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.
