Chapter 2 Review: Physiology
By the end of this section you should know about:
- Action potentials
- Basic neural circuitry
- Excitation and inhibition
- Receptive fields
- Sensory coding theories
Specialized cells for reception, conductions and transmission of electrochemical signals
Neurons: Basic Anatomy
Dendrites: receivers of information from other neurons
o Comes in the form of electricity from another neuron
o Neurons connected dendrite to axon and the information flows
End part of the axon – axon terminal/axon button
o Holds all the neurotransmitters that will transmit the signals
o The receptor: a specific neuron type that is specialized to respond to specific environmental
stimuli (e.g. light receptors = rods, cones)
o Figure 2.5 of text – each receptor is specialized to transmit a specific type of environmental
energy into electivity (vision, hearing, touch, smell, taste)
What’s a “Potential”?
Membrane potential: the difference in electrical charge between the inside and outside of the cell
o The membrane of a neuron maintains an electrical gradient– difference in electrical charge
between inside and outside
Resting potential: -70 mV
o In the absence of any outside disturbance, the membrane maintains an electrical polarization or
charge. The outside of the neuron is slightly negative with respect to the inside mainly because
of the negatively charged proteins inside the cell.
Potential: difference in charge from one to another
o Neurons are lightly charged all of the time
o The outside of the neuron is more negative than the inside
o Neurons are never happy doing nothing – always randomly firing off – always have a little bit of
Cell Membrane is Selectively Permeable
The neuron separates positive from negative charges
Each neuron is bounded by a membrane separating the intracellular and extracellular fluids, both of
which contain ions (atoms bearing an electrical charge)
We are only interested in 2 kinds of ions: positively charged ions and negatively charged ions.
o Inside the axon, there is a higher concentration of negative ions, making it negative overall
o Outside the axon, there is a higher concentration of positive ions, making it positive on the
o The membrane of the axon will not let some ions pass through (large positive ions).
The neuron likes to separate out the different charges by creating a membrane
o The membrane is selectively permeable – allows some things in and keeps other things out o Ion channels either open or closed – usually mostly closed but open to send an action potential or
create a neuron impulse
When a message for an action potential occurs, the channels begin to open and this allows positive
sodium ions on the outside to rush inside the cell
o Changes the membrane potential to more positive – increases from -70 to 40 in short amount of
o Forcing the negative ions out of the cell – lets out too many negative ions and needs to let a few
The Action Potential
When a neuron receives a signal from other neuron (sensory receptors), it fires an impulse. This
impulse is called the action potential.
When the neuron fires, it changes its resting potential by opening up some of the ion channels to
allow a flood of positive ions into the cell (sodium ions).
o This is called DEPOLARIZATION.
o Once enough ion channels open, the neuron reaches firing threshold, and triggers an action
potential, or nerve impulse.
This action potential is propagated to other neurons connected to the firing neuron.
The REFRACTORY period is the resting pause in which the neuron pumps all the positive ions back
outside to allow it to fire again.
o No action potentials can occur until the potential goes back to rest (-70)
The All or None Law
A neuron fires with the same potency each time, although frequency of firing can vary
There is no partial firing – either there is an action potential or there isn’t
o E.g. taking a picture – either you take a picture or you don’t; you can’t partially take one
The rate of firing depends on the strength of the stimulus that is creating the action potential in the
Interactions Between Neurons: Circuits
Neural circuit: groups of interconnected neurons
How inhibition and excitation work in terms of neural circuitry
The most simple circuit is one-to-one neural circuit: one neuron connection to another Convergence
Synapsing of more than one neuron onto a single neuron
All excitatory – bringing action potentials closer to firing
Convergence with Inhibition
Excitatory inputs converging onto an inhibitory synapse
3 excitatory inputs and 2 inhibitory inputs via A and C from 1,2, 6, and 7
Purely additive or subtractive view