Transcript peripheral nervous system - Evans Laboratory: Environmental

BIOL 2020
HUMAN
ANATOMY &
PHYSIOLOGY II
Dr. Tyler Evans
Email: [email protected]
Office: S Sci 350
Office Hours: F 8:30-11:30 or by appointment
Website: http://evanslabcsueb.weebly.com/
Phone: 510-885-3475
LAST LECTURE
ORGANIZATION OF THE NERVOUS SYSTEM
Fig 11.2 pg 388
LAST LECTURE
CELL TYPES OF THE NERVOUS SYSTEM
• although highly complex, the nervous system is made of two principal cell types:
1. NEURONS: excitable cells capable of transmitting electrical signal
2. NEURGOGLIA: supporting cells that surround more delicate neurons
TYPES OF NEUROGLIA:
a.
b.
c.
d.
ASTROCYTES: support, brace and anchor neurons
• called SATELLITE CELLS in the PNS
MICROGLIA: repair damages neurons and prevent
against infection
EPENDYMAL CELLS: line cavities of the central
nervous system separating cerebrospinal fluid from
nervous tissue
• can be ciliated to help circulate the fluid
OLIGODENDROCYTES: wrap around neurons to
produce an insulating cover called MYELIN SHEATH
• in the PNS, SCHWANN CELLS produce myelin
sheath
Fig 11.3 pg 390
LAST LECTURE
FUNCTION OF REGIONS OF THE BRAIN
• recall, CNS consists of the BRAIN and SPINAL CORD and is the integrating and
control center for the nervous system
• four regions in the adult brain:
1. CEREBRAL HEMISPHERES
2. DIENCEPHALON
3. BRAIN STEM
4. CEREBELLUM (includes midbrain, pons and medulla oblongata)
Fig 12.2 pg 430
LAST LECTURE
THE SPINAL CORD
• the spinal cord is enclosed in the vertebral column and
provides a two-way conduction pathway to/from the
brain
• surrounded by a protective membrane called DURA
MATER and CEREBROSPINAL FLUID
• it terminates at a structure called the CONUS
MEDULLARIS
• 31 pairs of spinal nerve fibers attach via foramen
(holes) in vertebrae
• these fibers connect to motor and sensory
neurons that control movement and the senses
Fig 12.28 pg 467
Fig 12.26 pg 465
THE PERIPHERAL NERVOUS SYSTEM
CHAPTER 13
SENSORY RECEPTORS
• the PERIPHERAL NERVOUS SYSTEM (PNS) provides links from and to the world
outside our bodies
• the PNS includes all neural structures outside the brain and spinal cord including
sensory receptors, peripheral nerves and motor endings
• input into the PNS comes from SENSORY RECEPTORS that are specialized to
respond to changes in their environment
• receptors can be classified in three different ways:
1.
STIMULUS THEY DETECT
MECHANORECPTORS: respond to mechanical force such as touch and pressure
THERMORECEPTORS: respond to temperature changes
PHOTORECEPTORS: respond to light and found in the retina of the eye
CHEMORECEPTORS: respond to chemicals in solution and involved in smell and taste
NOCICEPTORS: responds to damaging stimuli that result in pain
THE PERIPHERAL NERVOUS SYSTEM
SENSORY RECEPTORS
Receptors can be classified in three different ways:
2. LOCATION IN THE BODY
EXTERORECEPTORS: are sensitive to stimuli originating outside the body, so most of
these receptors occur near the surface for example in the skin and special sense
INTERORECEPTORS: also called VISCERORECEPTORS, respond to stimuli within the
body such as viscera and blood vessels.
• respond to chemical changes, temperature and tissue stretch
• usually unaware of their activities
PROPRIORECEPTORS: also respond to internal stimuli, but occur only in skeletal
muscles, tendons, ligaments, joints
• these receptors constantly relay information about our movement to the brain
THE PERIPHERAL NERVOUS SYSTEM
SENSORY RECEPTORS
Receptors can be classified in three different ways:
3. RECEPTOR STRUCTURE
NONENCAPSULATED (FREE) NERVE ENDINGS: typically nothing more that swellings
at the end of a nerve fiber
• are present throughout the body and in abundance in epithelia (especially the
skin) and connective tissue
Table 13.1 list examples of nonencapsulated receptors
• they include the Merkel
disks in the skin and hair
follicle receptors we have
already described (ROOT
HAIR PLEXUS)
THE PERIPHERAL NERVOUS SYSTEM
SENSORY RECEPTORS
Receptors can be classified in three different ways:
3. RECEPTOR STRUCTURE
ENCAPSULATED NERVE ENDINGS: terminals of sensory neurons enclosed in
connective tissue
• virtually all are mechanoreceptors involved in touch and pressure detection
• vary greatly in shape, size and distribution in the body
• found in joints, muscle, tendons and deeper parts of the skin
• require stronger stimulus to become activated
THE PERIPHERAL NERVOUS SYSTEM
SENSORY RECEPTORS
• regardless of detected, stimulus, location or structure of the receptor, each
sensory receptor takes incoming stimuli and converts them into changes in
membrane potential
• typically, specialized receptor proteins in the cell membrane absorb energy of
the incoming stimulus and undergo a conformation change
• this conformational change triggers a signal transduction pathway that opens or
closes ion channels in the membrane and creates an ACTION POTENTIAL
THE PERIPHERAL NERVOUS SYSTEM
CREATING STIMULUS MODALITY
• just said that whatever the stimulus, different types of sensory receptors
convert the signals to action potentials
• but if all action potentials are the same how can integrating centers (e.g. like
our brain) distinguish signals coming from different types of receptor?
THE PERIPHERAL NERVOUS SYSTEM
DETERMINING STIMULUS LOCATION
• sensory systems must also encode the location of the stimulus
• one main factor determining stimulus location is the location of the stimulated
receptor on the body
e.g. NEURONS INVOLVED IN TOUCH
• sensory neurons involved in touch have a RECEPTIVE FIELD, referring to a
region of the skin that triggers a particular set of sensory neurons
• however, information from a particular set of sensory neurons can only
determine whether a signal has occurred in the receptive field and cannot
provide more precise locations
• can be problematic because neurons can have very small or very large
receptive fields or differences in ACUITY
How could organisms gain more precise information
about the location of a stimulus?
THE PERIPHERAL NERVOUS SYSTEM
DETERMINING STIMULUS INTENSITY
• because action potentials will not change their signal intensity (recall this is an
all-or-nothing response), stimulus intensity is determined by the number of
activated receptors on the membrane of a sensory cell
• the weakest stimulus that will produce an action potential is called the
THRESHOLD OF DETECTION
• e.g. some photoreceptors can detect a single photon of light
• In contrast, at RECEPTOR SATURATION all of the receptors on a sensory cell are
activated and an increase in stimulus intensity will have no effect
• difference between
threshold of detection
and receptor saturation is
called the DYNAMIC
RANGE, which is depicted
in this graph
THE PERIPHERAL NERVOUS SYSTEM
DETERMINING STIMULUS DURATION
• sensory receptors, regardless of the stimulus they detect, can come in two forms
that allow information to be conveyed about stimulus duration
1. TONIC RECEPTORS:
• fire action potentials as long as the
stimulus is present
• while most tonic receptors will
continue to fire action potential, the
frequency usually declines
substantially over time, called
RECEPTOR ADAPTATION
• receptor adaptation is critically
important as it allows us to tune out
unimportant information
THE PERIPHERAL NERVOUS SYSTEM
DETERMINING STIMULUS DURATION
• sensory receptors, regardless of the stimulus they detect, can come in two forms
that allow information to be conveyed about stimulus duration
2. PHASIC RECEPTORS
• fire an action
potential only when
the stimulus begins,
even if the stimulus
persists
TODAY’S LECTURE:
THE SPECIAL SENSES
CHAPTER 15
• most people think of five basic senses: vision, taste, smell, hearing and touch
• your textbook considers touch a general sense and doesn’t include a discussion
in this chapter, but we have briefly described sense of touch in the
integumentary system lecture (e.g. root hair plexus)
• the remaining four senses plus EQUILIBRIUM are considered SPECIAL SENSES
• SPECIAL SENSORY RECEPTORS are distinct because these receptors are confined
to the head region and are housed within complex sensory organs (e.g. eyes,
ears) of epithelial structures (e.g. taste buds)
• keep in mind that we perceive the world through the
simultaneous use of several special senses
THE SPECIAL SENSES
THE EYE AND VISION: ACCESSORY STRUCTURES
• vision is our dominant sense and some 70% of sensory receptors in the body are
in the eyes and 50% of cerebral cortex involved in visual processing
• most structures are not actually involved in photoreception and there are several
important accessory structures in the eye:
1. EYEBROWS: shades eyes from UV light and prevent sweat form entering eye
2. EYELIDS: provides protection and moisture
• eyelids are associated with
LACRIMAL CARUNCLE that
contains sebaceous and sweat
glands that moisturize the eye
when eyelids blink
• eyelashes are heavily innervated
so that anything that contacts
eyelashes causes a reflexive
blink
Fig 15.1 pg 545
THE SPECIAL SENSES
THE EYE AND VISION: ACCESSORY STRUCTURES
3. CONJUNCTIVA: mucous membrane that lines the eyelid and part of eyeball
• produces a lubricating mucus that prevents the eyes from drying out
4. LACRIMAL APPARATUS: consists of LACRIMAL (TEAR) GLAND and ducts which
drain the gland.
• the lacrimal gland continually
releases LACRIMAL SECRETION (i.e.
tears), which contain mucus,
antibodies and lysozyme, an enzyme
that destroys bacteria
• tears move across the eye and drain
into the LACRIMAL PUNCTA which
leads into the nasal cavity (cause of
sniffles when tear production is
high)
Fig 15.2 pg 546
THE SPECIAL SENSES
THE EYE AND VISION: ACCESSORY STRUCTURES
5. EXTRINSIC EYE MUSCLES: six strap-like muscle control the movement of each eye
• four RECTUS MUSCLES insert into the eyeball and each moving the eye in the
superior, inferior, lateral and medial directions
• SUPERIOR OBLIQUE MUSCLE rotates eye downward and laterally, while the
INFERIOR OBLIQUE MUSCLE rotates eye upward and laterally.
• DIPLODIA (double vision) or STRABISMUS (cross-eyes) can
result from weakened eye muscles
Fig 15.3 pg 547
THE SPECIAL SENSES
THE EYEBALL
• composed of three regions: the outermost FIBROUS LAYER, the VASCULAR
LAYER and the innermost INNER LAYER
FIBROUS LAYER
• SCLERA: seen externally as “whites of eye” provides anchor points for muscles
• CORNEA: transparent structure that acts as window to bend and focus light
entering the eye
• interestingly, cornea has no blood supply or immune response
• the cornea can be transplanted without possibility of rejection
WHY?
THE SPECIAL SENSES
THE EYEBALL
• composed of three regions: the outermost FIBROUS LAYER, the VASCULAR
LAYER and the innermost INNER LAYER
VASCULAR LAYER (middle of eyeball)
• CHOROID: contains blood vessels that supply the eye and also produces melanin
that assists in absorbing light
• CILIARY BODY: circle the lens and houses muscles that control the lens shape
• IRIS: visible colored part of the eye and has an opening at the center called the
PUPIL
• associated muscles contract or relax to control the diameter of the pupil and
thereby regulating the amount of light entering the eye
• iris diameter is under
the control of the
autonomic nervous
system
Fig 15.5 pg 549
THE SPECIAL SENSES
THE EYEBALL
Fig 15.4 pg 548
THE SPECIAL SENSES
THE EYEBALL
• Composed of three regions: the outermost FIBROUS LAYER, the VASCULAR
LAYER and the innermost INNER LAYER
• the INNER LAYER is the RETINA: contains millions of photoreceptors that
transduce light energy. The retina has two main divisions:
1. PIGMENTED LAYER OF RETINA: absorbs light and prevents it from scattering
2. NEURAL LAYER OF RETINA: contains three major types of neurons:
PHOTORECEPTORS: sensory cells that detects incoming light
BIPOLAR CELLS and GANGLION CELLS: transduce light information to optic
nerve (which eventually leads to the brain)
Fig 15.6 pg 550
THE SPECIAL SENSES
PHOTORECEPTORS
PHOTORECEPTORS: are sensory cells that detect incoming light. A quarter billion are
found in the retina and come in two forms:
RODS: used for dim-light and peripheral vision because they are more numerous
and more sensitive to light.
• however, rods do not provide sharp vision or color vision
CONES: are photoreceptors for bright light and provide high resolution color vision
Fig 15.6 pg 550
THE SPECIAL SENSES
EYE HUMORS
HUMOR refers to the liquid that fills and supports the eye ball. There are two main
types that occur in different locations of the eye
1. VITREOUS HUMOR: covers the lens are cornea and helps transmit light and
maintain intraocular pressure
2. AQUEOUS HUMOR: covers posterior eye and is continually circulated because is
assists in supplying oxygen and nutrients to eye while draining metabolic wastes
• GLAUCOMA occurs when aqueous humor fails to drain and pressure builds
in the eye compressing the retina and optic nerve
Fig 15.8 pg 552
THE SPECIAL SENSES
LENS
• the LENS is a convex and transparent structure that can bend to precisely focus
light on the retina (ciliary muscle control its shape)
• like the cornea, it lacks a blood supply (it must to be transparent)
• is composed primarily of CRYSTALLIN proteins
• CATARACTS result from clouding of the lens. Cataracts are triggered by oxidative
damage that promotes the clumping of crystallin proteins which makes the lens
less transparent.
Cataracts, a clouding of the lens
Fig 15.9 pg 553
THE SPECIAL SENSES
LIGHT AND OPTICS
• eyes respond to the part of the ELECTROMAGNETIC SPETRUM called VISIBLE
LIGHT ( approx. 400-700 nm wavelengths)
• color is given to objected by the wavelengths they reflect. For example, an apple
reflects mostly red wavelengths of light between 650-700 nm
• Light travels in a straight line, but slows down when traveling through objects of
differing density which causes REFRACTION
Fig 15.10 & 15.11 pg 554
THE SPECIAL SENSES
LIGHT AND OPTICS
• light is refracted three times before contacting the photoreceptors in the retina:
entering the cornea, entering the lens and leaving the lens
• changing the curvature of the lens ensures that light converges on the retina
• CILIARY MUSCLES contract to cause the lens to become more rounded in order
to focus on close objects
• to focus on distant objects, ciliary muscles relax causing the lens to flatten
• this process is called ACCOMODATION
THE SPECIAL SENSES
VISUAL PROBLEMS
• MYOPIA: occurs when distant objects do not focus on the retina and cannot be
viewed clearly. Commonly called nearsightedness and can be fixed with flattened
contact lenses that better focus distant objects
• HYPEROPIA: or farsightedness occurs when light from close objects do not focus
on the retina, but can be corrected with convex corrective lenses.
Fig 15.14 pg 557
THE SPECIAL SENSES
PHOTOTRANSDUCTION
• PHOTOTRANSDUCTION is the process of converting light energy into changes in
membrane potential
• photoreceptors are arranged into outer and inner segments
• the outer segments contain the PHOTOPIGMENTS that absorb incoming light.
• inner segments contain the synaptic terminals that connect photoreceptors to
other retinal neurons that lead to the nervous system
• rods are very sensitive and
function in low light
conditions
• cones contain one of three
different photopigments,
each of which absorb a
different wavelength of color
(red, green or blue) and thus
provide color vision
Fig 15.15 pg 558
THE SPECIAL SENSES
PHOTOTRANSDUCTION
• photopigments are a combination of a light absorbing molecule called RETINAL
(a derivative of VITAMIN A) and an OPSIN protein
• for example, RHODOPSIN is the photopigment found in rods
• when retinal is struck by light it undergoes a conformational (shape) change:
• photoexcitation
causes 11-CISRETINAL to change
to 11-TRANSRETINAL
• same process
occurs in cones,
but using different
photopigments
Fig 15.16 pg 560
THE SPECIAL SENSES
PHOTOTRANSDUCTION
• the conformational change in retinal, triggers the opsin protein to change
shape, which in turn triggers a G-PROTEIN SIGNALING CASCADE that ultimately
leads to the opening or closing of ion channels that changes membrane
potentials
(not necessary to memorize the steps in this signaling pathway)
Fig 15.17 pg 561
THE SPECIAL SENSES
PHOTORECEPTOR PATHOLOGIES
• deficiencies in vitamin A occurs in countries where malnutrition is common
• lack of vitamin A in the diet prevents the synthesis of the photopigment
RETINAL
• this leads to rod degeneration which seriously hampers vision in low light,
more commonly called NIGHT BLINDNESS
• eat your carrots if you want to see at night!
• genetic conditions that affect the CONES cause color blindness
THE SPECIAL SENSES
CHEMICAL SENSES: OLFACTION (SMELL)
• although our sense of smell is far less acute than other animals, humans can still
distinguish a very large number of odors
• the organ of smell is a small patch of pseudostratified epithelium located in the
nasal cavity called the OLFACTORY EPITHELIUM
• the olfactory epithelium contains millions of OLFACTORY SENSORY NEURONS
• mucus helps capture chemical odors
Fig 15.20 pg 566
THE SPECIAL SENSES
CHEMICAL SENSES: OLFACTION (SMELL)
• there are about 400 “smell” genes active only in the nose, each encoding a
different olfactory receptor (in combination can discriminate about 10,000
scents)
• chemical must be volatile (gas) and be capable of dissolving in mucus to reach
receptor, otherwise it will be undetectable
OCTANOIC ACID
• smells like roses or
oranges
carboxylic
acid
OCTANOL
hydroxyl
• smells rancid or sweaty
THE SPECIAL SENSES
CHEMICAL SENSES: OLFACTION (SMELL)
• olfaction follows the same type of transduction pathway as was described for
photoreceptors
• binding of an ODORANT to an OLFACTORY RECEPTOR triggers a conformational
change in the receptor that opens ion channels and creates an action potential
that transmits this information to the OLFACTORY BULBS of the brain
• some “emotional” odors are
eventually processed by the
LIMBIC SYSTEM
• dangerous odors like smoke
can trigger fight or flight
response
• food odors can trigger
salivation and stimulate
digestion
THE SPECIAL SENSES
PATHOLOGIES OF SMELL
• most olfactory disorders result from head injuries that tears the olfactory
nerves or neurological disorders like Parkinson’s disease
• some people can have UNCINATE FITS, olfactory hallucinations in which they
experience a particularly bad smell (e.g. rotting meat)
• for example, can occur in epileptics just prior to a seizure
THE SPECIAL SENSES
CHEMICAL SENSES: GUSTATION (TASTE)
• the sensory organs for taste are TASTE BUDS, most of which are located on the
tongue (a few are scattered around the inside of the mouth)
• taste buds are located on the
top of FUNGIFORM
PAPILLAE, mushroom shaped
bumps scattered over the
surface of the tongue
• a few FOLIATE PAPILLAE are
found on the edges of the
tongue and fewer still
VALLATE PAILLAE are found
at the back of the tongue
Fig 15.22 pg 568
THE SPECIAL SENSES
CHEMICAL SENSES: GUSTATION (TASTE)
• taste buds contain GUSTATORY EPITHELIAL CELLS that are the receptors for taste
• these cells have long microvilli called GUSTATORY HAIRS that project into the
taste bud through a pore and act as the sensitive portions of the cell
• coiling around each gustatory epithelial cell are nerve fibers that provide a link to
the nervous system
Fig 15.22 pg 568
THE SPECIAL SENSES
CHEMICAL SENSES: GUSTATION (TASTE)
• normally, taste occurs as a complicated mixture of qualities
• can be grouped into five categories:
1.
2.
3.
4.
SWEET: elicited by organic substances including sugars, amino acids and
alcohols
SOUR: produced by acids and their high hydrogen ion content
SALTY: produced by metal ions like sodium and chloride
BITTER: elicited alkaloids like caffeine, morphine, quinine, nicotine
• these tastes have an important nutritional function, by signaling foods we
require for survival, for example:
• sugar indicates carbohydrates
• salts indicate minerals
• sour and bitter indicate things we should not eat
THE SPECIAL SENSES
ACTIVATION OF TASTE RECEPTORS
• for a chemical to taste, it must dissolve in saliva, diffuse through taste bud pores
and contact the gustatory hairs
• each taste is detected in a different way. For example, salty and sour do not
actually use receptors instead effects ion channels directly
1.
SALTY-triggered by Na+ influx through Na+ ion channels present in gustatory
hairs
2.
SOUR-triggered by H+ influx through H+ ion channels present in gustatory hairs
• thus salty and sour do not require receptors, but can alter membrane
potential directly
3.
SWEET-triggered by the binding of sugars to a receptor protein called
GUSTDUCIN
4.
BITTER-also requires receptors, but bitter receptors are much more complex
and humans have at least 25 different genes encoding for bitter taste receptors
THE SPECIAL SENSES
PATHOLOGIES OF TASTE
• causes of taste disorders include: upper respiratory tract infections, head
injuries, chemicals or medications and neck radiation for cancer treatment
• disorders of taste are rare because taste can be detected over a larger area than
smell and is transduced through multiple neuronal pathways
FOR REVIEW TONIGHT
• Understand the different types of sensory receptor and how
they are distinguished
• Understand how action potentials can be varied to create
different signals
• Understand the structure and function of various parts of the
eye
• Understand the role of rods and cones in the process of
phototransduction
• Understand the process of olfaction
• Understand how different tastes excite sensory cells (i.e.
receptors vs. direct effects on ion channels)
NEXT LECTURE
NERVOUS SYSTEM II
• Hearing, equilibrium and the Autonomic Nervous
System