1. 6.5 Neurons and synapses
Essential idea: Neurons transmit the message, synapses modulate the message.
Nature of science:
Cooperation and collaboration between groups of scientists— biologists are contributing to research into memory and learning. (4.3)

2. Understandings: Neurons transmit electrical impulses.
The myelination of nerve fibres allows for saltatory conduction.
Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
An action potential consists of depolarization and repolarization of the neuron.
Nerve impulses are action potentials propagated along the axons of neurons.
Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.
Synapses are junctions between neurons and between neurons and receptor or effector cells.
When presynaptic neurons are depolarized they release a neurotransmitter into the synapse.
A nerve impulse is only initiated if the threshold potential is reached.

3. Applications and skills:
Application: Secretion and reabsorption of acetylcholine by neurons at synapses.
Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.
Skill: Analysis of oscilloscope traces showing resting potentials and action potentials.

4. Nervous System
The master controlling and communicating system of the body
Functions
Sensory input – monitoring stimuli
Integration – interpretation of sensory input
Motor output – response to stimuli

5. Organization of the Nervous System
Central nervous system (CNS)
Brain and spinal cord
Integration and command center
Peripheral nervous system (PNS)
Paired spinal and cranial nerves
Carries messages to and from the spinal cord and brain

6. Histology of Nerve Tissue
The two principal cell types of the nervous system are:
Neurons – excitable cells that transmit electrical signals
Supporting cells – cells that surround and wrap neurons

7. Neurons (Nerve Cells) Structural units of the nervous system
Composed of a body, axon, and dendrites
Long-lived, amitotic, and have a high metabolic rate
Their plasma membrane function in:
Electrical signaling
Cell-to-cell signaling during development

8. Cell body (soma) Contains the nucleus and a nucleolus
Contains an axon hillock – cone-shaped area from which axons arise
Figure 11.4b

9. Dendrites Short, tapering, and diffusely branched processes
receptive regions of the neuron

10. Axons Slender processes of uniform diameter arising from the hillock
Usually there is only one unbranched axon per neuron
Axonal terminal – branched terminus of an axon

11. Neuron Classification
Functional:
Sensory (afferent) — transmit impulses toward the CNS
Motor (efferent) — carry impulses away from the CNS
Interneurons (relay neurons) — shuttle signals through CNS pathways

12. Neurophysiology Neurons are highly irritable
Action potentials, or nerve impulses, are:
Electrical impulses carried along the length of axons
Always the same regardless of stimulus
The underlying functional feature of the nervous system

13. Gated Channels When gated channels are open:
Ions move quickly across the membrane
Movement is along their electrochemical gradients
An electrical current is created
Voltage changes across the membrane

14. Electrochemical Gradient
chemical gradient - when ions move from high concentration to low concentration
electrical gradient - when ions move toward an area of opposite charge
electrochemical gradient – the electrical and chemical gradients taken together

15. Resting Membrane Potential (Vr)
potential difference (–70 mV) across the membrane of a resting neuron
Differential permeability to Na+ and K+
sodium-potassium pump

16. Changes in Membrane Potential
Changes are caused by three events
Depolarization – the inside of the membrane becomes less negative
Repolarization – the membrane returns to its resting membrane potential
Hyperpolarization – the inside of the membrane becomes more negative than the resting potential

17. Action Potentials (APs)
brief reversal of membrane potential
only generated by muscle cells and neurons
do not decrease in strength over distance

18. Resting Potential Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+

19. Action Potential: Depolarization Phase
Na+ gates are opened; K+ gates are closed
positive sodium flows in, depolarization begins. Then the influx creates a self-propagating depolarization. Explosive positive feedback. Lasts 1 millisec.

20. Action Potential: Repolarization Phase
Sodium channel close, K+ channel open
K+ exits the cell and internal negativity of the resting neuron is restored
Inactivation gates “swing shut” sodium influx stops. SLOW voltage gated potassium channels open and K+ flows OUT of cell. Repolarization “overshoots”

21. Action Potential: Hyperpolarization
Potassium gates remain open, causing an excessive efflux of K+
This efflux causes hyperpolarization of the membrane (undershoot)
The neuron is insensitive to stimulus and depolarization during this time
THIS RESTORES POTENTIAL BUT NOT THE CHEMICAL GRADIENTS!!!
Figure

22. Action Potential: Role of the Sodium-Potassium Pump
Repolarization
Restores the resting electrical conditions of the neuron
Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is restored by the sodium- potassium pump

23. Phases of the Action Potential
1 – resting state (-70 mV)
2 – depolarization phase ( mV)
3 – repolarization phase ( mV)
4 – hyperpolarization ( overshoots -70 mV)
graph shows the action potential and the permiability of the plasma membrane to Na and K
Figure 11.12

24. Propagation of an Action Potential (Time = 0ms)
Na+ influx causes a patch of the axonal membrane to depolarize
Positive ions in the axoplasm move toward the polarized (negative) portion of the membrane

25. Propagation of an Action Potential (Time = 2ms)
Ions of the extracellular fluid move toward the area of greatest negative charge
A current is created that depolarizes the adjacent membrane in a forward direction
The impulse propagates away from its point of origin

26. Propagation of an Action Potential (Time = 4ms)
The action potential moves away from the stimulus
Where sodium gates are closing, potassium gates are open and create a current flow