CSB332 Lecture 9
- Electrical impulses flow from first order sensory (afferent) neurons in the PNS to second order
sensory neurons located within the spinal cord. The signal is eventually relayed to the thalamus,
which is the major sensory relay station of the nervous system. From there, information is
delivered to different areas in the brain for further cortical and cognitive processing.
- One of the functions of cognitive processing is to be able to compare novel information with
information already stored in different areas of the cerebral cortex. Cognitive processing allows
the organism to activate the appropriate behavioural program in response to the stimulus
encountered. The behavioural output is encoded in outgoing electrical impulses that propagate
along motor neurons and motor fibres forming the cortical spinal tract. The signals terminate in
- The flow of information from one neuron to another is called synaptic transmission. Electrical
signals are communicated or transmitted from one region of the PNS until the signals get to the
CNS. Once it gets to the CNS, there is going to be cognitive processing. cognitive processing
refers to comparing information that is already stored in the brain with new information that
has just been received. This will generate a response to the stimulation received. The motor
response will be translated into behaviour.
- When a signal flows from one type of neuron to another, they typically change in terms of the
type of signals. The pattern of transmission of the signals follows a series of transformations
from graded potentials to action potentials. When the impulses reach another region of the
brain, then the same series of transformations occur.
- The neurons of the brain act like AD converters (analog-to-digital). Neurons convert analog
signals (e.g., graded potentials) in the form of continuous values that are disproportionate to
intensity or magnitude of the stimulation. Eventually, at the axon hillock or the AIS, the analog
signal gets converted to a digital signal (e.g., action potential). An action potential is a digital
signal because an action potential generates in an all or none fashion. The neuron either fires or
not at all (e.g., 1 = yes firing, 0 = no firing).
- In the PNS, environmental stimuli (e.g., light, sound, odorant chemicals, touch, pain mediating
stimuli, temperature, etc.) are converted to electrical signals called receptor potentials. This
conversion is termed sensory transduction and occurs in two subtypes of receptors.
- Short receptor cells have short axons. The distance between the receiving area and the synaptic
region is less than 0.1 mm. Short receptor cells do not generate action potentials, rather they
use graded or electrotonic potentials. They release neurotransmitters tonically or continuously.
- Long receptor cells are typically pseudounipolar cells. Pseudounipolar cells have cell bodies that
project two segments of axons. One segment extends toward the receiving area. The other
segment extends toward the second order sensory neurons. The distance between the receiving
area and the terminal axons is more than 0.1 mm. - The first step in transmission occurs in sensory receptors. An ecological environmental stimulus
gets converted into electrical impulses that can be understood and propagated into the nervous
system through a process called sensory transduction. Sensory transduction occurs in different
types of sensory receptors that are present in different parts of the body. There are two types of
- Short receptors typically either do not have axons or have very short axons. The distance from
the receiving region and the synaptic region is less than 0.1 mm. The release of
neurotransmitters from sensory receptors is based upon whether the membrane potential of
the receptor is depolarized or hyperpolarized. Neurotransmitters being released from these
receptors are continuous that is modulated by whether the receptor is depolarized or
hyperpolarized. Depolarization may result in a decrease in the release of neurotransmitters and
hyperpolarization may result in an increase in the release of neurotransmitters, or vice versa.
- For long receptors, the distance from the sensory receiving area to the synaptic region is more
than 0.1 mm. They are considered pseudounipolar cells, which have two bifurcated axons
projecting from the cell body. One region of the axon is directly connected to the sensory area.
The other region of the axon will comprise your terminal axons that would synapse to the
second or third order neurons.
- Receptors that mediate mechanoreception, pain, temperature, proprioreception in the limbs
and trunk, proprioreception in the jaw, and olfaction are all examples of long receptor cells.
- Receptors that mediate gustation (taste buds), audition (hair cells), and vision (photoreceptors)
are all examples of short receptors.
- The molecular receptors for odorants are found in sensory cilia that project into the mucus layer
of olfactory epithelium. Depolarizing receptor potentials in the long receptors gives rise to
action potentials that propagate along the olfactory receptor neuron’s axon into the CNS.
- Odorant molecules bind to specific GPCR in the plasma membrane of the sensory cilia. This frees
the alpha subunit to activate AC, which increases the concentration of cAMP. This causes non-
selective cation channels to open, which depolarizes the membrane. Ca2+-gated current can
enhance this effect.
- Other pathways may involve the activation of another enzyme called PLC. The consequent
increase in IP3 concentration acts directly on plasma membrane Ca2+ channels.
- The sensory area is located on sensory cilia, which are thread-like projections that emanate
from the cell body. Chemical odorants are received by receptors located along the sensory cilia.
These receptors are typically metabotropic receptors, which are coupled to G proteins and many
other signaling molecules inside the cell, so that when these receptors get activated by chemical
odorants, the receptor will change its conformation, resulting in the activation of many signaling
molecules in the cell, resulting in changes in the conductance of ion channels.
- This type of receptor results in an increase in calcium conductance (calcium influx) and an
increase chloride conductance (chloride efflux) resulting in the depolarization of the cytosolic
environment, leading to the depolarization of the cell, and resulting in the generation of an
action potential. CSB332 Lecture 10
- Electrical impulses generated in higher neural pathways (in second and third order sensory
neurons, in interneurons, and in motor neurons in the cerebral cortex) occur via postsynaptic
receptors that produce graded potentials that summate as they travel towards the axon
hillock/axon initial segment.
- In electrochemical synapses, presynaptic excitatory axons release excitatory neurotransmitters
(e.g., glutamate) that activate excitatory postsynaptic receptors. Activation of excitatory
postsynaptic receptors would generate EPSPs.
- Presynaptic inhibitory inputs release inhibitory neurotransmitters (e.g., GABA). These
neurotransmitters activate inhibitory postsynaptic receptors that would then generate IPSPs.
- All generated EPSPs and IPSPs would then travel towards the axon initial segment or the axon
hillock. As they travel, they will summate.
o Temporal summation occurs when two or more EPSPs or IPSPs are generated almost at
the same time or close to each other in time.
o Spatial summation occurs when two or more EPSPs or IPSPs are generated by two or
more neighbouring postsynaptic regions or close to each other in location.
- When the sum of EPSPs and IPSPs at the AIS reaches a threshold of excitation, then the neuron
will fire an action potential. If it is below the threshold of excitation, then the neuron will not fire
an action potential.
- In higher order neurons in the brain and the spinal cord, you won’t find any sensory receptors.
Synaptic transmission occurs via postsynaptic receptors. Postsynaptic receptors are expressed
on the cell body of neurons. There are two types of postsynaptic receptors: ionotropic and
o An ionotropic receptor is an ion channel itself. When a ligand binds to an ionotropic
receptor, it allows the influx or efflux of ions.
o A metabotropic receptor does not form a pore, but are simpler in terms of function.
They are composed of a continuous sequence of amino acids producing seven
transmembrane domains. It is called a serpentine receptor because the structure looks
like a serpent. The N-terminus is located outside of the cell. The C-terminus is located
inside the cell. They have special regions within the intracellular domain that interacts
with G proteins. This is why metabotropic receptors are called G proteins. Their
intracellular domain directly interacts and binds G proteins. There are also other
domains that are sensitive to phosphorylation.
- When ionotropic or metabotropic receptors are activated by a ligand, they produce two types of
current: EPSC or IPSC. It depends on the type of receptor.
o If an ionotropic receptor (e.g., GABAergic receptor) is a chloride channel, then activation
of the receptor results in the generation of IPSPs. If the ionotropic receptor is an ion
channel that is permeable to sodium or calcium, then activation of the receptor results
in the generation of EPSPs. Slide 10
- As opposed to chemical synapses, postsynaptic receptor potentials are not generated in
electrical synapses (gap junctions). Current flows from the cytosolic environment of neuron to
the cytosolic environment of the other neuron via specialized hemichannels called connexons.
This type of transmission is called direct synaptic transmission.
- Connexons interconnect the cytoplasmic environment of two or more neurons. Each connexon
or hemichannel is composed of six subunits called connexins. A connexin is composed of four
membrane spanning domains.
- Electrical synapses (gap junctions) allow for the continuous flow of current from one neuron to
another. This is how current is transmitted in electrochemical signals.