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Peripheral Peripheral Nervous System

Organization of Nervous System: The peripheral nervous system links Nervous system the to the “real” world

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

Brain Motor division Sensory division (efferent) (afferent)

Nerve Types: 1) Sensory nerves (contains only afferent fibers) (bundles of ) Autonomic nervous system Somatic nervous system 2) Motor nerves (contains only efferent fibers) (involuntary; smooth & cardiac muscle) (voluntary; ) 3) Mixed nerves (contains afferent / efferent fibers) Sensory receptors Most nerves in the body are mixed nerves Sympathetic division Parasympathetic division Motor output

Peripheral Nervous System Peripheral Nervous System

Nerve Structure: Endoneurium Classification of Nerve Fibers:

A. Epineurium: • Classified according to conduction velocity • Outside nerve covering • Dense network of fibers Perineurium Relative Relative conduction Classification Type diameter velocity Myelination B. Perineurium: • Divides nerve into fascicles A alpha (A) Largest Fastest (120 m / s) Yes • Contains blood vessels A beta (A) Medium Medium Yes Epineurium C. Endoneurium: Sensory and A gamma (A) Medium Medium Yes • Surrounds individual axons and ties Motor A delta (A) Small Medium Yes them together (Erlanger & Gasser) B Small Medium Yes

C Smallest Slowest (0.2 m / s) No

Fascicle Ia Largest Fastest Yes Ib Largest Fastest Yes Sensory only II Medium Medium Yes (Lloyd & Hunt) III Small Medium Yes IV Smallest Slowest No Marieb & Hoehn – Figure 13.26

Peripheral Nervous System Sensory Systems

Organization of Nervous System: “Nothing is in the mind that does not pass through the ” Nervous system Aristotle (~ 350 B.C.) Integration Central nervous system Peripheral nervous system (CNS) (PNS) • Sensory receptors provide the only channels of from the external Motor Sensory world to the nervous system: output input • Detections: Electrical impulses - triggered by cells Brain Motor division Sensory division • Sensations: Electrical impulses - reach brain via Spinal cord Subjective (efferent) (afferent) • : Interpretation of electrical impulses by brain

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

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

1 Sensory Systems Sensory Systems : Category of sensation Sensory Pathways in NS: Fourth-order Sensory Receptors: midline neuron 1) Receptor: Type of Receptor: Modality: Location: • Converts stimuli to electrochemical energy • Structure variable Cerebral • (touch) • Specialized epithelial cells (e.g., vision) Third-order neuron • Inner (audition) • Modified neurons (e.g., olfaction) • (vestibular) 2) First-order afferent neuron: Touch Audition Vestibular • Cell body located in ganglion (PNS) Second-order Photoreceptors • (vision) 3) Second-order afferent neuron: neuron Vision • Synapse with first-order neuron at relay nuclei • Many first-order synapse with one second-order Brain stem • () • Interneurons present (signal processing) • Nose (olfaction) • Cross at midline (decussate) Taste Olfaction Osmolarity • Vessels (osmolarity) 4) Third-order afferent neuron: • Reside in thalamus (relay nuclei) Relay nucleus • Skin () • Interneurons present (signal processing) Temperature

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

Guyton & Hall – Figure 46.2 / 46.3 Costanzo – Figure 3.5 / 3.6 Sensory Systems Sensory Systems

Sensory Receptors: Sensory Receptors:

Sensory transduction: The process by which environmental stimuli activates a Receptor Field: An area of the body that when stimulated results in a change in receptor and is converted into electrical energy firing rate of a

• Receptor excitation = Change in membrane potential (Receptor potential) Receptor fields may be excitatory or inhibitory (usually via opening of channels) Receptor fields exist for first-, Excitatory receptor fields increase firing rate second-, third-, and fourth-order neurons Inhibitory receptor fields decrease firing rate 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 The smaller a receptor field, the more defines boundaries ( Intensity =  Receptor potential =  AP frequency) 3) Receptor potential triggers action potential precisely the sensation can be localized and provides a contrasting border

Sensory Systems Sensory Systems depends on how quickly receptor potential drops below threshold Sensory Receptors: Sensory Receptors:

Sensory coding: Various aspects of perceived stimuli are encoded and sent to the Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency proper location in the CNS of action potentials generated declines over time

Features encoded: Tonic receptors (slowly adapting receptors) 1) Modality 2) Location 3) Threshold

Labeled Line Principle: Threshold of Detection: Can encode Receptors respond to Weakest signal that will stimulus intensity single modality produce a receptor response 50% of the time Touch Somatosensory : receptor cortex Vision: Bending of equal to 1 photon of light Will adapt to ; Receptor fields diameter of H atom may take hours / days

4) Intensity 5) Duration

• Number of receptors activated Length of time a sensory Pattern of adaptation differs among • Transmit impulses as long as • Firing rate of sensory neurons neuron fires different types of receptors stimulus present • Types of receptors activated During prolonged stimulus, receptors “adapt” to the stimulus • Keep brain appraised of body status

Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7

2 Costanzo – Figure 3.8 Sensory Systems Sensory Systems Adaptation depends on how quickly receptor potential drops below threshold Rapid adapters = Change in velocity Sensory Receptors: : Slow adapters = Intensity; duration Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency • Processes information about touch, position, pain, and temperature of action potentials generated declines over time Mechanoreceptors: Phasic receptors Two-point discrimination (rapidly adapting receptors)

Pacinian corpuscle Meissner’s corpuscle Can not encode stimulus intensity Location = Subcutaneous; intramuscular Location = Non-hairy skin Detection = Vibration; tapping Detection = Tapping; fluttering Adaptation = Very rapidly Adaptation = Rapidly

Pattern of adaptation differs among • Transmit impulses only when different types of receptors change is taking place Hair plexus Ruffini’s corpuscle Location = Hairy skin Location = Skin; capsules • Important for predicting future Detection = Velocity; movement direction Detection = Stretch; joint rotation position / condition of body Adaptation = Rapidly Adaptation = Slowly Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7

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

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

Merkel’s receptor / Tactile disc Mechanical nociceptors: • Respond to mechanical stimuli Location = Non-hairy skin / Hairy skin Detection = Vertical indentation • A delta (A) afferent neurons (myelinated) Adaptation = Slowly Polymodal nociceptors: Thermoreceptors: • Respond to multiple stimuli: • High-intensity chemical stimuli • Slowly adapting receptors; located in skin • Hot / Cold stimuli • C afferent neurons (unmyelinated)

Chemical cues released at damaged / : Intense activity areas include: • Cold receptors vs. warm receptors Process where axons of nociceptors release • H+ • Prostaglandins substances that sensitize the nociceptors to • Overlap in response ranges • K+ • Bradykinins stimuli that were not previously painful • Do not respond at extreme ranges (nociceptors) • ATP • Substance P

Sensory Systems Sensory Systems

Somatosensory Pathways: Somatosensory Pathways: midline midline 1) Dorsal column system 2) Anterolateral (spinothalamic) system • Information transmitted: • Information transmitted: Cerebral Cerebral a) Discriminative touch cortex a) Pain cortex b) Pressure / Vibration b) Temperature / Light touch c) Two-point discrimination • Consists mainly of groups III and IV fibers d) • Fast pain (e.g., pin prick) • Consists mainly of groups I and II fibers Thalamus • A delta fibers; group II & group III fibers Thalamus • Fibers decussate in brain stem (second-order neurons) • Rapid onset / offset; precisely localized • Nucleus gracilis = Info from lower body • Slow pain (e.g., ) • Nucleus cuneatus = Info from upper body Brain • C fibers; group IV fibers Brain stem stem • Aching / throbbing; poorly localized Fasciculus Fasciculus cuneatus gracilis Lateral spinothalamic (pain / temperature)

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

3 Sensory Systems Sensory Systems

Somatosensory Pathways: Vision: midline 2) Anterolateral (spinothalamic) system (~ 1 ml / day) 70% of sensory receptors Referred Pain: Cerebral 50% of cortex

Thalamus

Brain stem Contained in of :

1) Orbital fat (cushions / insulates eye) 2) Extrinsic eye muscles (6) 3) Lacrimal gland: Produces tears Dematomal rule: • Lubricates eye : Sites on the skin are innervated by nerves arising • Supplies nutrients / oxygen from the same spinal cord segments as those Receptor A condition in which an eye rotates Spinal • Provides antibacterial enzymes innervating the visceral organs cord medially / laterally

Marieb & Hoehn – Figure 14.8 Marieb & Hoehn – Figure 15.3

Marieb & Hoehn – Figure 15.4 / 15.6 Sensory Systems Sensory Systems is the only tissue that can be transplanted with limited rejection issues Vision: Vision: Optic Disk (): Origin of ; no Layers of the Eye: Cornea: : Layers of the Eye: photoreceptors present Transparent window “White of the eye” 1) Fibrous Layer: (outermost) 3) : (innermost) • Offers structurally support / protection • Pigmented layer (light absorption) Sclera Sclera • Acts as anchoring point for muscles • Neural layer (light detection) Cornea Cornea • Assists in light focusing Choroid 2) Vascular Layer: (middle) • Contains blood vessels / vessels Incoming

light Pigmented Layer Fovea • Regulates light entering eye Iris • Controls shape of Optic Disk Iris:

Ciliary Ciliary

circular Regulates size of radial body body muscles muscles Retina constrict constrict Choroid: Pigmented layer; delivers nutrients to retina • Photoreceptors (light detectors) : • Rods (light sensitive - ~ 125 million / eye) Parasympathetic Sympathetic Smooth, muscular ring; controls lens shape • Cones ( sensitive - ~ 6 million / eye) (sharp focus) • Only contain brown Marieb & Hoehn – Figure 15.4 Cones clustered in fovea of macula lutea

Sensory Systems Sensory Systems Lasik eye Vision: Posterior Vision: surgery segment Segments of the Eye: For clear vision, light must be focused on the retina

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

Vitreous Humor: Anterior segment Gelatinous mass in posterior segment Close objects =  focal distance Rounder lens =  focal distance • Transmits light : • Stabilize eye shape The bending of light when it passes Canal of Schlemm: • Holds retina firmly in place between mediums of different density Drains : aqueous humor 1) Cornea: 85% of refraction (fixed) Changing the shape of a lens = 12 – 21 mm Hg 2) Lens: 15% of refraction (variable) to keep an image in focus (constant ) Ciliary body produces • Composed of lens fibers (cells) : : aqueous humor (cloudy lens) • (transparent ) Disease where intraocular pressure (1 – 2 l / minute) becomes pathologically high (~ 70 mm Hg) Marieb & Hoehn – Figure 15.8

4 Sensory Systems Sensory Systems Near point of vision: 20 / 20 = Standard Vision: Inner limit of clear vision Vision: 20 / 200 = Legally blind

Accommodation: Children = 7 – 9 cm When Things Go Wrong: Loss of lens Young Adults = 15 – 20 cm elasticity Elderly Adults = 70 – 85 cm Near-sighted Far-sighted

tightens relaxes suspensory suspensory ligaments ligaments

Emmetropia Hyperopia

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

In Addition: : • constrict Warping of the lens or cornea leading to image distortion Correct with diverging lens Correct with converging lens • Eyeballs converge

Randall et al. – Figure 7.38 Sensory Systems Sensory Systems Horizontal cells / Amacrine cells Vision: 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 • Synapses between receptors Photon: and interneurons Basic unit of visible light • Nuclei of interneurons

• Synapses between interneurons and ganglion cells High Energy Low Energy (short wavelength) (long wavelength) • Nuclei of ganglion cells

Optic nerve layer • Leave eye via Serial link

Costanzo – Figure 3.13

Sensory Systems Sensory Systems

Vision: Vision: A single photon of light can activate a rod; several hundred photons of light are Retinal Layers: Photoreception: required to activate a cone (Day vision) Cones demonstrate high acuity Rod Structure: Cone Structure: and low sensitivity (R) = Receptor cells (more abundant In rods) • Only a few cones synapse on a single bipolar cell Light-sensitive (H) = Horizontal cells photochemicals • In fovea, a single cone synapses on (B) = Bipolar cells a single bipolar cell Outer (A) = Amacrine cells segment • A single bipolar cell synapses on a single ganglion cell

() Rods demonstrate low acuity and high sensitivity Inner Cytoplasm / segment • Many rods synapse on a single bipolar cell cytoplasmic Connected via individual • Light striking any one rod will activate the bipolar cell • Multiple bipolar cells may synapse on a single Connection to Synaptic terminal ganglion cell neural elements

Costanzo – Figure 3.13 Costanzo – Figure 3.14

5 Sensory Systems Sensory Systems

Vision: Vision: Photoreception: Photoreception:

Photochemistry of Vision: Rod Receptor Potential:

• Rods and cones contain chemicals that decompose on exposure to light • At rest (in dark…), rod receptor potential ~ - 40 mV (receptor average = - 80 / - 90 mV) • Rods = Rhodopsin (visual purple) • Cause: Outer segment highly permeable to Na+ Similar compositions Dark Current • Cones = Cone (color pigments) Rhodopsin decomposition triggers Example: Rhodopsin hyperpolarization of receptor cell Structure: • Opsin = membrane

• Retinal = light-absorbing pigment Hyperpolarize (aldehyde of vitamin A) The greater the amount of light, the greater the Two sterically distinct retinal states in the retina hyperpolarization

Light

(photoisomerization) constant NT release NT release cis-form trans-form NT = Glutamate

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

Rhodopsin Activation: Regeneration of Rhodopsin: cGMP no longer Storage location present to bind (picked up from blood) to ion channels 1) Conformational change in opsin 4) Closure ion channels (Free of opsin)

Trans-retinal detaches

Opsin converted to metarhodopsin II

3) Activation of phosphodiesterase (enzyme) Bleaching: The fading of photopigment upon absorption of light 2) Activation of transducin (G protein)

Randall et al. – Figure 7.44 Randall et al. – Figure 7.45

Sensory Systems Sensory Systems

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

Color Vision:

• Three classes of cones recognized • Trichromacy Theory • : , Green, Orange Normal • Sensation of color = CNS processing Protanopia Deuteranopia Tritanopia (lacking red cones) (lacking green cones) (lacking blue cones) • Color sensitivity depends on opsin structure, not light-absorbing molecule Test Yourself… • 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

6 Martini & Nath – Figure 13.18 Sensory Systems Sensory Systems

Nasal Vision: field Auditory: Temporal L R L R Temporal Optic Pathways: field field Oval Inner ear window Tympanic Sensory receptors Hemianopia: / membrane Loss of vision in half the Vestibule (equilibrum)

of one or both eyes (hearing) Optic nerve Visual Field 1) Optic chiasma Left Right 2) External 1) 3) acoustic meatus

Lateral geniculate nuclei 2) (thalamus) Geniculocalcarine tract Converts waves Into 3) mechanical movements External ear (amplification) Auditory = deficit Collect / direct sound waves tube

Costanzo – Figure 3.17 Primary

Marieb & Hoehn – Figure 15.28 Marieb & Hoehn – Figure 15.30 Sensory Systems Sensory Systems Cochlea (“snail”): Sound: Spiral-shaped chamber that A pressure disturbance produced Auditory: houses the receptors for hearing Auditory: by a vibrating object The Nature of Sound: High pitch Low pitch Scala Scala media Vestibuli Cochlea (contains ) (contains ) Frequency: Number of waves passing a given point in a given time Pitch = Sensory of frequency

Organ of Corti Soft Loud

Tectorial membrane of Corti: Amplitude: Intensity of sound

As Loudness = Sensory perception of amplitude vibrates, hair cells bump up against Average human speech = 65 dB Potential damage = > 100 dB Scala tempani (contains perilymph) Basilar membrane Pain = > 120 dB

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

Tympanic membrane vibrates

High frequency displace Basilar membrane differs basilar membrane near base in stiffness along length Ossicles vibrate : 20 Hz to 20,000 Hz vibrates (most sensitive = 2000 – 5000 Hz) Medium frequency sounds displace basilar membrane near middle Cochlear microphonic Conduction Deafness: potential Pressure waves exit Pressure waves develop via Sound is not able to be in scali vestibuli transferred to internal ear Tonotopic map Hyperpolarization Sensorineural Deafness: vibrates Low frequency sounds displace Damage occurs in neural • Shear force bends NT basilar membrane near end pathway (e.g., hair cells) • K+ conductance changes • Depolarization leads to NT release

7 Marieb & Hoehn – Figure 15.34 Sensory Systems Sensory Systems Auditory: (tonotopic map preserved) : Vestibule: midline Region of inner ear housing Auditory Pathways: receptors that respond to gravity • Provides stable visual image for retina sensation / linear acceleration Medial Cerebral • Adjust posture to maintain geniculate cortex (static equilibrium) Central lesions do not cause nuclei (thalamus) deafness because fibers from Vestibule Maculae anatomy: each ear are intermixed in CNS

Inferior Thalamus colliculus Otolithic nuclei membrane (midbrain) Brain stem Cochlear Hair nerve cell (horizontal) (vertical)

Cochlear Maculae: nuclei Sensory receptors for static equilibrium (medulla) acceleration (e.g, forward) causes Spinal cord 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 Sensory Systems

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

Crista ampullaris anatomy: 1) Cupula begins to move; endolymph Semicircular canal Cupula is stationary Ampulla 2) Eventually, endolymph catches up with cupula (hair cells quiescent) 3) When stopped, endolymph briefly moves hair cells in opposite direction Endolymph (reverses firing rates) (fluid) Rotation of head stimulates semicircular cells on same side rotation is occurring : Hair cell ( bent (stereocilia bent Sensory receptor for dynamic toward ) away from kinocilium) equilibrium Head rotation (e.g., spinning) causes endolymph to push against cupula; subsequent bending of hair cells modifies AP firing rate Costanzo – Figure 3.23

Sensory Systems Sensory Systems

Vestibular System: Visual reflexes Olfaction: (e.g., ) midline Vestibular Pathways: Olfactory Organ: (nasal cavity) Postural reflexes Basal Nucleus Input Output Cerebral cell cortex Olfactory Medial nucleus Semicircular Extraocular receptor Superior nucleus canals muscles cell

Vestibule Spinal cord Lateral nucleus Supporting Nasal (utricles) () Thalamus cell Vestibule / Inferior nucleus Semicircular Cerebellum canals Brain stem

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

8 Sensory Systems Sensory Systems Across-fiber pattern code: Olfaction: Each odorant produces unique Olfaction: pattern of activity across a midline Olfactory Transduction: population of receptors Olfactory Pathways: Olfactory • G protein (Golf) coupled to adenylate cyclase ~ 1000 receptor cells synapse cortex Cerebral • ATP  cAMP with a single mitral cell cortex • cAMP triggers ligand-gated Na+ channels Mitral cells Glomeruli (olfactory tract) (Olfactory bulb) Thalamus Limbic system Olfactory Encoding: Cribiform plate Brain stem We don’t exactly know how…

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

Marieb & Hoehn – Figure 15.23 Sensory Systems Sensory Systems

Gustation: Gustation: Taste bud: (~ 10,000 / tongue) Taste Transduction:

Basal cell

Taste pore transient receptor potential Gustatory Bitter cell Sour Salty Sweet • Gustatory cells (taste receptors) Taste buds spread across tongue; • Slender microvilli (taste hairs) different thresholds in different regions • 10 day lifespan • Basal cells (stem cells  new receptors) • Taste buds on lingual papillae:

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

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