neurons and synapses

nervous system

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)
Figure 11.4b
Contains the nucleus and a
nucleolus
Contains an axon hillock –
cone-shaped area from
which axons arise

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 (V
r)
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

20. Action Potential: Repolarization Phase
Sodium channel close, K
+
channel open
K
+
exits the cell and internal negativity of the resting
neuron is restored

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
Figure 11.12.4

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 (-70 +30 mV)
3 –repolarization phase (+30 -70 mV)
4 –hyperpolarization ( overshoots -70 mV)
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