Professor Dave again, let’s discuss the peripheral nervous system.
The nervous system is divided into two main divisions, the central nervous system, which
consists of the brain and spinal cord, as we just described in the previous clip, and
the peripheral nervous system.
This is the rest of it, the part that receives information from the world outside our bodies,
the information that the brain needs to be able to tell the body what to do, so the peripheral
nervous system is the interface between a human being and its surroundings.
It consists of tiny white nerves that thread through every single part of the body, and
this includes all neural structures apart from the brain and spinal cord.
Let’s learn a bit more about how this system is organized.
The peripheral nervous system is divided into a sensory, or afferent division, and a motor,
or efferent division.
The first of these is what gives us the ability to sense and perceive our surroundings, and
the second is what gives us the ability to physically respond to them.
Let’s talk about the sensory system first, since that’s the entry point for any stimulus.
A major component of this is called the somatosensory system, which is comprised of all the receptors
along the exterior of your body.
If something touches you, on your leg, or arm, or the back of your head, you have this
system to thank for being able to feel it.
Some processing occurs even before the signal reaches the brain, and information about the
stimulus is encoded in the resulting nerve impulses.
These impulses are integrated at the circuit level so as to send them to the correct area
of the cerebral cortex, so that the stimulus can be perceived properly.
It is here where the information is interpreted, its magnitude is estimated, and its quality
is assessed.
Sometimes the perception of a stimulus is pain, and although unpleasant, this is a valuable
biological response that causes us to protect ourselves from something that is actively
damaging us, like a flame, or a sharp object, by moving away from it.
There are a wide variety of sensory receptors that have the ability to respond to stimuli.
We can categorize these by the type of stimulus they detect, their location on the body, and
their structure.
Going by the first method, we can identify mechanoreceptors, that respond to touch or
pressure, thermoreceptors, that respond to temperature change, photoreceptors, that respond
to light, and chemoreceptors, that respond to specific chemicals.
There are also nociceptors, which respond to stimuli that can damage the body, and in
turn stimulate other receptors of the varieties we mentioned to elicit a response.
If going by location, exteroceptors respond to stimuli outside of the body, interoceptors
respond to stimuli inside the body, and proprioceptors respond to stimuli inside the body as well,
although specifically in skeletal muscles and related connective tissue.
Lastly, if considering structure, there are many, including nonencapsulated or encapsulated
nerve endings, lamellar and bulbous corpuscles, muscle spindles, tendon organs, and joint
kinesthetic receptors.
These are each best suited for monitoring their own particular type of stimulus.
Of course, most of our sensory perception comes from the senses, each of which requires
a complex organ to receive and transmit data.
These include the eyes for vision, a nose for smelling, a tongue for tasting, and ears
for hearing.
Beyond this there is the sense of touch that is not as localized, and can be divided into
different modalities associated with pain, temperature, vibration, and other phenomena.
But looking at the other senses and their respective organs, each of these structures
could be discussed for several hours, so we will offer just a brief description here,
and expand upon them later.
First, the eyes.
These come with accessory structures, those being the eyebrows, which prevent sweat from
trickling into the eyes, and eyelids, which cover and protect the eyes.
They are activated reflexively to blink every few seconds, which spreads secretions across
the eyes to keep them moist.
Eyelashes project from the eyelids, which are full of nerve endings, and if anything
touches them, reflex blinking will occur.
Lining the eyelids and folding back across part of the eyeball is a mucous membrane called
the conjunctiva.
There is also a lacrimal apparatus, comprised of a gland that produces a secretion that
we call tears, and ducts that drain into the nasal cavity.
Tears are meant to clean the eye and destroy pathogens that enter the area.
Six muscles control the motion of the eye, while the eyeball itself is a hollow sphere
filled with a clear fluid called the vitreous humor.
The eye has an outer fibrous layer, made of a sclera and a cornea.
Next is a vascular layer comprised of the choroid, ciliary body, and iris.
The iris is the colored part of the eye, with its central pupil, which is where light enters.
The inner portion of the eye is the retina, and this is where the photoreceptors sit,
which convert photons into information for the brain, thus allowing us to see our surroundings.
The retina has an outer pigmented layer and an inner neural layer.
The photoreceptors are of two types, rods and cones.
The sensitive rods are for dim light and peripheral vision, while the cones are for bright light
and colors.
The lens is the biconvex, transparent structure that can focus light onto the retina, and
it is comprised of the lens epithelium and lens fibers.
The actual mechanisms of refraction and phototransduction are very complex and will not be discussed
at this time, so let’s move on to the nose.
The nose is full of chemoreceptors.
If a substance is volatile, meaning easily evaporated at room temperature, some of it
can interact with these receptors, allowing us to decide whether that substance is to
be avoided.
The structure responsible for this is called the olfactory epithelium, up in the roof of
the nasal cavity.
This contains millions of olfactory sensory neurons, which are receptors capable of distinguishing
thousands of odors.
If an odorant binds to a receptor in the olfactory cilium membrane, an impulse is generated that
runs up to an olfactory bulb and then down an olfactory tract all the way to the olfactory
cortex in the brain, where a number of responses may be triggered depending on the nature of
the odor.
Moving on to the tongue, this is the main structure that allows for the perception of
taste, as the majority of the ten thousand or so taste buds we possess are found on the
papillae of the tongue.
A taste bud is made of fifty to a hundred cells, which are a combination of gustatory
epithelial and basal epithelial cells.
The former of these are the ones that act as chemoreceptors, while the latter act as
stem cells, replacing gustatory epithelial cells that get damaged or burnt.
The five taste sensations that can be produced by these receptors are sweet, sour, salty,
bitter, and umami, the last of which is not familiar to everyone, but it means savory,
like a steak.
Most foods produce some combination of these sensations to generate a unique perception.
Lastly, let’s look at the ear.
This is divided into an external ear, a middle ear, and internal ear.
The external ear contains the familiar features, an aurical, helix, and lobule or earlobe.
The auditory canal also begins here, which leads up to the tympanic membrane, or eardrum.
Acoustic energy in the form of sound waves will reach the eardrum and cause it to vibrate,
and this energy is transferred to the bones of the middle ear.
This area is known as the tympanic cavity, which is spanned by three tiny bones called
auditory ossicles.
These are the malleus, the incus, and the stapes.
This leads to the internal ear, also known as the labyrinth, due to its complex structure.
It is divided into the bony labyrinth and the membranous labyrinth.
The main cavity in the bony labyrinth is the vestibule, which connects semicircular ducts
within semicircular canals.
Also extending from the vestibule is the cochlea, a snail-like spiral chamber that coils around
the modiolus.
The cavity of the cochlea is divided into three chambers, or scalae.
These are the scala vestibuli, scala media, and scala tympani.
And running through the center is the cochlear duct, which houses the spiral organ, which
contains cochlear hair cells, the receptors for hearing.
The ear also contains the vestibular apparatus, which helps us to maintain equilibrium, or
balance.
That covers the sensory organs, so before we move forward, let’s just mention a few
things about nerves.
A nerve is a bundle of axons enclosed by connective tissue.
These are the highways that allow for the transfer of information around the body.
Each axon within a nerve is surrounded by some loose connective tissue called an endoneurium
that also encloses the Schwann cells.
Another layer of connective tissue called the perineurium binds groups of fibers together
to form fascicles, and a tough fibrous sheath called the epineurium encloses all the fascicles
to form the nerve.
Apart from neurons, there are also blood vessels and lymphatic vessels in the nerve.
Most nerves are mixed nerves that contain both sensory and motor neurons, and thus transmit
impulses both to and from the central nervous system.
But there are also sensory nerves, that transmit towards the central nervous system, and motor
nerves, that transmit away from the central nervous system.
There are twelve pairs of cranial nerves that are associated with the brain, most of which
are associated with the brain stem.
These run between the brain and the sensory organs, and several other places.
Then there are thirty one pairs of spinal nerves that go from the spinal cord to most
of the areas of the body, forming complicated networks called nerve plexuses, with each
plexus innervating a particular region of the body.
We will get more specific with these plexuses and nerves a bit later, as well as all of
the reflexes they produce, when we take a look at kinesiology, which is the study of
body movement.
For now let’s move forward and wrap up our survey of the nervous system.