing neurotransmitter, congregate at the presynaptic membrane.
The action potential causes voltage-gated calcium (Ca2+) channels to open; Ca2+ ions flood in.
vesicles
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+

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

38. 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

40. Ionotropic Receptors

41. Metabotropic Receptors (G-Protein)

43. Postsynaptic potentials
Depending on the type of ion channel which opens, the postsynaptic cell membrane becomes either depolarized or hyperpolarized.
Ions will tend to follow the concentration gradient from high to low concentration, and the electrostatic gradient towards the opposite charge.

44. 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.)
Na+
Ca2+ +
outside
inside -

45. 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)
Cl- +
outside
inside K+ -

46. Postsynaptic Ion motion

47. 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

48. 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

49. 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.

50. 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.

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

53. Motor Control: Basics

55. Hierarchical Organization of Motor System
Primary Motor Cortex and Premotor Areas

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

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

58. postsynaptic
neuron
science-education.nih.gov

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

62. Gaits of the cat: an informal computational model