• University of Toronto

• BIO 304

BIO304 Physiology of Neurons and Muscle
Lecture 1 Notes – Introduction and Visualizing the Nervous System
What is the Nervous System?
1. The nervous system is made up of a network of cells that can sense information
a. About the “outside” world (light, chemicals, temperature, touch)
b. About the “inside” world (internal states, signals from other cells)
2. Cells (neurons) are able to propagate information in the form of electrical impulses
a. Graded potentials
b. Action potentials
3. Cells convey information to each other via chemical and/or electrical synapses s
4. The nervous system generates outputs to coordinate physiology and behaviour
a. Ex. feeding, courtship, locomotion
What are the cells of the nervous system?
 Motor neuron
 Mitral cell
 Pyramidal cell
 Purkinje cell
 Ganglion cell
What are photoreceptor cells?
 Cells in the retina that have special proteins that detect light to trigger a change in voltage
across the cell membrane
o Light is converted into an electrical signal in the receptor cells
 Electrical signals travel to the synapse, where they trigger the release of chemicals
(neurotransmitters) that bind neurotransmitter receptors on post-synaptic bipolar cells
o i.e. chemical synapses convert electrical signals into chemical signals
 the neurotransmitter receptors on bipolar cells produce graded electrical responses
o stronger light  more NT secretion by receptor cells
o more NT  stronger change in membrane voltage (depolarization) of the bipolar
cell
o i.e. we are back to electrical signal
 graded responses reach bipolar cell nerve terminals which synapse onto ganglion cells
o neurotransmitters are secreted again
o bind to NT receptors on ganglion cells to trigger graded electrical responses
 when graded depolarization is strong enough, most neurons (like ganglion cells)
generation action potentials (Aps)
o All-or-None electrical responses that can travel very fast along nerves (i.e. axons)
What is the difference between Graded Potentials vs. Action Potentials?
 Graded potentials act locally and “fizzle out” over time
 Graded potentials can be excitatory (depolarizing) or inhibitory (hyperpolarizing)
o Excitatory Post-Synaptic Potentials = EPSPs
 Action potentials arise when graded potentials activate voltage-gated ion channels
 x/
2
o Unlike graded potentials, APs don’t dissipate and travel long distances
o Can travel up to 120 m/s along myelinate (insulated) axons
 APs are driven by voltage-gated Na+ and K+ ion channels
 Main function of action potentials in neurons:
o Transmit signals along axons
o Trigger pre-synaptic Ca2+ influx through voltage-gated calcium channels for the
release of neurotransmitters (fast exocytosis)