1. Neurophysiology

2. Nervous System Functions:
Sensory Input – monitoring stimuli occurring inside and outside the body
Integration – interpretation of sensory input
Motor Output – response to stimuli by activating effector organs

3. Organization of the Nervous System
CNS
Brain and Spinal Cord (in dorsal body cavity)
Integration and command center – interprets sensory input and responds to input
PNS
Paired Spinal and Cranial nerves
Carries messages to and from the spinal cord and brain – links parts of the body to the CNS

4. PNS - Two Functional Divisions
Sensory (afferent) Division
Somatic afferent nerves – carry impulses from skin, skeletal muscles, and joints to the CNS
Visceral afferent nerves – transmit impulses from visceral organs to the CNS
Motor (efferent) Division
Transmits impulses from the CNS to effector organs, muscles and glands, to effect (bring about) a motor response

5. Motor Division: two subdivisions
Somatic Nervous System (voluntary)
Somatic motor nerve fibers (axons) that conduct impulses from CNS to Skeletal muscles – allows conscious control of skeletal muscles
Autonomic Nervous System (ANS) (involuntary)
Visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands
Two functional divisions – sympathetic and parasympathetic

6. Levels of Organization in the Nervous System

10. Membrane Potentials: Signals
Two types of signals are produced by a change in membrane potential: graded potentials (short-distance) action potentials (long-distance)

11. Graded Potentials
1-Short-lived, local changes in membrane potential (either depolarizations or hyperpolarizations)
2-Cause currents that decreases in magnitude with distance
3-Their magnitude varies directly with the strength of the stimulus – the stronger the stimulus the more the voltage changes and the farther the current goes
4-Sufficiently strong graded potentials can initiate action potentials

12. Action Potentials (APs)
An action potential in the axon of a neuron is called a nerve impulse and is the way neurons communicate.
The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to +30mV
APs do not decrease in strength with distance
The depolarization phase is followed by a repolarization phase and often a short period of hyperpolarization
All-or-None phenomenon – action potentials either happen completely, or not at all

13. Propagation of an Action Potential
The action potential is self-propagating and moves away from the stimulus (point of origin)

14. Stimulus Intensity
All action potentials are alike and are independent of stimulus intensity
How can CNS determine if a stimulus intense or weak?
Strong stimuli can generate an action potential more often than weaker stimuli and the CNS determines stimulus intensity by the frequency of impulse transmission

15. Axon Conduction Velocities
Conduction velocities vary widely among neurons
Determined mainly by:
Axon Diameter – the larger the diameter, the faster the impulse (less resistance)
Presence of a Myelin Sheath – myelination increases impulse speed (Continuous vs. Saltatory Conduction)

16. Saltatory Conduction
Current passes through a myelinated axon only at the nodes of Ranvier
Voltage-gated Na+ channels are concentrated at these nodes
Action potentials are triggered only at the nodes and jump from one node to the next
Much faster than conduction along unmyelinated axons

17. Saltatory Conduction
Current passes through a myelinated axon only at the nodes of Ranvier (Na+ channels concentrated at nodes)
Action potentials occur only at the nodes and jump from node to node

18. Erlanger and Gasser divided mammalian nerve fibers into A, B, and C groups, further subdividing the A group into α, β, γ, and δ fibers.

19. Fiber Type Origin Number A α Ia Ib A β II A δ III Dorsal root C IV
Muscle spindle, annulospinal ending. Ia
Golgi tendon organ. Ib
A β
Muscle spindle, flower-spray ending; touch, pressure. II
A δ
Pain and cold receptors; some touch receptors.
III
Dorsal root C
Pain, temperature, and other receptors. IV

20. Synapses

21. Synapse
A junction that mediates information transfer from one neuron to another neuron
Presynaptic neuron – conducts impulses toward the synapse (sender)
Postsynaptic neuron – transmits impulses away from the synapse (receiver)

22. Types of Synapses
Axodendritic – synapse between the axon of one neuron and the dendrite of another
Axosomatic – synapse between the axon of one neuron and the soma of another
Other types:
Axoaxonic (axon to axon)
Dendrodendritic (dendrite to dendrite)
Dendrosomatic (dendrites to soma)

23. Synapses

24. Synapses can be…
Electrical
CHEMICAL!

25. Electrical Synapses Less common than chemical synapses
Gap junctions allow neurons to be electrically coupled as ions can flow directly from neuron to neuron - provide a means to synchronize activity of neurons

26. Electrical Synapse Electrical synapses gap junctions (connexins)
smooth and cardiac muscles, glial cells
Only a few examples of GJ have been found in the central nervous system

27. Electrical Synapse

28. Chemical Synapse One way conducton
Functional connection between a neuron and another neuron (or effector cell such as muscle, gland).
One way conducton

29. Chemical Synapses
Specialized for the release and reception of chemical neurotransmitters
Typically composed of two parts:
Axon terminal of the presynaptic neuron containing membrane-bound synaptic vesicles
Receptor region on the dendrite(s) or soma of the postsynaptic neuron

30. The synaptic terminal contains numerous vesicles that enclose a neurotransmitter for which the postsynaptic neuron has membrane receptors. When an action potential enters the synaptic terminal of the presynaptic neuron, the vesicles dump their neurotransmitter into the gap between the neurons. The neurotransmitter diffuses rapidly across the space, binds to postsynaptic receptors, and causes ion channels to open. Ions flow through these open channels, causing a postsynaptic potential in the postsynaptic cell.

31. Synaptic Cleft
Fluid-filled space separating the presynaptic and postsynaptic neurons, prevents nerve impulses from directly passing from one neuron to the next
Transmission across the synaptic cleft:
Is a chemical event (as opposed to an electrical one)
Ensures unidirectional communication between neurons

34. Synapse AP comes down the axon.
At the synapses, VG Ca++ channels let in calcium.
This triggers the release (exocytosis!) of the contents
of vesicles in the axonal bouton.
The contents are: NEUROTRANSMITTERS.

35. Synapse
Neurotransmitters (NT) cross the narrow synaptic space and bind to receptors on the dendrite (or other cell).
This causes a response in the postsynaptic cell.
The whole cycle starts again in this second cell!

36. Postsynaptic Potentials
EPSP (excitatory postsynaptic potential):
Depolarization.
Brings cell closer to threshold for an AP.
Often Na+ channels.
IPSP (inhibitory postsynaptic potential):
Hyperpolarization.
Takes cell further away from threshold for an AP.
Often Cl- and K+ channels.

37. Excitatory Postsynaptic Potentials
EPSPs are local graded depolarization events that can initiate an action potential in an axon
Postsynaptic membranes do not generate action potentials. The currents created by EPSPs decline with distance, but can spread to the axon hillock and depolarize the axon to threshold leading to an action potential

38. Inhibitory Postsynaptic Potentials
Neurotransmitter binding to a receptor at inhibitory synapses reduces a postsynaptic neuron’s ability to generate an action potential
Postsynaptic membrane is hyperpolarized due to increased permeability to K+ and/or Cl- ions. Leaves the charge on the inner membrane face more negative and the neuron becomes less likely to “fire”.

40. Summation
A single EPSP cannot induce an action potential EPSPs must summate (add together) to induce an AP
Temporal Summation – presynaptic neurons transmit impulses in quick succession
Spatial Summation – postsynaptic neuron is stimulated by a large number of terminals at the same time
IPSPs also summate and can summate with EPSPs.

41. Summation

42. EPSP

44. So… AP flies down axon of first neuron. NT are released at synapse.
Receptors bind NT and produce an EPSP or IPSP in postsynaptic neuron.
The sum of the inputs -> AP in this second neuron.

45. Neurotransmitters
Chemicals used for neuron communication with the body and the brain
More than 50 different neurotransmitters have been identified
Classified chemically and functionally

46. Neurotransmitters Small molecules, Rapidly acting
Cause most acute responses of the nervous system such as:
Transmission of sensory signals to the brain
Transmission of motor signals to the muscles

47. Small molecules Rapidly acting
Synthesized in presynaptic terminals
Absorbed by means active transport to the vesicle

48. Neurotransmitters – Chemical classification
Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Novel messengers: ATP and dissolved gases NO and CO

49. Neurotransmitters: Acetylcholine
Released at the neuromuscular junction
Enclosed in synaptic vesicles
Degraded by the acetylcholinesterase (AChE)
Released by:
All neurons that stimulate skeletal muscle
Some neurons in the autonomic nervous system

50. Neurotransmitters: Biogenic Amines
Include: Catecholamines – dopamine, norepinephrine, and epinephrine Indolamines – serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and our biological clock

51. Neurotransmitter Receptor Mechanisms
Direct: neurotransmitters that open ion channels
Promote rapid responses
Examples: ACh and amino acids
Indirect: neurotransmitters that act through second messengers
Promote long-lasting effects
Examples: biogenic amines, peptides, and dissolved gases

53. GABAA Receptor

54. Termination of Neurotransmitter Effects
Neurotransmitter bound to a postsynaptic neuron produces a continuous postsynaptic effect and also blocks reception of additional “messages”
Terminating Mechanisms:
1- Degradation by enzymes
2- Uptake by astrocytes or the presynaptic terminals
3- Diffusion away from the synaptic cleft

55. Neuropeptides Large molecules, slowly acting
Cause more prolonged actions such as:
Long term changes in number of neuronal receptors
Long term opening / closure or of certain ion channels
Long term changes in numbers of synapses or sizes of synapses

56. Neuropeptides:
Are generally thousand or more times as potent as small molecules

57. Neuropeptides Are synthesized by ribosome in cell body
The vesicles are transported to the terminal
Much smaller quantities released than small molecules

58. Neuropeptide Transmission

59. Synaptic Delay
Neurotransmitter must be released, diffuse across the synapse, and bind to receptors ( ms)
Synaptic delay is the rate-limiting step of neural transmission

60. Fatigue of synaptic transmission
Exhaustion or partially exhaustion of neurotransmitter stores
Progressive inactivation of postsynaptic receptors
Slow development of abnormal concentrations of ions inside the postsynaptic neuron

61. Effect of acidosis and alkalosis on synaptic transmission
Alkalosis increases neuronal excitability
-Overbreathing can precipitate an epileptic attack
Acidosis depresses the neuronal activity
-in very sever diabetic acidosis, coma always develops

62. Effect hypoxia on synaptic transmission
Neuronal excitability is highly dependent on adequate supply of oxygen
Cessation of oxygen for only a few seconds can cause inexcitability of some neurons
When brain blood flow interrupted the person becomes unconscious

63. Effect drugs on synaptic transmission
Caffeine (found in the coffee), theophylline( tea) and theobromine(cocoa) increase neuronal excitability
-By reducing threshold for excitation of neurons
Anesthetics increase threshold