1. Part 1 of 2 Nervous communication
Coordination Chapter 15
Part 1 of 2
Nervous communication

2. Nervous communication
In animals there are two types of information transfer
Electrical impulses – nerves
Chemical messengers – hormones
Communication systems within animals
Receptors (input) receive information from within and outside the organism
Effectors (output) include muscles and glands

3. Receptors/effectors

6. Mammalian nervous system
Central nervous system (CNS) – brain & Spinal cord
Peripheral nervous system (PNS) – cranial & spinal nerves

7. Neurons Nerves = neurons 3 types of neurons
Sensory neurons – impulse from receptor to CNS
Intermediate neurons – impulse from sensory neurons to motor neurons (a.k.a. relay or connector neurons)
Motor neurons – impulse from CNS to effector

8. Types of neurons

9. Motor neurons
Body of motor neuron lies within the spinal cord or brain
Dendrites – often short cytoplasmic extensions with many branches, impulse receiving end of neuron
Axons – long extensions carrying the output impulse. Mitochondria are found along the axon with many mitochondria at the axonal terminal with vesicles containing neurotransmitter chemicals.

10. Sensory neuron Same basic structure as motor neuron
Body near source of stimuli or ganglion (cluster of neurons)
Long axon

11. Relay neurons
Relay neurons or intermediate neurons are only found in the CNS

12. Pyramidal neurons Pyramidal cells are motor and relay neurons
Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex. Pyramidal neurons are also one of two cell types where the characteristic sign, Negri bodies, are found in post-mortem rabies infection.
Pyramidal neurons in the prefrontal cortex are implicated in cognitive ability.

13. Myelin (a.k.a. myelin sheath)
Myelin is the term used to describe Schwann cells wrapped around the axon of neurons
1/3 of motor and sensory neurons are myelinated
This wrapping is referred to as myelin sheath
Myelin sheath is made of lipid and protein
Myelin sheath affects the speed of impulse conduction

14. Myelin (a.k.a. myelin sheath) cont.
The small uncovered areas between Schwann cells are called nodes of Ranvier
Nodes of Ranvier are 2-3 micrometers long
They occur every 1-3 millimeters

15. Myelin Sheath cont.

16. Oligodendrocytes
Equivalent to the function performed by Schwann cells in the peripheral nervous system.
Oligodendrocytes do this by creating the myelin sheath, which is 80% lipid and 20% protein.
A single oligodendrocyte can extend its processes to 50 axons
Schwann cells, on the other hand, can wrap around only one axon.

17. Oligodendrocyte

18. Nerve fiber

19. Reflex arc
A reflex arc is the pathway by which an involuntary response to danger signals occurs (reflex action)
Touching hot object
Sight of an object flying towards you

20. Transmission of nerve impulse
Fast traveling electrical impulses along the surface membrane of neuron
Very brief changes in the distribution of electrical charge across the membrane (action potential)
Rapid movement of sodium and potassium ions into and out of axon

21. Resting potential
The inside of axons are always slightly negative compared to the outside (potential difference)
60-70 mV lower inside (-60 to -70 mV)
Produced and maintained by sodium-potassium pumps in membrane
3 Na+ out for every 2 K+ in
A form of active transport (ATP is hydrolysed)

22. Resting potential cont.

23. Resting potential cont.
The axons of squid and earthworms are wide enough to insert tiny electrode into cytoplasm to measure the change in electrical charge

24. Resting potential
Membrane protein channels for K+ and Na+ remain open all the time
More for K+ than for Na+
Membrane relatively impermeable to Na+
Steep concentration gradient + negatively charged inside make for an electrochemical gradient

25. Electrochemical gradient

26. Action potential
In addition to the Na+/K+ pumps that are always open there are channels that allow the reverse movement of ions. These are called voltage-gated channels
Voltage-gated channels remain closed until the action potential threshold is reached.
-70 mV to +30 mV back to -70 mV in 3 milliseconds (ms)

27. Action potential (positive feedback)
Action potentials in mammals are only generated if the stimulus causes a potential difference to reach about 55 mV – the threshold potential
At 55 mV channels open and the inside of the axon reaches +30 mV compared to outside axon
This is termed depolarization

28. Repolarization of axon
After 1 millisecond, Na+ voltage gated channels close
Simultaneously, K+ channels open returning the potential difference to – 70 mV
As the action potential moves down the axon towards the axonal terminal, there is a refractory period when the axon is unresponsive to another stimuli

30. Consequences of refractory period
Action potentials don’t merge into one another
There is a minimum time between action potentials
The length of the refractory period determines the maximum frequency impulses can be transmitted

32. Action potential

33. How action potentials carry information
Action potentials do not change in size
Nor they change in size due to intensity of stimulus no matter how long the axon is
Speed of action potential is always the same
The difference from strong or weak stimulus is the frequency – or how many neurons are carrying the signal
Action potentials are an all-or-none response (all or none law)

34. Speed of conduction
Unmyelinated neurons – 0.5 meters per second (m s-1)
Myelinated neurons – 100 m s-1
Depolarization can only occur at nodes of Ranvier where all the channel proteins and pump protein are concentrated
Action potentials jump from node to node (saltatory conduction)

35. Speed of conduction cont.
Diameter affects speed of transmission
Thick axons faster than thin axons

36. Starting an action potential
Receptor cells respond to a stimulus (light, touch, sound, temperature, chemicals) receptor potential
Receptor cells turn stimulus into electrical impulse
If receptor potential is large enough, it will open calcium ion (Ca2+) channels
Ca2+ enter cytoplasm and cause exocytosis of vesicles containing neurotransmitters
Neurotransmitters stimulate action potential in sensory neuron

37. synapses
There is a 20 nm gap called the synaptic cleft separating neurons
Action potential starts at the pre-synaptic neuron
Action potential causes the release of neurotransmitters into the synaptic cleft
Neurotransmitters bind to receptor proteins on the post synaptic neuron
If the threshold is reached, then it will initiate depolarization of the subsequent neuron

38. neurotransmitters More than 40 neurotransmitters are known
Noradrenaline and acetylcholine (ACh) are found throughout the nervous system
Dopamine, glutamic acid and gamma-aminobutyric acid (GABA) only occur in the brain

39. Cholinergic synapses Cholinergic synapses use acetylcholine (ACh)
Action potential arrives at axonal terminal
Voltage-gated channels open for calcium ions (Ca2+)
This causes the vesicles containing Ach to fuse with membrane and through exocytosis releases ACh which crosses the synaptic cleft
ACh binds with receptors on in on the post-synaptic neuron causing Na+ channels to open
This depolarizes membrane causing an action potential

41. Synapse cont.
ACh is recycled by acetylcholinesterase into acetate and choline
Choline is taken back into the presynaptic neuron and combine with coenzyme A to make ACh again
This all happens in 5-10 ms
Synapses are a one-way street