BIO310H p. 113-117 & 155-157 01/10/13
Lecture 2 & 3
Communicate information using a combination of electrical and chemical signals.
Most are electrically excitable electrical signals can be generated across the plasma
membrane and transmitted along the length of the cells without loss of signal strength, as
a result of the movement of ions.
Individual neurons are most studied of all types because:
1) Their electrical properties can be easily investigated.
2) Neurons function similarly in all types of animals.
3) Neurons process information, while relying on a small number of basic physical and
neurons + glial cells = supporting cells
Collects and processes information, analyzes it, and generates output to control the
animal’s responses from the simplest to the most complex.
Structure, Function, and the Organization of the Neuron
Soma/cell body: responsible for the metabolic maintenance of the cell. Most neurons
possess multiple dendrites and a single axon.
Dendrites: branched; receivers that gather signals from other neurons and carry them
toward the soma.
Axons/nerve fibers: specialized nerve extensions of the neuron that conduct signals away
from the soma.
During the embryonic development of each neuron, the dendrites and the axon grow
outward from the soma.
If an axon in an adult animal is damaged, it typically degenerates back to the soma within
a few days or weeks. In mammals, regeneration of axons takes place in the periphery of
the body. In cold-blooded vertebrates, regeneration of axons takes place within the CNS.
Damaged neurons in many invertebrates readily regenerate and reestablish connections
with their original targets.
Spike-initiating zone: integrates signals from many input neurons to determine whether
the neuron will initiate its own signal action potential (AP). Located at or near the
junction between the axon and the soma, a region called axon hillock.
Action potentials/spikes/nerve impulses: the voltage across the membrane rapidly rises
and then falls.
The axon carries an AP from its point of origin in the spike-initiating zone to the axon
terminals, which transmit the signal to other cells.
Motor neurons: signal travels from the axon terminals to skeletal muscle fibers.
Passive electrical properties: capacitance and resistance
Active electrical properties: allow them to conduct electrical signals without decrement
(i.e. with no loss of signal strength). Depends on the presence of voltage-gated ion
channels in the plasma membrane.
The axonal membrane is specialized for the conduction of APs by virtue of fast acting,
voltage-gated ion channels that selectively allow Na+ and K+ to cross the membrane. In
addition, the plasma membrane of the axon terminals contains voltage-gated Ca2+ BIO310H p. 113-117 & 155-157 01/10/13
channels and other specializations that allow neurons to transmit signals to other cells
when APs invade the terminals.
Afferent fiber: axon of a sensory neuron; conducts a signal inward toward higher
processing centers in the brain.
Efferent neurons: carry information from the processing regions of the CNS outward to
Synapses: location where information is passed between neurons.
Neuronal circuit: afferent neurons, efferent neurons, and interneurons.
Presynaptic cell: passes information to a particular neuron.
Postsynaptic cell: receives information transmitted across a synapse.
Most synaptic transmission is carried by chemical neurotransmitters, which are released
from the axon terminals of the presynaptic neuron.
The plasma membrane of the postsynaptic neuron’s dendrites and soma contains ligand-
gated ion channels that bind neurotransmitters and cause the postsynaptic cell to respond
to the presence of the chemical signal.
Information is carried in neuronal circuits via alternating chemical and e