Lecture Final: Chapter 24

Question 1

Sensory pathway basic functions (4)

+ Overall

Answer 1

1. Sensory recognition: capture a form of energy (stimulus) that leads to a graded potential in receptor cells → Sensation: being aware of a stimulus
2. Transduction: converting the graded receptor potential into an action potential → basically, sensation is now transduced into a nerve impulse
3. Transmission: action potential travels to the central nervous system (brain and spinal cord)
4. Perception: interneurons provide the meaning of the stimulus → brain builds perception of the taste / molecular structure (can also extend to hearing, seeing, etc)
   - - influenced by specific pathway of innervation (Principle of Labeled Lines)

Overall: detection and processing of sensory information allows for generation of motor responses, which is the physiological basis of animal behavior
– affected by frequency of AP and the number of sensory neurons involved

Question 2

Categories of Sensory Receptors (3)
1sub2
2sub2
3sub3

Dominant Sense

• Statistics
• Three things to know

Answer 2

Neuronal: receptor is afferent neuron

• • chemoreceptor: olfactory
• • electromagnetic

Epithelial: receptor regulates afferent neuron

• • chemoreceptors: taste
• • mechanoreceptors: auditory (organ of Corti); equilibrium and movement (vestibular apparatus)

General Senses: neurons with naked dendrites / associated with connective tissue / in skin

• • tactile
• • thermoreceptors
• • pain (can also be generated when there is too much of a stimulus)

VISION bc 70% of all sensory receptors are in eyes ;; 50% cerebral cortex involved in visual processing – evolved from a common ancestor

1. All eyes use RHODOPSIN (photopigment)
2. Disparate eyes that look nothing in common STILL share homologous genes that regulate eye development
3. Single gene PAX6 initiates development in diverse animals including both vertebrates and invertebrates

Question 3

EYEBALL STRUCTURE

• fluids involved = VH
• three layers FVR
• each have structures / things inside of them

F3
V3
R2

Answer 3

EYEBALL IS FILLED WITH FLUID

• • Aqueous humor: anterior and posterior regions; can build up and increase pressure on optic nerve, causing glaucoma
• • Vitreous humor: btwn lens and retina; never changed in your lifetime

FIBROUS LAYER:

• • Sclera: white, avascular connective tissue; continuous dura mater of brain
• • Cornea: transparent; “lens” that participates in refracting / bending light rays onto retina
• • Crystallins: transparent proteins that form body of lens

VASCULAR LAYER:

• • Choroid: vascular to nourish all eye layers; dark brown to minimize light scattering
• • Ciliary body: encircles lens with muscles that control shape of lens → affects curvature to influence refraction
• • Iris: colored part of eye; smooth muscles that act as a smooth diaphragm → constricts in bright; dilates in dim

RETINA: innermost layer

• • Outer Pigmented layer + inner neural layer with rods and cones
• • Fovea: center of visual acuity (sharpest imagery); best color and images

Question 4

How does your eye change to see things?

• Nearby
• Far away

Answer 4

Near vision (ACCOMMODATION): Focus on nearby objects

1. Ciliary muscles contract, pulling border of choroid toward lens
2. Circular muscles around the eye constrict → suspensory ligaments relax but lens bulge (become thicker and tounder)

Distance vision: focus on far away objects

1. Ciliary muscles relax and border of choroid moves away from lens
2. Circular muscles relax → Suspensory ligaments pull against lens, making it flatter

Question 5

Retinal Cells

• functions of Outer (4)
• things within Inner (4)

Answer 5

OUTER PIGMENT LAYER: disk like membrane (lamellae) contains photopigments (rhodopsin)

• • absorb scattered light
• • phagocytic (will eat outer parts of the rod or cones for recycling purposes)
• • Isomerizes bleached retinal
• • Stores Vitamin A

INNER NEURAL LAYER:

• • Photoreceptors: modified neurons; millions in each eyeball; act as an extension of the brain → generate graded potentials to be sent to bipolar
• • Bipolar cells: generate graded potential of EPSP or IPSP to be sent to ganglion
• • Horizontal cells: includes amacrine cells; lateral inhibition to detect contrasts; perception of shape and motion
• • Retinal ganglion cells (RGC): axons make up the OPTIC NERVE → generate AP

Question 6

Photoreceptors

• Rods
• Cones

Rhodopsin
+ Signal Transduction Pathway

Answer 6

Rods: most sensitive to light; provide low resolution; non color images in dim light and peripheral vision
Cones: less sensitive to light; provides high resolution and color vision
** Cone like bright light – rods like dim.

Rhodopsin: photopigment that contains retinal (chromophore) and opsin (protein); specific class of GPCR that captures photons 
-- isomerizes cis retinal into trans retinal to activate opsin = bleaching

1. Cis-retinal absorbed light; is converted to trans-retinal
2. Opsin is activated, triggering activation of transducin (G protein)
3. Activated transducin activates phosphodiesterase
4. Activated phosphodiesterase breaks down cGMP to 5’ GMP, resulting in Na+ channel with no cGMP bound to it (closes up)
5. Membrane hyperpolarizes (becomes more negative) and reduces neurotransmitter release

Question 7

Dark vs Light Responses

• intensity levels
• moving to different lights

Answer 7

No light = rhodopsin inactive → rod depolarization (Na+ channels are open); glutamate released, causing inhibitory postsynaptic potential (IPSP) → bipolar cell becomes hyperpolarized, no neurotransmitter release → no action potential / nerve impulse

Light = rhodopsin active → rod hyperpolarization (Na+ channels are open); no glutamate released → bipolar cell is depolarized due to lack of IPSP, neurotransmitter released → yields AP / nerve impulse

Low intensity light: small amount of rhodopsin is bleached; retina continues to respond
High intensity: wholesale bleaching occurs; rods become non functional (cones still active) → transducers are released and move into inner segment of rod cell, uncoupling rhodopsin from transduction cascade

Moving from dark to bright light, cones take over and visual acuity and color vision improve over the next 5 to 10 minutes
Moving from bright to dark light, cones cease functioning; transducers return to the outer segment of the rods across 30 minute period, slowly becoming active again

Question 8

Integration of Visual Information

Answer 8

Individual photoreceptors are sensitive to levels of resolution → Ganglion cells respond to light stimulation from numerous photoreceptors (aka receptive field)

RODS: part of converging pathways; up to 100 feed into a single Ganglion cell → effects are thus summated, meaning that there is a fuzzy image with high sensitivity

CONES: 100x less sensitive to light than rods BUT each cone (esp in the fovea) has a “straight-through” pathway via its own bipolar cell to a Ganglion cell, allowing for high acuity and detailed views of a very small area of visual field

Question 9

Neural Pathways for Vision

Answer 9

Lens reverse images: temporal provides light for nasal

• • RIGHT EYE sees LEFT SIDE via right temporal retina and left nasal retina
• • LEFT EYE sees RIGHT SIDE via left temporal retina and right nasal retina

Optic nerve projections in many mammals partially cross at optic chiasm (partial decussation); nasal will cross over again to go to contralateral (opposite) side BUT temporal will stay on ipsilateral (same) side
– Synapse with neurons in thalamus (LGN) → neurons project to the primary visual cortex

Mixing of input from two eyes allows mammals with forward facing eyes to merge binocular input for depth perception.

Question 10

Skeletal Muscle

• Composition
• Myosin and Actin

Answer 10

Derived from mesoderm; striated and voluntary; cells are electrically isolated from each other

Comprised of multinucleated muscle fibers
> Myofibril: longitudinal bundles containing sarcomeres; enveloped by sarcoplasmic reticulum
> Sarcomeres: functional units of skeletal muscles; represent the orderly arrangement of actin and myosin; cause the look of striations

Myosin: two subunits organized into a head and a double helical tail
Actin: subunits form twisted, double helixes with tropomyosin arranged head to tail in groove of helix
– troponin bound to tropomyosin along thin filaments

Question 11

Contraction Pathway for Skeletal Muscle

Answer 11

@ NEUROMUSCULAR JUNCTION –

1. ACh molecules released from axon terminal will bind to receptors on sarcolemma of muscle (T tubules), opening it.
2. Na+ rushes inwards, generating a graded potential that will spread to voltage Na channels and initiate AP.
3. K+ efflux out will repolarize muscles.
4. AP spreads down tubules and change shape of voltage-sensitive tubule proteins.
5. Shape change opens Ca2+ channels in SR.
6. Ca2+ will bring to troponin and remove the blocking actions of tropomyosin, allowing muscle contractions to begin.

Question 12

SLIDING FILAMENT THEORY OF MUSCLE CONTRACTIONS

+ CROSS BRIDGE CYCLE

Answer 12

SLIDING FILAMENT THEORY OF MUSCLE CONTRACTIONS
Myosin heads will grab actin and pull them to the midline

CROSS BRIDGE
Myosin: two binding sites

ATP binding site (ATPase)

1. Energy released by hydrolysis of ATP stored in Myosin-ADP-Pi in relaxed muscled
2. All myosin heads have STORED ENERGY but temporarily unable to bind actin (in the absence of Ca2+)
   - - energized and ready to go!

Actin binding site

1. Myosin ADP-Pi bind actin (in presence of Ca2+) – at first weak, then Pi are released first then binding becomes TIGHTER, followed by power stroke
2. At the end of power stroke, myosin releases ADP
3. Myosin remains tightly bound to actin until ATP attaches (rigor confirmation)

Question 13

Summation of Contraction

• motor unit, defined
• twitching
• temporal
• spatial

Answer 13

A motor neuron and all muscle fibers it innervates are collectively termed “MOTOR UNIT”
– Axon of each motor neuron branches to innervate several muscle fibers (cells)

When a motor neuron generates an AP, ALL OF THE MUSCLE FIBERS IN THE MOTOR UNIT generate AP and contract to produce a twitch (team response) → trains of AP of increasing frequencies fuse twitches into tetanic contraction (holding of a contraction)

TEMPORAL SUMMATION

• • Increasing firing of motor neurons sums twitches (Each 100 msec) into smooth, sustained contractions (tetanus)
• • Small time difference btwn nerve impulses allows for fusing of twitches
• • High [Ca2+] in cytoplasm permits summation and tetanus

SPATIAL SUMMATION

• • Increased stimulation in CNS recruits multiple motor units serving the muscle to increase the tension of the contraction
• • Small muscle fibers recruited first; larger muscle fibers recruited by the highest threshold of neurons
• • Light response then more intense contractions → gotta warm up!

Question 14

Functions of ATP in muscle (3)

+ sources (3)

Answer 14

FUNCTIONS:

1. Detach myosin from actin when ATP binds
2. Hydrolysis to ADP-Pi cocks myosin
3. Hydrolysis powers SR Ca2+ - ATPase pump

SOURCES:

1. Creatine phosphate donates high energy Pi to ADP
2. Anaerobic glycolysis uses glucose / glycogen
3. Aerobic catabolism uses lipids, carbohydrates, and proteins as sources of energy