在本课程中，学生将学习认识和应用说明人体九个器官系统中整体人体机能（作为完整有机体）的基本概念。
简介和结构
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From the course by Duke University
人体生理学导论（中文版）
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From the lesson
感觉和躯体神经系统
在此模块中，我们探讨两种类型的细胞：一种将信息转送到中枢神经系统（大脑）以进行解读，第二种从中枢神经系统转送信息来控制随意运动，它们是运动神经元。 通向大脑的输入通路由叫做感觉细胞的特定细胞来介导。 感觉细胞将能量（如光或热）转化为大脑中的神经元可识别的能量形式（电位）。 接下来，大脑解读这些信息（解读为视觉和痛觉），然后通过躯体神经系统的运动神经元将运动反应发送至身体的效应细胞。运动神经元激活骨骼肌来控制呼吸和四肢的运动。
Meet the Instructors
Jennifer CarbreyAssistant Research Professor
Department of Cell Biology
Emma JakoiAssociate Research Professor
Department of Cell Biology
Hello.
We're going to finish our consideration of the nervous system
by now considering an efferent pathway, the somatic nervous system.
So we talked about the afferent pathways coming into
the central nervous system, and now we're going to kind of
combine the two when we, after we'd considered just the
efferent pathway, the somatic nervous system that activates skeletal muscle.
Then in the next session, we're going to talk
about reflexes, which is where we bring in sensory information.
The central nervous system integrates that information, decides what to do.
And then we're going to consider the efferent pathway through the somatic
nervous system and how that controls things like reflexes and locomotion.
So in this session, we're going to talk about general principles of
the somatic nervous system, some aspects of spinal cord
structure, and just touch again on the neuromuscular junction.
But a fair amount of this will be
some review from just the nervous system in general.
And then in the next section, we're going to talk more about how the afferent
pathways and efferent pathways come together to control movement.
So
you've seen this figure before.
Just to place this in the context of the larger nervous
system where we're going to have sensory
information coming in through afferent pathways.
That's what we've talked about.
Most recently the CNS is going to integrate and
then we are going to be talking about
the somatic nervous system which is the part
of the efferent nervous system that controls skeletal muscle.
That's going to be in contrast to the autonomic's neurvous
system which is going to be what controls the organs.
And this again is reviewed to remind you that
with the autonomic nervous system, such as the sympathetic portion
and the parasympathetic portion; we have two neurons in
series, where the first neurons in the central nervous system.
And the second one is sitting out in the
periphery, but with the somatic nervous system, we have the
neuron, the single neuron sitting in the central nervous system and sending
its axon all the way out to the muscle to stimulate it.
So it's a single neuron system in terms of the efferent portion.
And also keep in mind that this skeletal muscle is
going to perform a variety of actions, not just voluntary actions.
So, it's going to be involved in reflexes and
breathing, which are going to be somewhat involuntary actions.
Let's review our spinal cord structure, where we've already talked
about how we're, going to have a afferent neuron coming in.
Itself cell body is sitting usually in the
Dorsal root ganglion for the body sensory information.
And then there will be neurons sitting in the spinal cord called interneurons
That are going to be part of the system that is going to interpret the
stimuli coming in and we'll see in
the next session, concrete examples of this circuitry.
So we're going to have inner neurons that are Collecting information.
And then based on what they decide we will have firing of the ether neuron
for the somatic neverous system that's going to be sitting in the ventral.
What we call the ventral horn of the spinal chord.
So this is the ventral portion of the spinal
chord and here we're going to have all of our
motor, somatic motor neurons with their cell bodies sending
their axon all the way out to the skeletal muscle.
We need to now consider that somatic motor neuron that were
just looking at in the spinal chord, what is it synapsing with?
And so, that comes to the definition of a motor unit, where a motor unit is a single
motor neuron, and all the muscle fibers, which is
the same thing as a muscle cell, that it controls.
So, that's what a motor unit is and we'll be talking
about this more when we talk about skeletal muscle as well.
Where a, usually a neuron is going to control anywhere from three
to around a thousand different, muscle
cells, skeletal muscle cells or, which is the same thing as skeletal muscle fibers.
All of those fibers are going to be in the same muscle.
But they're usually going to be spread out.
So that's what's shown here in this muscle.
Where we're saying that these blue muscle cells are the
ones that are innervated and are receiving signals from one neuron.
This is this ones motor neuron motor unit. Keep in mind,
that one, a single muscle cell is only
going to be innervated by one neuron which means
it's only being controlled or regulated by one neuron.
So our, our decision making about whether or not a muscle itself
should contract is coming at the level of this somatic motor neuron.
If it sends an action potential down its axon to the muscle, the muscle is going
to contract, and we'll talk about that in just a minute.
And it's that muscle cell is only getting inputs from that one neuron.
So the muscle cell is not having to decide whether or not
its going to contract, it's because it's only being controlled by one neuron.
And whenever that neuron fires, that muscle cell is going to contract.
So the, decision making is happening at, with the somatic motor
neuron, not with the muscle cell receiving multiple inputs
from multiple neurons and deciding whether or not to contract.
So let's talk about the neuromuscular junction which is just going to
be a certain type of synapse which we've already talked about.
There's nothing so unusual with it.
We're going to have the axon from the somatic motor neuron coming to contact
the plasma membrane of the skeletal muscle cell.
So that action potential will travel down that axon, and when it does
just like in a normal synapse it's going to open voltage gated
calcium channels, that will let calcium into the terminus of the axon.
Which is going to be the signal to because vesicles of neuro transmitter, and more
specifically, acetylcholine, to fuse and to be released into the synaptic cleft.
Cleft.
So, acetylcholine is going to be what's released by
all somatic motor neurons, the muscle motor end
plate, which is just the portion of the
muscle plasma membrane, that is at the synapse.
It's going to connect, contain nicotinic acetylcholine receptors.
So these are nicotinic synapses. And we know that nicotinic acetylcholine
receptors are going to be acetylcholine Gated sodium channels.
So that means that if this neuron fires, it's going
to open sodium channels, that's going to because a graded potential.
However, keep in mind that right next to this synapse
on the skeletal muscle plasma membrane we're going to have voltage
gated, ion
channels. Which
is going to be somewhat unique compared to what we
saw with Neurons, so that means that basically we are having a synapse
what to, what would be equivalent to the axon
initial segment.
And so that means that whenever we have firing of the
somatic motor neuron, for all intents and purposes we're also going to
have an action potential that occurs in the skeletal muscle membrane as
well, and we'll be talking more about that once we cover muscle.
But again, it's this idea that if that neuron fires we're going
to have an action potential in the muscle membrane and that's going to because
contraction, just because we're having a graded
potential right next to voltage gated channels.
And so it's a pretty much given that
they're going, one's going to follow the other.
So we've just had an introduction to the somatic
nervous system, where we're going to control many different types of movement.
One, because for instance, the diaphragm
that controls our breathing is skeletal muscle.
So, things like that as well as obviously
the muscles that are important for posture and locomotion.
And the somatic motor neurons are going to act.
They're sitting in the spinal cord and they're going to act on skeletal muscle.
A motor unit is going to be a somatic motor neuron and the fibers that it
innervates or controls.
And we're going to have these motor neurons in the
grey matter of the ventral horn of the spinal cord.
And yet we mentioned just briefly but
we'll talk more about it, this interneurons in
the spinal cord and how they're going to
be important in coordinating responses and integrating information.
And this is a subject
that we're going to really focus on in the next section
in terms of talking about how we make reflexive movements.
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