1. The Nervous System
6.5 Neurons and Synapses

2. The Nervous System
Neurons– specialized cells that carry rapid electrical impulses.
The nervous system is composed of neurons (the brain alone is estimated to have 100 billion)
Central nervous system (CNS)~ brain and spinal cord
Peripheral nerves-- sensory and motor neurons
Effector cells~ muscle or gland cells

3. Neuron Structure Neuron~
Dendrites~ receive and carry impulses to cell body
Cell body~ contains nucleus and organelles
Axon~ carries impulses away from cell body to other neurons or muscle.

4. Neuron animation

5. Neuron function
Neurons transmit electrical impulses.

6. Neuron Function details

7. Neuron Function (Resting Neuron)
Neurons pump sodium and potassium ions across their membranes to generate a resting potential.(-70mV inside neuron)
Sodium potassium pump (3 Na+ out/ 2 K+ in)
Note: The cell is POLARIZED at rest!

8. Neuron Function (Response to stimulus)
Stimulus causes:
Depolarization (inflow of Na+ through voltage-gated ion channels)
This causes the cell to become less negative
A nerve impulse is only initiated if the threshold potential is reached.

9. Neuron Function (Action Potential)
• An action potential consists of:
depolarization (as Na+ rushes in)
repolarization of the neuron (as K + rushes out)
Propagation of the action potential down the axon is a nerve impulse!
local currents cause each successive part of the axon to reach the threshold potential.
ALL-OR-NONE– full action potential or none at all…
Action potential practice and animation:

10. Overview
STAGES
1-Resting state •both channels (sodium and potassium) closed. Sodium Potassium pump maintains gradient.
2-Threshold • if reached, a stimulus opens Na+ channels
3-Depolarization •action potential generated •Na+ channels open; cell becomes positive (K+ channels closed)
4-Repolarization •Na+ channels close, K+ channels open; K+ leaves •cell becomes negative
5-Undershoot •both gates close, but K+ channel is slow; causes hyperpolarization
Refractory period~ insensitive to depolarization due to closing of Na+ gates
Start over…

11. The Action Potential

13. The Resting Potential vs. Action Potential

14. Annotate the action potential diagram

15. Neuron Function: Myelin sheath
Myelin sheath speeds nerve impulses (multiple sclerosis results from degeneration of myelin)
The myelination of nerve fibres allows for saltatory conduction. 150m/sec
Nodes of Ranvier (ion channels- between Schwann cells of myelin sheath)
Animation:

16. Synapses
Synapses are junctions between neurons and between neurons and receptor or effector cells.

17. Synapse structure
Animation:
Presynaptic neuron: transmitting cell (releases neurotransmitters)
Postsynaptic cell: receiving cell
Synaptic cleft: gap between axon’s synaptic terminal and next cell.
Synaptic vesicles: neurotransmitter releasers

18. Synapse Function
Cool animation
Ca+2 influx: caused by action potential reaching synapse
vesicles fuse with presynaptic membrane and release….
Neurotransmitters
Diffuse across synapse and attach to receptors– can cause:
depolarization of next cell (excitatory) or
hyperpolarization (inhibitory)
After stimulation, neurotransmitters are degraded or taken up by presynaptic cell.

19. Application: Secretion and reabsorption of acetylcholine by neurons at synapses.
Watch this animation and take notes:

20. Application: neonicotinoid pesticides
Block synaptic transmission at cholinergic synapses in insects
irreversibly bind to postsynaptic acetylcholine receptors
These synapses occur in insect brains, so blocking transmission = death.
In mammals, neonicotinoids don’t bind as strongly…

22. Utilization: An understanding of the workings of neurotransmitters and synapses has led to the development of numerous pharmaceuticals for the treatment of mental disorders.

23. Nature of science: Cooperation and collaboration between groups of scientists—biologists are contributing to research into memory and learning. (4.3)

24. Essential idea: Neurons transmit the message, synapses modulate the message.

25. End of IB stuff

26. Draw and Label: a Motor Neuron
Include:
Dendrites
Cell body w/ nucleus
Axon
Myelin sheath
Nodes of Ranvier
Motor end plates (the end of the motor neuron– transmits signal to muscle)

27. Simple Nerve Circuit
Sensory neurons: conduct information from receptors to CNS
Relay Neurons (Interneurons): interpret information in CNS
Motor neurons: conduct signals from CNS to effector cells (muscle or gland)
Reflex: simple response; sensory to motor neurons (doesn’t reach brain, just spinal cord)

28. Glial cells
Supporting cells/glia: non-conducting cell that provides support, insulation, and protection
Myelin sheath speeds nerve impulses (multiple sclerosis results from degeneration of myelin)
Blood brain barrier– tight junctions caused to form by astrocytes (type of glial cell)-- restricts passage of most substances into brain.

29. Neurotransmitters Acetylcholine (most common) •excites skeletal muscle
Biogenic amines (derived from amino acids) Mostly in CNS •epinephrine/norepinephrine •dopamine – sleep, mood, attention, learning
*lack– Parkinson’s
*excess– schizophrenia
•serotonin--sleep, mood, attention, learning
Amino acids
GABA, Glycine = inhibitory
Glutamate, aspartate= excitatory
Neuropeptides (short chains of amino acids)
•endorphins– usu. Inhibitory
effect of drugs on neurotransmission

30. Vertebrate PNS Divisions of PNS
somatic system--voluntary, conscious control
autonomic system a.parasympathetic division conservation of energy (calm)
b. sympathetic division increase energy consumption (fight or flight)
Cranial nerves (brain origin)
Spinal nerves (spine origin)

31. The Vertebrate Brain Forebrain •cerebrum~memory, learning, emotion
•cerebral cortex~sensory and motor nerve cell bodies, complex behavior, thought.
corpus callosum~connects left and right hemispheres
Thalamus– sensory input, motor output for cerebrum;
Hypothalamus– homeostasis! (how?)
Midbrain •inferior (auditory) and superior (visual) colliculi
Hindbrain •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions (breathing, circ, digestion etc.

32. Cut Stuff
Schwann cells~ support cells that form myelin sheath (in PNS)
Oligodendrocytes-- support cells that form myelin sheath (in CNS)
1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative