1. Human Anatomy / Physiology
Nervous System

2. The human nervous system
Composed of cells called neurons:
Can carry rapid electrical impulses
Two main parts:
Central nervous system: brain, spinal cord
Peripheral nervous system: somatic (sensory and motor neurons) and autonomic neurons
Main functions:
CNS: integrate information and determine reaction, thought, memory, learning, instinct, balance, emotion, etc.
PNS: transfer information from body to CNS, and “decisions” to muscles, glands, etc.

3. Central Peripheral

4. Neurons Specialized cells that carry electrical impulses
node of Ranvier
Motor Neuron

5. Structure of a Motor Neuron
Dendrites receive information
Cell body / nucleus controls metabolism, functions
Axon sends signal
Myelin insulates neuron
Nodes of Ranvier carry electrical signal (saltatory conduction)
Motor end plate passes message to muscle cell
(motor end plate)
Node of
Ranvier

7. Types of Neurons
Sensory Neurons: transmit electrical impulses from sensory receptors to the CNS
Relay Neurons (Interneurons): move impulses inside the CNS
Motor Neurons: take impulses from CNS to effectors (glands/muscles)
Sensory Neuron
Motor Neuron

8. How we perceive information
A simple reflex
Stimulus
receptor
sensory neurons
relay neurons
(In the CNS)
motor neurons
Effectors
response 3 2 4 1 5 7 6

9. Reflex

10. Resting potential
The electrical potential across the membrane of a neuron that is NOT conducting an impulse
The neuron is “waiting”
The cell is negative inside
-70 millivolts (mV)
Plus Cl- and other negative ions 

11. Resting Potential and Active Transport
Sodium-potassium (Na+/K+) pump maintains the electrical potential
requires constant ATP
active transport, against concentration gradient
~20% of ATP is used to maintain readiness of neurons (resting potential)
SODIUM OUT!
POTASSIUM IN!

12. How the impulse is transmitted
Impulse begins when a neuron is stimulated by another neuron or by the environment
Electrical impulse moves in one direction:
Dendrites → Cell Body → Axon
Synapse: gap between 2 neurons
Neurotransmitters send the signal to the following neuron
No myelin = 5-25m/s
With myelin = m/s

13. Action Potential
The reversal (depolarization) and restoration (repolarization) of the electrical potential across the membrane of a nerve cell as an electrical impulse passes along it
Shows the electrical potential at one point of the axon over time.

15. Passage of a nerve impulse: RESTING
First the cell is at resting potential
The Na+/K+ pump is maintaining mV

16. Passage of a nerve impluse: DEPOLARIZATION
The cell becomes less negative
Diffusion of Na+ ions from the preceding portion of the cell or signal from cell body
Na+ channels open ONLY when -70 mV rises to -55mV (threshold)
As Na+ enters, the potential reaches +30mV

17. Passage of a nerve impulse: REPOLARIZATION
K+ channels open AFTER Na+ channels.
Diffusion of K+ out of the cell makes the potential negative again (-70mV) and Na+ channels close
For a short time the cells goes beyond -70mV to about -80mV (hyperpolarization)
During this time, the Na+ gates will not open (refractory period). This prevents action potential from traveling backwards.

18. Passage of a nerve impulse: Back to RESTING
The Na+ that entered will diffuse and open the next Na+ channels, and the signal continues
The resting potential is restored by Na+/K+ pump and K+ leakage

19. Passage of a nerve impulse: summary

21. Practice: Action Potential
Harvard Outreach Animation
Many animations available for this unit

22. Myelinated v. Nonmyelinated

23. Synaptic transmission
Along the axon, the signal is electrical
To pass from one neuron to another, it must cross a small space at a neural junction (the synapse); this signal is chemical

24. The Synapse Synapse = gap between neurons
Action potential cannot cross gap: neurotransmitters carry the impulse
Neurotransmitters: stored at the end on axons (glutamate, GABA, acetylcholine, norepinephrine, dopamine, serotonin, nitric oxide, etc)
Voltage Ca+2 gated ions open → calcium flows inside neuron
Calcium help vesicles fuse with membrane → neurotransmitters are released
These bind with neuroreceptors
Voltage gated ions are activated = depolarization
Impulse is passed on to post-synaptic neuron
Neurotransmitters = broken by enzymes and reabsorbed by pre-synaptic neuron

25. Synaptic transmission

26. Synaptic transmission (common example)
Action potential opens Ca2+ gates
Ca2+ cause vesicles filled with neurotransmitter (ex. Acetylcholine, dopamine, seratonin) to release them through exocytosis
The neurotransmitters diffuse across the synapse and bind to receptors.
Na+ gates open in the post-synaptic neuronand start a new action potential
The neurotransmitter is broken down (ex. By cholinesterase) and recycled and Ca2+ is removed
This would excite the post-synaptic neuron, but other systems can inhibit / hyperpolarize it.

27. Visual summary of synaptic transmission