Download Neuroanatomy Part 2

ANSC 2401 – Lecture
Special Senses
Vision
Image Formation
Cornea and Lens: Focus an image of a distant object on the retina
Ciliary muscles: Contract, changing the shape of the lens which brings closer object into
focus.
Pupil maintains the proper amount of light exposure of the retina.
Refraction of Light Rays: Retraction is the bending of light.
As the light enters the eye it is refracted at both the anterior and posterior surfaces of the
cornea. Both surfaces of the lens further refract the light focusing the image on
the retina.
Lens: Fine tunes the image and changes the focus for near or distant objects.
Accommodation for Near Point Vision
When the eye is focusing on a near object, the lens curves more to bend the rays toward
the central fovea. This is called accommodation.
Near Point of Vision: minimum distance of 10cm (4 in.) in a young adult
As we age the lens loses elasticity and therefore its ability to accommodate.
This is called prebyopia.
Abnormal Refraction
Emmetropic eye: normal eye with object at 20ft (6m) focused clearly
Myopia: nearsighted
Hypermetropia: farsighted
Astigmatism: irregularities of the surface of the lens
Physiology of Vision
Step One: light is absorbed by the photopigment (colored proteins the undergo structural
changes upon light absorption)
Rods: photopigment is rhodopsin and it absorbs blue to green light
Cones: have one of three photopigments (therefore three types of cones) and
they absorb blue, green or yellow-orange light
Photopigments: are made of two parts, a glycoprotein and a derivative of
Vitamin A known as retinal (the light absorbing portion of all
photopigments). Nyctalopia: night blindness (inability to see well in
low light) causes by a deficiency in Vitamin A. Light immediately
begins to degrade the photopigments which causes visual stimulation.
Step Two: When light stimulates the photoreceptors, the impulse causes the rods and
cones to release neurotransmitters.
Step Three: The neurotransmitters cause the bipolar to be excited. ( The horizontal cells
can inhibit signals to the bipolar cells which enhances contract. These cells also
assist in differentiating various colors.)
Step Four: Bipolar cells can stimulate amacrine cells which synapse with the ganglion
cells or synapse directly with the ganglion cells.
Step Five: The ganglion depolarizes and initiates a nerve impulse.
Step Six: The nerve impulses are passed on the optic nerve and the pulse travels to
the thalamus.
Step Seven: The thalamus relays the impulse to the occipital lobe where the image is
interpreted. There are at least three systems in the cerebral cortex that process
the information: one for shape of objects, one for color, and one for movement,
location, and spatial organization.
Hearing
Step One: The auricle directs sound into the external auditory meatus.
Step Two: Sound waves strike the ear drum and cause it to vibrate. It vibrates slowly in response
to low-frequency (low-pitch) sound and rapidly to high-frequency (high-pitch).
Step Three: Because the malleus is connected to the eardrum, it also starts to vibrate. The
vibrations are then transmitted through the inus to the stapes.
Step Four: As the stapes vibrates it the oval window. The oval window vibrates ~ 20 times more
vigorously than the ear drum.
Step Five: The movement of the oval window sets up fluid pressure waves in the perilymph of
the cochlea.
Step Six: The perilymph flows toward the scala vestibuli. The pressure waves are transmitted
from the scala vestibuli to the scala tempani and eventually to the round window.
Step Seven: As the pressure deforms the walls of the scala vestibuli and scala tympani, they push
the vestibular membrane back and forth. This results in the pressure of the endolymph in
the cochlear duct to increase and decrease.
Step Eight: The pressure fluctuations cause slight vibrations in the basilar membrane which
causes the hair cells in the spiral organ to move against the tectorial membrane. The
bending of the microvilla (hair cells) leads to the generation of a nerve impulse in the
cochlear nerve fibers.
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Step Nine: The impulse is then transmitted via the cochlear branch of the Vestibularcochlear
nerve to the thalamus where the pulse is transferred on to the temporal lobe of the
cerebrum.
Equilibrium
Static Equilibrium: maintenance of body position (mainly the head) relative to the force of
gravity.
The walls of the saccule and utricle (within the vestibule) contained a thickened region
called the maculae (receptor for static equilibrium and some aspects of dynamic
equilibrium)
They detect linear acceleration and deceleration. Ex: Feeling as an elevator car
speeds up or slows down.
The maculea contain hair cells imbedded in a layer of support cells. The long
extensions of the hair cells are embedded in the otolithic membrane.
When the head tilts, the heavy otolithic membrane and the otoliths are pulled by
gravity downhill in the direction of the tilt. This stimulates the hair
cells.
The hair cells release neurotransmitters that stimulate the sensory neurons in the
vestibular branch of the Vestibulocochlear nerve.
Dynamic Equilibrium: maintenance of body position (mainly the head) in response to
sudden movements such as rotation, acceleration, and deceleration.
The three semicircular canals and the saccule and urticle maintain dynamic equilibrium.
The three canals lie in three planes at approximately right angle to each other. Their
position allows for the detection of rotational acceleration and deceleration.
In the ampulla of each canal is the crista. Each crista contains hair cells that are
embedded in supporting cells and are covered by a gelatinous mass of material
called cupula.
When the head moves the endolymph in the semicircular canals flows over the hair cells
and bends them. This stimulates the sensory neuron and a nerve impulse is
generated that passes along the vestibular branch of the Vestibulocochlear nerve.
Smell (Olfactory Sensation)
Olfactory receptors respond to chemical stimulation of an odor molecule causes a nerve impulse to
begin. The nerve impulse is passed on to the olfactory nerve which terminates in the
olfactory bulbs on the ventral side of the frontal lobe of the cerebrum.
The nerve pulse is then transferred on to the temporal lobe and to the frontal lobe either directly or
indirectly through the thalamus. This is where the information is process.
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Once it was thought that there were seven primary scents (camphor, musk, floral, peppermint,
ether, pungent, and putrid). However, evidence now suggests that there may be several
hundred primary scents.
Taste (Gustatory Sensation)
There are nearly 10,000 taste buds in young adults. Most are on the tongue, but some can be
found on the soft palate, pharynx, and larynx.
Taste and smell are often associated. This is because odors from food will pass through the
nasopharynx and nasal cavity and stimulate the olfactory receptors. A given
concentration of a substance will stimulate the olfactory system thousand times more
than taste.
Each taste bud is an oval body made up of ~ 50 gustatory receptor cells and a number of
supporting cells and forms a capsule. The gustatory hair (microvillus) projects from the
surface of the gustatory receptor cell into the taste pore.
When a chemical dissolves in saliva, it can makes contact with the gustatory hairs. This
stimulates the release of neurotransmitters which cause an impulse in the sensory
neurons. Depending on the location of the taste bud, the nerve impulse is passed along to
either the Facial Nerve, the Glossopharyngeal, or the Vagus.
Cutaneous Sensations
Touch: General results from a stimulation of tactile receptors in the skin or tissues immediately
under the skin.
Crude Tough: the ability to perceive something that has touched the skin although its
exact location, size, shape, or texture cannot be determined.
Discriminative Tough: the ability to determine exactly what point on the body is being
touched.
Corpuscles of Touch: egg-shaped receptors for discriminative touch. They are
most plentiful at the fingertips, palms, and soles. They are also
abundant in the eyelids, tip of tongue, lips, nipples, clitoris, and the tip
of the penis.
Hair Root Plexuses: Dendrites are arranged in a network around the hair
follicle. Movement of hair shaft stimulates the dendrites.
Type I Cutaneous Mechanoreceptors (Tactile or Merkel Disks): flattened
portions of dendrites of sensor neurons that make contact with
epidermal cells called Merkel cells. That are distributed in many of the
same locations as the corpuscles of tough and also serve in
discriminating touch.
Type II Cutaneous Mechanoreceptors (End Organ of Ruffini): deeply imbedded
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in the dermis and deeper tissue of the body. They detect heavy and
continuous touch sensations.
Pressure: a sustained sensation that is felt over a larger area than touch.
Pressure receptors are type II cutaneous mechnoreceptors and lamellated corpuscles.
Lamellated Corpuscle: oval structures composed of a connective tissue capsule,
layered like an onion around a dendrite. They are located in
subcutaneous tissues, deep submucosal tissues and serous membranes;
around joints, tendons, and muscles; and in the mammary glands,
external genitalia, and certain viscera like the pancreas and urinary
bladder.
Vibration: results from rapid, repetitive sensory signals from tactile receptors.
Corpuscles of Touch: detect lower-frequency vibrations
Lamellated Corpuscles: detect higher-frequency vibrations
Itch and Tickle
Itch results from stimulation of free nerve endings by certain chemicals and is often result
of a local inflammatory response.
Tickle sensation is to come from the free nerve endings. This is the only sensation that
you cannot elicit yourself.
Thermal Sensations: free nerve endings detect warmth and coolness
Pain Sensation: Necessary for normal life to protect ourselves from greater damage.
Nociceptors: the receptors for pain (free nerve endings) and are found in almost every
tissue of the body and they can respond to any type of stimulus if it strong
enough to cause tissue damage
Tissue damage can release chemicals (ex: prostaglandins) that stimulate the
nociceptors. Pain can persist even after the initial trauma due to the
lingering of these substances.
Types of Pain
Acute Pain: occurs very rapidly (usually within 0.1 second) after a stimulus is
applied and is not felt in deeper tissues of the body. This pain is
described as sharp, fast , and pricking pain. (ex: needle prick)
Chronic Pain: begins after a second or more and gradually increases in
intensity over a period of several seconds or minutes. This pain can be
excruciating and is described as burning, aching, throbbing, and slow
pain. Can occur in skin and deeper tissues or internal organs. (ex:
toothache)
Referred Pain: Pain that may be felt in a surface area far from the stimulated
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organ. (ex: the pain of a heart attack is typically felt over the skin over
the heart and along the left arm)
Phantom Pain: experienced by patients that have had a limb removed. Nerve
impulses arise from the remaining proximal portions of the sensory
neurons that previously received impulses from the limb.
Pathway for Cutaneous Sensations
First-order neurons carry signals from sensor receptors to either the brain stem or spinal
cord.
Second-order neurons carry the signals from the spinal cord and brain stem to the
thalamus. The axons of the second order neurons crossover to the opposite side
in the spinal cord or brain stem before ascending to the thalamus.
Third-order neurons project from the thalamus to the primary somatosensory area of the
cortex where conscious perception of sensations results.
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