Physiology 2130 Lecture Notes - Lecture 6: Axon Hillock, Neuromuscular Junction, Axon Terminal
8 Dec 2016
School
Department
Course
Professor
Monday, May 25, 2015
Module 6: The Nervous System
In the last two weeks we learned about action potentials and signalling to skeletal
muscle. But, how are the actions of many muscles coordinated?
1) A general look at the nervous system:
•You should have a good idea of the organization of the nervous system; the central
nervous system and the peripheral nervous system. In addition you should know the
lobes of the brain and their functions, some of which are touched on in a bit greater
depth than others.
Because the language of the nervous system is action potentials, we should fully
understand action potentials before we can talk about the nervous system in detail.
2) How does a neuron tell another neuron to fire an action potential?
In the muscle, you saw that there was an information transfer in the form of an end plate
potential. Here in the neurons, something a little different occurs.
i) Basic anatomy- Neurons meet or synapse with each other via the axon
terminal of one neuron (the presynaptic neuron- since it is the neuron BEFORE
the synapse) meeting up with dendrites of another neuron (the postsynaptic
neuron since it is the neuron AFTER the synapse). At the synapse, some of the
events that take place are similar to what you learned for the neuromuscular
junction. You learned that as an action potential reaches the axon terminal, Ca++
voltage-gated channels open and Ca++ enters the axon terminal, which causes
the release of the neurotransmitter from the synaptic vesicles into the synaptic
cleft.
ii) Neurotransmitter release- From neuron to neuron, the same thing happens
except the neurotransmitter doesn’t have to be acetylcholine, rather it can be
norepinephrine, GABA, glycine or a host of other neurotransmitters that exist in
the body. These neurotransmitters cross the synaptic cleft and bind to their
receptors that are associated with chemically-gated ion channels, just as you had
seen in the neuromuscular junction.
Here is when things can become different. In the muscle, an action potential in a
normal healthy motor neuron ALWAYS caused an action potential in the muscle.
!1
find more resources at oneclass.com
find more resources at oneclass.com
Monday, May 25, 2015
Neuron to neuron transmission is different because some neurotransmitters are
excitatory and some are inhibitory. Just as their description implies, excitatory
neurotransmitters want to try to excite the postsynaptic neuron and bring it closer to
threshold (but may not reach threshold) by depolarization, while inhibitory
neurotransmitters want to inhibit the postsynaptic neuron and bring it further away from
threshold by hyperpolarization.
An important note is that one neuron only releases one specific neurotransmitter.
A) Excitatory Neurotransmitters
Since you need to reach threshold in order to fire an action potential at the axon hillock,
there must be a large enough change in the membrane potential to open up Na+voltage
gated channels and bring the neuron at this region from –70 mV to a membrane
potential of –55 mV. Therefore there must be excitement or depolarization of the
neuron. This event that triggers the depolarization is called excitatory postsynaptic
potentials (EPSP for short). The properties of EPSP are VERY DIFFERENT from action
potentials (see below for EPSP/IPSP properties). EPSP’s are generated from excitatory
neurotransmitters being released from the presynaptic neuron. These neurotransmitters
bind to their receptors causing the opening of chemically-gated ion channels that are
specific for positive ions (predominantly Na+), which enter the dendrites of the
postsynaptic neuron and cause a local depolarization.
B) Inhibitory Neurotransmitters
We can’t ignore the fact that there are also inhibitory neurotransmitters that bring about
inhibitory postsynaptic potentials (IPSP for short). These work in a similar fashion to
EPSP’s except in this case chemically-gated channels specific for either Cl- or K+ will
open to bring about a local hyperpolarization in the dendrites.
3) If you have an EPSP, do you automatically have an action potential?
The short answer is NO. Because first of all action potentials are only generated at the
axon hillock region where there you find your first voltage-gated channels: remember
you need these voltage-gated Na+ channels to open to have an action potential. These
EPSP’s occur in the dendrites … and therefore must spread down towards the axon
hillock. Secondly, the size of your EPSPs matter.
Properties of the EPSP and IPSP
A) Occur in the dentrite region of the neuron (they are local)- these are local
depolarizations or hyperpolarizations that must spread to the axon hillock. If by the axon
!2
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
In the last two weeks we learned about action potentials and signalling to skeletal muscle. But, how are the actions of many muscles coordinated: a general look at the nervous system, you should have a good idea of the organization of the nervous system; the central nervous system and the peripheral nervous system. In addition you should know the lobes of the brain and their functions, some of which are touched on in a bit greater depth than others. In the muscle, you saw that there was an information transfer in the form of an end plate potential. At the synapse, some of the events that take place are similar to what you learned for the neuromuscular junction. These neurotransmitters cross the synaptic cleft and bind to their receptors that are associated with chemically-gated ion channels, just as you had seen in the neuromuscular junction.
