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Peripheral Nervous

Organization of :

Nervous system

Integration Peripheral nervous system (CNS) (PNS) Motor Sensory output input

Brain Motor division Sensory division (efferent) (afferent)

Autonomic nervous system nervous system (involuntary; smooth & cardiac muscle) (voluntary; )

Sympathetic division Parasympathetic division

Peripheral Nervous System

The peripheral nervous system links the to the “real” world

Ganglia ( cell bodies)

Nerve Types: 1) Sensory nerves (contains only afferent fibers) (bundles of ) 2) Motor nerves (contains only efferent fibers)

3) Mixed nerves (contains afferent / efferent fibers) Sensory receptors Most nerves in the body are mixed nerves

Motor output

Peripheral Nervous System

Nerve Structure: A. : • Outside nerve covering • Dense network of fibers

B. Perineurium: • Divides nerve into fascicles • Contains vessels Epineurium C. Endoneurium: • Surrounds individual axons and ties them together

Fascicle

Marieb & Hoehn – Figure 13.26

1 Peripheral Nervous System

Classification of Nerve Fibers:

• Classified according to conduction velocity

Relative Relative conduction Classification Type diameter velocity Myelination

A alpha (A) Largest Fastest (120 m / s) Yes A beta (A) Medium Medium Yes Sensory and A gamma (A) Medium Medium Yes Motor A delta (A) Small Medium Yes (Erlanger & Gasser) B Small Medium Yes

C Smallest Slowest (0.2 m / s) No

Ia Largest Fastest Yes Ib Largest Fastest Yes Sensory only II Medium Medium Yes (Lloyd & Hunt) III Small Medium Yes IV Smallest Slowest No

Peripheral Nervous System

Organization of Nervous System:

Nervous system

Integration Central nervous system Peripheral nervous system (CNS) (PNS) Motor Sensory output input

Brain Spinal cord Motor division Sensory division (efferent) (afferent)

Autonomic nervous system (involuntary; smooth & cardiac muscle) (voluntary; skeletal muscle)

Sympathetic division Parasympathetic division

Sensory

“Nothing is in the that does not pass through the ”

Aristotle (~ 350 B.C.)

• Sensory receptors provide the only channels of communication from the external world to the nervous system: • Detections: Electrical impulses - triggered by cells • Sensations: Electrical impulses - reach brain via Subjective • : Interpretation of electrical impulses by brain

Sensations / perceptions are not inherent in stimuli themselves; instead they depend on the neural processing of the stimuli

2 Sensory Systems

Sensory Pathways in NS: Fourth-order midline neuron 1) Receptor: • Converts stimuli to electrochemical energy • Structure variable Cerebral cortex • Specialized epithelial cells (e.g., vision) Third-order neuron • Modified neurons (e.g., olfaction) 2) First-order afferent neuron: • Cell body located in (PNS) Second-order 3) Second-order afferent neuron: neuron • with first-order neuron at relay nuclei • Many first-order synapse with one second-order Brain stem • present (signal processing) • Cross at midline (decussate) 4) Third-order afferent neuron: • Reside in thalamus (relay nuclei) Relay • Interneurons present (signal processing)

Receptor 5) Fourth-order afferent neuron: Spinal • Reside in appropriate sensory area First-order cord neuron

Sensory Systems : Category of sensation Sensory Receptors:

Type of Receptor: Modality: Location:

• Skin (touch) • Inner (audition) • (vestibular) Touch Audition Vestibular

Photoreceptors • (vision) Vision

• () • Nose (olfaction) Taste Olfaction Osmolarity • Vessels (osmolarity)

Thermoreceptors • Skin (temperature) Temperature

Nociceptors • Skin () Pain

Guyton & Hall – Figure 46.2 / 46.3 Sensory Systems

Sensory Receptors:

Sensory transduction: The process by which environmental stimuli activates a receptor and is converted into electrical energy

• Receptor excitation = Change in (Receptor potential) (usually via opening of channels)

Example:

1) Pressure causes a change in the membrane properties (e.g., mechanical deformation) Receptor Potential > Threshold  2) Ion channels open; receptor potential develops • Amplitude correlates with size ( Intensity =  Receptor potential =  AP ) 3) Receptor potential triggers action potential

3 Costanzo – Figure 3.5 / 3.6 Sensory Systems

Sensory Receptors:

Receptor Field: An area of the body that when stimulated results in a change in firing rate of a

Receptor fields may be excitatory or inhibitory Receptor fields exist for first-, Excitatory receptor fields increase firing rate second-, third-, and fourth-order neurons Inhibitory receptor fields decrease firing rate

The smaller a receptor field, the more Lateral inhibition defines boundaries precisely the sensation can be localized and provides a contrasting border

Sensory Systems

Sensory Receptors:

Sensory coding: Various aspects of perceived stimuli are encoded and sent to the proper location in the CNS

Features encoded:

1) Modality 2) Location 3) Threshold

Labeled Line Principle: Threshold of Detection: Receptors respond to Weakest signal that will single modality produce a receptor response 50% of the time Touch Somatosensory : receptor cortex Vision: Bending of hair equal to 1 photon of light Receptor fields diameter of H atom

4) Intensity 5) Duration • Number of receptors activated Length of time a sensory • Firing rate of sensory neurons neuron fires • Types of receptors activated During prolonged stimulus, receptors “adapt” to the stimulus

Sensory Systems Adaptation depends on how quickly receptor potential drops below threshold Sensory Receptors: Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency of action potentials generated declines over time

Tonic receptors (slowly adapting receptors)

Can encode stimulus intensity

Will adapt to extinction; may take hours / days

Pattern of adaptation differs among • Transmit impulses as long as different types of receptors stimulus present • Keep brain appraised of body status

Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7

4 Sensory Systems Adaptation depends on how quickly receptor potential drops below threshold Sensory Receptors: Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency of action potentials generated declines over time

Phasic receptors (rapidly adapting receptors)

Can not encode stimulus intensity

Pattern of adaptation differs among • Transmit impulses only when different types of receptors change is taking place • Important for predicting future position / condition of body

Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7

Costanzo – Figure 3.8 Sensory Systems Rapid adapters = Change in velocity : Slow adapters = Intensity; duration • Processes information about touch, position, pain, and temperature

Mechanoreceptors: Two-point discrimination

Pacinian corpuscle Meissner’s corpuscle Location = Subcutaneous; intramuscular Location = Non-hairy skin Detection = Vibration; tapping Detection = Tapping; fluttering Adaptation = Very rapidly Adaptation = Rapidly

Hair plexus Ruffini’s corpuscle Location = Hairy skin Location = Skin; capsules Detection = Velocity; movement direction Detection = Stretch; joint rotation Adaptation = Rapidly Adaptation = Slowly

Costanzo – Figure 3.8 / 3.9 Sensory Systems Rapid adapters = Change in velocity Somatosensory System: Slow adapters = Intensity; duration • Processes information about touch, position, pain, and temperature

Mechanoreceptors:

Merkel’s receptor / Tactile disc Location = Non-hairy skin / Hairy skin Detection = Vertical indentation Adaptation = Slowly

Thermoreceptors: • Slowly adapting receptors; located in skin

• Cold receptors vs. warm receptors • Overlap in response ranges • Do not respond at extreme ranges ()

5 Sensory Systems

Somatosensory System: • Processes information about touch, position, pain, and temperature

Nociceptors: • Respond to noxious stimuli that can produce tissue damage

Mechanical nociceptors: • Respond to mechanical stimuli • A delta (A) afferent neurons (myelinated)

Polymodal nociceptors: • Respond to multiple stimuli: • High-intensity chemical stimuli • Hot / Cold stimuli • C afferent neurons (unmyelinated)

Chemical cues released at damaged / : Intense activity areas include: Process where axons of nociceptors release • H+ • Prostaglandins substances that sensitize the nociceptors to • K+ • stimuli that were not previously painful • ATP •

Sensory Systems

Somatosensory Pathways: midline 1) Dorsal column system • Information transmitted: Cerebral a) Discriminative touch cortex b) Pressure / Vibration c) Two-point discrimination d) • Consists mainly of groups I and II fibers Thalamus • Fibers decussate in brain stem (second-order neurons) • Nucleus gracilis = Info from lower body • Nucleus cuneatus = Info from upper body Brain stem Fasciculus Fasciculus cuneatus gracilis

Receptor Spinal cord

Marieb & Hoehn – Figure 12.33

Sensory Systems

Somatosensory Pathways: midline 2) Anterolateral (spinothalamic) system • Information transmitted: Cerebral a) Pain cortex b) Temperature / Light touch • Consists mainly of groups III and IV fibers • Fast pain (e.g., pin prick) • A delta fibers; group II & group III fibers Thalamus • Rapid onset / offset; precisely localized • Slow pain (e.g., burn) • C fibers; group IV fibers Brain stem • Aching / throbbing; poorly localized

Lateral spinothalamic (pain / temperature)

Receptor Spinal Ventral cord spinothalamic Marieb & Hoehn – Figure 12.33 (light touch)

6 Sensory Systems

Somatosensory Pathways: midline 2) Anterolateral (spinothalamic) system

Referred Pain: Cerebral cortex

Thalamus

Brain stem

Dematomal rule: Sites on the skin are innervated by nerves arising Receptor from the same spinal cord segments as those Spinal innervating the visceral organs cord

Marieb & Hoehn – Figure 14.8

Sensory Systems

Vision:

(~ 1 ml / day) 70% of sensory receptors 50% of cerebral cortex

Contained in of :

1) Orbital (cushions / insulates eye) 2) Extrinsic eye muscles (6) 3) : Produces tears • Lubricates eye Strabismus: • Supplies nutrients / oxygen A condition in which an eye rotates • Provides antibacterial medially / laterally

Marieb & Hoehn – Figure 15.3

Sensory Systems is the only tissue that can be transplanted with limited rejection issues Vision: Layers of the Eye: Cornea: : Transparent window “White of the eye” 1) Fibrous Layer: (outermost) • Offers structurally support / protection Sclera • Acts as anchoring point for muscles Cornea • Assists in light focusing 2) Vascular Layer: (middle) • Contains blood vessels / lymph vessels • Regulates light entering eye • Controls shape of Iris: Ciliary circular Regulates size of radial body muscles muscles constrict constrict Choroid: Pigmented layer; delivers nutrients to : Parasympathetic Sympathetic Smooth, muscular ring; controls lens shape

• Only contain brown pigment Marieb & Hoehn – Figure 15.4

7 Marieb & Hoehn – Figure 15.4 / 15.6 Sensory Systems

Vision: Optic Disk (blind spot): Origin of ; no Layers of the Eye: photoreceptors present 3) Retina: (innermost) • Pigmented layer (light absorption) Sclera • Neural layer (light detection) Cornea Choroid

Incoming

light Pigmented Layer Fovea

Iris Optic Disk

Ciliary

body Retina

Fovea centralis • Photoreceptors (light detectors) • Rods (light sensitive - ~ 125 million / eye) • Cones (color sensitive - ~ 6 million / eye) Cones clustered in fovea of macula lutea (sharp focus)

Sensory Systems

Vision: Posterior segment Segments of the Eye:

Aqueous Humor: Circulating fluid in anterior segment • Nutrient / waste transport • Cushioning of eye • Retention of eye shape

Vitreous Humor: Anterior Gelatinous mass in posterior segment segment • Transmits light • Stabilize eye shape Canal of Schlemm: • Holds retina firmly in place Drains aqueous humor

Intraocular Pressure = 12 – 21 mm Hg Ciliary body produces Glaucoma: aqueous humor Disease where intraocular pressure (1 – 2 l / minute) becomes pathologically high (~ 70 mm Hg) Marieb & Hoehn – Figure 15.8

Sensory Systems Lasik eye Vision: surgery

For clear vision, light must be focused on the retina

Close objects =  focal distance Rounder lens =  focal distance Refraction: The bending of light when it passes between mediums of different density : 1) Cornea: 85% of refraction (fixed) Changing the shape of a lens 2) Lens: 15% of refraction (variable) to keep an image in focus (constant focal length) • Composed of lens fibers (cells) Cataract: (cloudy lens) • Crystallins (transparent )

8 Sensory Systems Near point of vision: Vision: Inner limit of clear vision

Accommodation: Children = 7 – 9 cm Loss of lens Young Adults = 15 – 20 cm elasticity Elderly Adults = 70 – 85 cm

tightens relaxes suspensory suspensory ligaments ligaments

Lens flattens for distant object focus; Lens bulges for near object focus; Ciliary body RELAXES Ciliary body CONTRACTS

In Addition: • constrict • Eyeballs converge

Sensory Systems 20 / 20 = Standard visual acuity Vision: 20 / 200 = Legally blind When Things Go Wrong:

Near-sighted Far-sighted

Emmetropia Myopia Hyperopia

Astigmatism: Warping of the lens or cornea leading to image distortion Correct with diverging lens Correct with converging lens

Randall et al. – Figure 7.38 Sensory Systems

Vision:

Photon: Basic unit of visible light

High Energy Low Energy (short wavelength) (long wavelength)

9 Sensory Systems Horizontal cells / Amacrine cells Vision: (facilitate / inhibit serial links – retinal processing) Retinal Layers: • Absorbs stray light Pigment cell layer • Storage site for retinal

(R) = Receptor cells Photoreceptor layer • Rods / cones

(H) = Horizontal cells • Nuclei of receptor cells (B) = Bipolar cells (A) = Amacrine cells • between receptors and interneurons

Inner nuclear layer • Nuclei of interneurons

• Synapses between interneurons and ganglion cells

Ganglion cell layer • Nuclei of ganglion cells

Optic nerve layer • Leave eye via Serial link

Costanzo – Figure 3.13

Sensory Systems

Vision: Retinal Layers: (Day vision) Cones demonstrate high acuity and low sensitivity (R) = Receptor cells • Only a few cones synapse on a single bipolar cell (H) = Horizontal cells • In fovea, a single cone synapses on (B) = Bipolar cells a single bipolar cell (A) = Amacrine cells • A single bipolar cell synapses on a single ganglion cell

() Rods demonstrate low acuity and high sensitivity • Many rods synapse on a single bipolar cell • Light striking any one rod will activate the bipolar cell • Multiple bipolar cells may synapse on a single ganglion cell

Costanzo – Figure 3.13

Sensory Systems

Vision: A single photon of light can activate a rod; several hundred photons of light are Photoreception: required to activate a cone

Rod Structure: Cone Structure:

(more abundant In rods) Light-sensitive photochemicals

Outer segment

Inner Cytoplasm / segment cytoplasmic organelles Connected via individual

Connection to Synaptic neural elements terminal

Costanzo – Figure 3.14

10 Sensory Systems

Vision: Photoreception:

Photochemistry of Vision: • Rods and cones contain chemicals that decompose on exposure to light • Rods = Rhodopsin (visual purple) Similar compositions • Cones = Cone pigments (color pigments)

Example: Rhodopsin

Structure: • = membrane • Retinal = light-absorbing pigment ( of vitamin A)

Two sterically distinct retinal states in the retina

Light

(photoisomerization) cis-form trans-form

Sensory Systems

Vision: Photoreception:

Rod Receptor Potential:

• At rest (in dark…), rod receptor potential ~ - 40 mV (receptor average = - 80 / - 90 mV) • Cause: Outer segment highly permeable to Na+ Dark Current

Rhodopsin decomposition triggers hyperpolarization of receptor cell

Hyperpolarize The greater the amount of light, the greater the hyperpolarization

constant NT release NT release NT = Glutamate

Sensory Systems

Vision: Photoreception:

Rhodopsin Activation: cGMP no longer present to bind to ion channels 1) Conformational change in opsin 4) Closure ion channels

Trans-retinal detaches

Opsin converted to metarhodopsin II

3) Activation of ()

2) Activation of transducin ()

Randall et al. – Figure 7.44

11 Sensory Systems Night Blindness: Vision: Reduced photosensitivity of (nutritional deficiency of vitamin A) Photoreception:

Regeneration of Rhodopsin: Storage location (picked up from blood)

(Free of opsin)

Bleaching: The fading of photopigment upon absorption of light

Randall et al. – Figure 7.45

Sensory Systems

Vision: Photoreception:

Color Vision:

• Three classes of cones recognized • Trichromacy Theory • : Blue, Green, • Sensation of color = CNS processing • Color sensitivity depends on opsin structure, not light-absorbing molecule • Humans (~ 50% analogy with a.a. of rods) • Blue = autosomal chromosome • Red / Green = X chromosome •  similarity (gene duplication)

Colorblindness: Defect in cone opsin gene

Guyton & Hall – Figure 50.10

Sensory Systems

Vision: Sex-linked (1% of males affected) Very rare (autosomal recessive) Photoreception: 8% of women carriers

Normal Protanopia Deuteranopia Tritanopia (lacking red cones) (lacking green cones) (lacking blue cones)

Test Yourself…

12 Sensory Systems

Nasal Vision: field Temporal L R L R Temporal Optic Pathways: field field

Hemianopia: Loss of vision in half the visual field of one or both eyes Optic nerve Visual Field 1) Optic chiasma Left Right 2) 1) 3)

Lateral geniculate nuclei 2) (thalamus)

Geniculocalcarine tract 3)

= deficit

Costanzo – Figure 3.17 Primary

Martini & Nath – Figure 13.18 Sensory Systems

Auditory:

Oval Inner ear window Tympanic Sensory receptors / membrane Vestibule (equilibrum)

Cochlea (hearing)

External acoustic meatus

Middle ear Converts waves Into mechanical movements External ear (amplification) Collect / direct sound waves Auditory tube

Marieb & Hoehn – Figure 15.28 Sensory Systems (“snail”): Spiral-shaped chamber that Auditory: houses the receptors for hearing

Cochlear nerve Scala Scala media Vestibuli Cochlea (contains ) (contains )

Organ of Corti

Tectorial membrane of Corti:

As vibrates, hair cells bump up against Scala tempani (contains perilymph) Basilar membrane

13 Marieb & Hoehn – Figure 15.30 Sensory Systems Sound: A pressure disturbance produced Auditory: by a vibrating object

The of Sound:

High pitch Low pitch

Frequency: Number of waves passing a given point in a given time Pitch = Sensory of frequency

Soft Loud

Amplitude: Intensity of sound Loudness = Sensory perception of amplitude

Average human speech = 65 dB Potential damage = > 100 dB Pain = > 120 dB

Marieb & Hoehn – Figure 15.31 Sensory Systems Oscillating NT release produces Auditory: intermittent firing of afferent nerves Amplification of signal: 1) Lever action of Sound waves enter auditory meatus 2) Large window  small window

Tympanic membrane vibrates

Ossicles vibrate

Oval window vibrates

Cochlear microphonic potential Pressure waves exit Pressure waves develop via in scali vestibuli

Hyperpolarization Depolarization vibrates • Shear force bends hairs NT • K+ conductance changes • Depolarization to NT release

Marieb & Hoehn – Figure 15.31 Sensory Systems

Auditory:

High frequency displace Basilar membrane differs basilar membrane near in stiffness along length

Hearing Range: 20 Hz to 20,000 Hz (most sensitive = 2000 – 5000 Hz) Medium frequency sounds displace basilar membrane near middle Conduction : Sound is not able to be transferred to internal ear Tonotopic map Sensorineural Deafness: Low frequency sounds displace Damage occurs in neural basilar membrane near end pathway (e.g., hair cells)

14 Sensory Systems Auditory: (tonotopic map preserved) midline Auditory Pathways:

Medial Cerebral geniculate cortex Central lesions do not cause nuclei deafness because fibers from (thalamus) each ear are intermixed in CNS

Inferior Thalamus colliculus nuclei () Brain stem

Cochlear nuclei (medulla) Spinal cord

Marieb & Hoehn – Figure 15.34 Sensory Systems

Vestibular System: Vestibule: Region of inner ear housing receptors that respond to gravity • Provides stable visual image for retina sensation / linear acceleration • Adjust posture to maintain balance (static equilibrium)

Vestibule Maculae anatomy:

Otolithic membrane

Hair cell (horizontal) (vertical)

Maculae: Sensory receptors for static equilibrium acceleration (e.g, forward) causes For every position of the head, there is a unique to slide; subsequent bending pattern of AP activity for the utricle & saccule of hair cells modifies AP firing rate

Marieb & Hoehn – Figure 15.34 / 15.36 Sensory Systems

Vestibular System: Semicircular canals: Region of inner ear housing receptors that respond to rotational Analyze three (3) rotational planes movements of head (“yes” / “no” / head tilt) (dynamic equilibrium)

Crista ampullaris anatomy: Semicircular canal Cupula

Ampulla

Endolymph (fluid)

Crista ampullaris: Sensory receptor for dynamic equilibrium Head rotation (e.g., spinning) causes endolymph to push against cupula; subsequent bending of hair cells modifies AP firing rate

15 Sensory Systems

Vestibular System:

Modified hair cell

Example: Counterclockwise rotation of head

1) Cupula begins to move; endolymph is stationary 2) Eventually, endolymph catches up with cupula (hair cells quiescent) 3) When stopped, endolymph briefly moves hair cells in opposite direction (reverses firing rates)

Rotation of head stimulates semicircular cells on same side rotation is occurring

( bent (stereocilia bent toward ) away from kinocilium)

Costanzo – Figure 3.23

Sensory Systems

Vestibular System: Visual (e.g., nystagmus) midline Vestibular Pathways: Postural reflexes

Nucleus Input Output Cerebral cortex Medial nucleus Semicircular Extraocular Superior nucleus canals muscles

Lateral nucleus Vestibule Spinal cord (utricles) () Thalamus

Vestibule / Inferior nucleus Semicircular canals Brain stem

Vestibular nerve

Vestibular nuclei Spinal (medulla) cord

Vestibulospinal tract

Sensory Systems

Olfaction: Olfactory Organ: ()

Basal cell

Olfactory receptor cell

Supporting Nasal cell

• cells (~ 1 million / cm2) • Primary afferent neurons (bipolar) • Ciliated (~ 20 cilia / cell) • Odorant-binding proteins • Odorants must by / lipid soluble • Supporting cells •  4 molecules = receptor activation • Basal cells (stem cells  new receptors) • ~ 1000 receptor types (discriminate ~ 10,000) • Continuous

16 Sensory Systems Across-fiber pattern code: Olfaction: Each odorant produces unique pattern of activity across a Olfactory Transduction: population of receptors

• G protein (Golf) coupled to adenylate cyclase • ATP  cAMP • cAMP triggers -gated Na+ channels

Olfactory Encoding:

We don’t exactly know how…

• What is known: 1) Olfactory receptors are not dedicated to a single odorant 2) Olfactory receptors are selective (variable response…) 3) Different receptor proteins have different responses to the same

Sensory Systems

Olfaction: midline Olfactory Pathways: Olfactory ~ 1000 receptor cells synapse cortex Cerebral with a single mitral cell cortex

Mitral cells Glomeruli (olfactory tract) () Thalamus Cribiform plate Brain stem

Nasal epithelium

Spinal cord

Marieb & Hoehn – Figure 15.23 Sensory Systems

Gustation: : (~ 10,000 / tongue)

Basal cell

Taste pore Gustatory Bitter cell Sour Salty Sweet • Gustatory cells (taste receptors) • Slender microvilli (taste hairs) • 10 day lifespan • Basal cells (stem cells  new receptors) • Taste buds on :

Circumvallate Fungiform Foliate (majority of taste buds) (anteriorly located) (laterally located)

17 Sensory Systems

Gustation: Taste Transduction:

transient receptor potential

Taste buds spread across tongue; different thresholds in different regions

Costanzo – Figure 3.37 / 3.28

Sensory Systems

Gustation: midline Taste Pathways: Ventral Taste cortex posteromedial Cerebral nucleus cortex Second-order neurons (thalamus) project ipsilaterally

Solitary nucleus (medulla) Thalamus

Brain stem Taste bud Posterior 1/3 of tongue (bitter / sour): Glossopharyngeal nerve (IX)

Anterior 2/3 of tongue (sweet / salty / umami): (VII)

Spinal cord

18