Hi everyone, this is topic 8 of the Human
Anatomy class and this is the third lecture on Nervous System.
In this lecture we are going ot learn about the transmission of nerve impulses. There
are four stages in this transmission. The first stage is called resting neuron, and
from the name itself, you know that the neuron is resting. So, the plasma membrane of the
neuron at rest is to be in a polarized state and the voltage of the plasma membrane is
called resting membrane potential. Broadly at th
is stage, the plasma membrane has more
positive outside than inside. That means inside is less positive. What kind of positive ions
are there, sodium ions, denoted by Na and potassium ions which is noted by K. Both of
these are positively charged ions the concentration of sodium ions is higher outside the cell,
and the potassium ions are more inside the cell.
The second stage of nerve impulse polarization is called depolarization. In this stage a
stimulus comes and is said to cause depolarizatio
n of the neurons membrane. In this stage the
membrane of the neuron becomes permeable to sodium or NA+ ions because the sodium channels
open. This is passive transport and the depolarized membrane allows the sodium ions to start flowing
in passively. The movement of sodium ions start an action
potential in the neuron, and in the place where the inside of the membrane is positive,
it produces a graded potential or localized depolarization.
If the number of sodium ions entering the cell is above t
he action potential then the
action starts traveling over the entire length of the neural axon. And these action potential
or neuron impulses travel faster if the axon is covered by myelin sheath.
The third stage is called repolarization. In this stage potassium ions which are also
positively charged, denoted by K+, they start rushing out to the restore the inside of the
membrane to a negative charge and the outside to a positive charge. This is because the
entry of the sodium ions made the insi
de more positive. This is known as repolarization.
Sometimes hyperpolarization occurs, this means potassium ions go outside than the amount
of sodium ions inside. The fourth and the last stage of nerve impulse
involves refractory period. In this phase the initial ionic conditions that was presented
in the resting potential stage is restored and active transport is used for this purpose.
Sodium potassium pump is used, this pump uses ATP, so ATP is broken down, for every one
molecule of ATP that i
s broken down to energy, 3 sodium ions are transported out of the cell
and two potassium ions are brought inside. This graph shoes the action potential in a
neuron. If you look at the y axis it denote the action potential in mV, and anything above
0 is positive and below 0 is negative. The x axis shoes the time in milliseconds, at
the beginning of the graph it is the resting membrane potential, the membrane potential
is at baseline, then around 0 time the membrane potential starts increasing it
reaches the
threshold potential and it gets more and more positive. As the graph is rising up, it is
depolarization. That means sodium ions are moving in. After a maximum value, the graph
starts going down, this is repolarization, this is where potassium ions are moving out
to make the outside more positive again. Sometimes hyperpolarization may occur, you see the graph
dipping the baseline, below the resting membrane potential and finally the refractory period,
the resting membrane potential is
restored. The graph shoes hoe the different channel
gates help in the four stages of the action potential.
This figure shows the proprogation of movement of the action potential. Look at the figure
on the left side, a tiny portion of the axon, the neuron is shown here in yellow color,
so you see the tiny potion of the axon is enlarged here. So below that axon, action
potential, so that means localized depolarization is shown with purple color. You see that the
ionic distribution has become more
positive inside the membrane then outside. This is
because the sodium ions have entered, and this is only localized, at a certain location.
Then gradually the action potential is moving in one direction, it is moving to the right
side. So the purple portion has moved, and the portion that was in purple before is in
repolarization, that is because potassium ions are moving out of the cell to the outside.
Then in the third phase, in the third part of this figure, action potential has moved
furthe
r and the repolarization has moved right behind the action potential. While the first
part that was purple in the beginning is now back to the resting potential. The figure
on the right side is also explaining similar things.
When the action potential reaches the axon terminal either vesicular or nonvesicular
synaptic action occurs. This figure shows a vesicular synapse.
This figure shoes neurotransmitter molecules diffusing across the synaptic cleft containing
synaptic fluid. Neurotransmitters
will eventually generate
and action potential in the post synaptic neuron.
Another image of a chemical or vesicular synapse. Vesicular synapse again.
Reflex arc, Reflex is a rapid, predictable, but involuntary response to a stimulus or
trigger. And this occurs over specific pathways known as reflex arcs. Reflex arcs consist
of a direct route from a sensory neuron to ultimately a effector neuron via an interneuron.
There are 5 basic elements or parts of a reflex arc. The first one is a receptor,
for an example
a receptor on the skin if you touch something hot is the sensor that detects it. Then the
signal from the sensor is taken by the sensory neuron. Here it is shown in turquoise blue.
The sensory neuron takes the information from the sensor to the CNS. The third element is
an interneuron or an association neuron present in the CNS. So, here shown in dark blue, the
interneuron is receiving the for the interneuron is receiving the message from the sensory
neuron and it will relay the m
essage to the motor neuron after the message has been processed.
SO, the motor neuron is the fourth element, it is shown in red here. It takes the message
from the interneuron back to the PNS to one of the effectors. And finally the fifth component
is the effector organ which is usually a muscle or a gland.
There are two types of reflexes, somatic reflex and autonomic reflex. Somatic reflexes stimulate
voluntary skeletal muscles, for example touch something hot, you pull your hand away. Autonomi
c
reflexes control the activity of involuntary organs, such as smooth muscle, heart, and
glands. This could be when you see something scary and your heart starts beating faster,
or your smooth mucsle after you have eaten start doing peristalsis. These are all autonomic
reflexes. This is a list of important definitions that
I have asked all of you to memorize in the last lecture. This is nuclei, ganglia, tracts
versus nerves, what is gray matter and white matter.
Now we are going to learn about t
he human brain, there are four regions in the human
brain. These are the cerebral hemispheres, also known as the cerebrum, the diencephalon,
the brain stem and the cerebellum. Inside the brain there are also some hollow spaces
which are called ventricles and they are filled with a fluid called cerebral spinal fluid
or CSF whoís component is very similar to blood plasma.
This is the adult human brain with all 4 regions. The most superior portion of the brain consists
of two cerebral hemispheres,
collectively called the cerebrum. The cerebrum is responsible
for the brains cognitive functions, including learning and language, consciously processing
sensory information, conscious planning of movement and personality. Its surface consists
of elevated ridges called gyri and shallow grooves called sulci. Deep grooves called
fissure separate major regions of the cerebral hemisphere. For example, the longitudinal
fissure separates the left and right hemisphere. The central sulcus separates the
frontal and
partial lobes of the cerebrum. This figure shoes the cerebrum,
The cerebrum is divided into 5 lobes by fissures, theses lobes are the frontal lobe, parietal,
occipital, temporal, and deep insula lobes. This figure shows the lobes of the cerebrum.
This is the photograph of a real human brain, and look at the different lobes.
The cerebrum contains several specialized areas. For example the, primary somatic sensory
area located in the parietal lobe information from the bodies sensory re
ceptors. The primary
motor area is located in the frontal lobe and send impulses to the skeletal muscles.
Brocaís area, is involved with the ability to speak, this is located in the frontal lobe.
This figure shoes the different specialized areas.
This slide lists other cerebral areas that are involved with special senses or interpretation.
For example, the gustatory area which is for taste, visual, auditory, olfactory, olfactory
means the sense of smell. The interpretation areas include, languag
e comprehension and
general interpretation. This figure is pointing out again, the olfactory
area, language comprehension area, and some other areas.
The cell bodies and unmyelinated axons and dendrites of the cerebral neurons lie in the
cerebrums outer 2mm, in a region called the cerebral cortex. These portions of the neurons
are unmyelinated with give this portion of the cerebral cortex a gray color, and for
this reason it is called gray matter. The cell bodies and cerebral processes communica
te
with other parts of the nervous system with bundles of myelinated axons, which is called
the white matter. This is because the myelin sheath is white in color. The largest tract
of cerebral white matter is called the corpus callosum. It connects the right ant the left
cerebral hemispheres, and it allows interhemispheric communication. Another prominent tract of
white matter is inferior to the corpus callosum and is known as the fornix. However the gray
matter isnít confined to the cerebral co
rtex only, there are clusters of cell bodies called
nuclei, which are found throughout the white matter of the cerebrum. An important group
of nuclei, the basal nuclei monitor voluntary motor function. These parts of the nuclei
are connected to other parts of the nervous system by various tracts of cerebral white
matter. This figure points out the cerebral cortex
contain the gray matter and then the white matter deep inside.
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