Human Phys Exam 2

Sensory Unit

A group of receptors that receive a particular stimulus and the afferent neuron associated with those receptors. The type of stimulus perceived is determined in part by the type of receptor activated.

Phasic Receptors

Rapidly adapting, responds strongly when a stimulus begins/changes, AP frequency decreases quickly

Tonic Receptors

Slowly adapting, respond continuously to a constant stimulus, maintaining AP frequency over time.

Mechanoreceptors

Mechanical forces such as touch, pressure, vibration, and stretching

Chemoreceptors

Chemicals, including odorants and tastants

Nociceptors

Potentially tissue-damaging stimuli associated with pain

Photoreceptors

Light (photons)

Proprioceptors

Body position and movement

Proprioception

The unconscious sense of body position and movement. Alcohol impairs this which helps explain why balance and coordination are tested during field sobriety tests

Receptive Field

The area of the body that activates a particular afferent sensory neuron when stimulated

Small Receptive Field

High acuity, better ability to locate a stimulus and distinguish two nearby points

Large Receptive Field

Low acuity, less accurate localization and greater difficulty distinguishing two points

Two-Point Threshold

The minimum distance at which two simultaneous touches are perceived as separate

Convergence

High increases sensitivity but decreases spatial acuity

TRP Channels

Ion channels involved in detecting temperature and certain painful stimuli

Thermoreceptors

Sensory receptors activated by changes in temperature (hot and cold)

Capsacin

Chemical found in hot peppers that activate heat-sensitive TRP channels. Activates TRP channels even when the actual temperature is not high. The brain interprets the resulting signals as burning or heat. It activates temperature-sensitive receptors rather than physically heating the tissue

Referred Pain

Pain originating in one location but received in another. Visceral and cutaneous first-order sensory neurons can converge onto the same second-order neuron in the spinal cord. The brain incorrectly interprets the signal as coming from the skin rather than the internal organ.

Ex: during a heart attack, visceral sensory signals from the heart and somatic signals from the skin converge

Phantom Limb Pain

Pain perceived in an amputated or missing limb. The nervous system can continue generating or interpreting signals associated with the missing limb

Taste Receptors

Separate from the afferent sensory neurons, the stimulation leads to neurotransmitter release and activation of sensory neurons

Sweet

Sugars and other sweet-tasting compounds

Salty

Salts, sodium ions

Sour

Hydrogen ions, associated with acids

Bitter

Various bitter compounds, many alkaloids

Umami

Amino acids, glutamate

Olfaction

Sense of smell, activated when odorants bind receptors on the olfactory sensory neurons. Unlike taste, vision, hearing, and equilibrium, this neuron itself contains the sensory receptor

Olfactory and Limbic System

The system is closely associated with structures involved in emotion and memory. Explains why particular smells can trigger strong memories and emotional responses.

Vestibular System

Located in the inner ear and detects head position and movement. It contributes to balance by providing the CNS with information about head movement and acceleration, head orientation relative to gravity, changes in body position.

The brain integrates this information with visual input and proprioception to maintain posture, balance, and coordinated movements.

Utricle macula

Detects horizontal linear acceleration and head tilt. The otolith membrane shifts, bending hair cells.

Ex: driving in a car

Saccule macula

Detects vertical linear acceleration and head tilt. The otolith membrane shifts, bending hair cells

Ex: riding in an elevator

Crista ampullaris

Detects angular (rotational) acceleration. The endolymph movement deflects the cupula and bends hair cells.

Basilar Membrane

Located within the cochlea and responds differently depending on sound frequency

Base, near oval window

Narrowest and stiffest, high frequency (high pitch) sounds

Apex, near helicotrema

Widest and least stiff, low frequency (low pitch) sounds

Tonotopic Organization

Frequency-dependent organization. As sound pitch increases, maximum basilar membrane displacement occurs close to the oval window. Inner hair cells are responsible for hearing, while outer hair cells adjust cochlear responses to different frequencies.

Darkness: photons, cGMP, PDE, membrane potential

High cGMP → cation channels open → Na+ and Ca2+ enter → depolarized (-35mV) → continuous glutamate release

Light: photons, cGMP, PDE, membrane potential

Photon activates visual pigment → PDE activated, cGMP decreases → cation channels close → hyperpolarized (-70mV) → glutamate release decreases

**Photoreceptors hyperpolarize in response to light

Vitamin A-derived Retinal Response to Phototransduction

Retinal is a light-sensitive molecule derived from vitamin A. The process begins when:

1. A photon is absorbed by a visual pigment containing retinal

2. 11-cis-retinal changes into all-trans-retinal

3. This changes the shape and activity of the associated opsin protein

4. The activated pigment initiates the signaling cascade that activates PDE

5. PDE decreases cGMP concentration

6. Cation channels close, hyperpolarizing the photoreceptor

7. Glutamate release decreases

Phototransduction

Conversion of light energy into an electrical signal

Signaling Pathways from Photoreceptor Cells to Ganglion Cells

Rods and cones (photoreceptors) → bipolar cells → ganglion cells → optic nerve → optic chiasm → optic tract → lateral geniculate nucleus (thalamus) → optic radiation → primary visual cortex

Graded Potentials

Photoreceptors and bipolar cells produce…

Action Potentials

Ganglion cells produce…because they contain voltage-gated Na+ channels

Membrane Potential of ON Bipolar Cells

Stimulated by light - Decreased glutamate release from photoreceptors allows them to depolarize

Darkness (high glutamate) = hyperpolarized

Light (reduced glutamate) = depolarized

Response to light = activates ON pathway

Membrane Potential in OFF Bipolar Cells

Inhibited by light.

Darkness (high glutamate) = depolarized

Light (reduced glutamate) = hyperpolarized

Response to light = inhibits OFF pathway

Emmetropia

Light focuses directly on retina, no correction needed, normal refractive vision

Hyperopia

Light focuses behind the retina, convex lens correction, makes nearby objects more difficult to focus on, farsightedness

Myopia

Light focuses in front on the retina, concave lens correction, makes distant objects blurry, nearsightedness

Parasympathetic Near Response

Occurs when the eyes focus on a nearby object, the PNS produces 3 changes: convergence, pupil constriction, lens accommodation

Convergence

Both eyes turn inward toward the nearby object

Pupil Constriction

Reduces peripheral light rays and improves image sharpness

Lens Accommodation

Ciliary muscles contract, suspensory ligaments relax, and the lens becomes thicker and more convex

Neurons and Somatosensory Sensation to the Postcentral Gyrus

1. First-order neuron: detects the stimulus and transmits information from the peripheral receptor into the CNS

2. Second-order neuron: relays the signal toward the thalamus and crosses to the opposite side at the appropriate level

3. Third-order neuron: transmits the information from the thalamus to the primary somatosensory cortex

Dorsal Column Pathway

Sensations: fine/deep touch, vibration, proprioception, stereognosis

Myelination: heavily myelinated

Decussation: medulla

Initial ascent: same side (ipsilateral)

Destination: thalamus → somatosensory cortex

Anterolateral Pathway

Sensations: pain, temperature, crude touch, pressure, tickle, itch

Myelination: fewer myelinated fibers

Decussation: spinal cord

Initial ascent: opposite side (contralateral) after crossing

Destination: thalamus → somatosensory cortex

Primary Sensory Cortex

Receives and initially processes sensory information from specific sensory pathways. It helps identify the location and basic characterisitcs of stimuli

Association Cortex

Integrates and interprets sensory information to create meaningful perceptions, including recognition and understanding

Sarcolemma

Plasma membrane of a muscle fiber; conducts electrical signals

Sarcoplasm

Cytoplasm of the muscle fiber containing myofibrils and other cellular components

Sarcoplasmic Reticulum

Modified smooth endoplasmic reticulum that stores and releases Ca2+

Transverse Tubules (T-tubules)

Extensions of the sarcolemma that carry action potentials deep into the muscle fiber

Triad

One T-tubule positioned between two terminal cisternae of the sarcoplasmic reticulum. Allows electrical excitation to trigger Ca2+ release throughout the muscle fiber

Events at NMJ to calcium release from SR to sarcomere shortening

1. Action potential arrives at the alpha motor neuron terminal

2. Acetylcholine is released into the synaptic cleft

3. ACh binds nicotinic ACh receptors on the motor end plate

4. Ion movement produces a graded endplate potential

5. Threshold is reached, generating a muscle action potential

6. Action potential spreads along sarcolemma and T-tubules

7. Voltage-sensitive DHPR activates RYR channels to the SR

8. SR releases calcium into the sarcoplasm

9. Ca2+ binds troponin, moving tropomyosin away from the actin binding sites

10. Myosin binds actin and cross-bridge cycling shortens the sarcomere

Acetylcholine

Neurotransmitter released by the motor nauron

Nicotinic Acetylcholine Receptor

Receptor on the motor end plate that binds acetylcholine

Acetylcholinesterase

Enzyme that breaks down acetylcholine, terminating its action

Endplate Potential

Local graded depolarization of the muscle membrane

Muscle Action Potential

Propagated electrical signal

DHPR

Voltage-sensitive protein in the T-tubule membrane

RYR

Ryanodine receptor that releases Ca2+ from the SR

Myasthenia Gravis

Autoimmune impairment of nicotinic ACh receptors at the NMJ, effects are muscle weakness and fatigability

Curare

Blocks nicotinic ACh receptors, prevents normal muscle excitation and contraction

Botulinum Toxin

Prevents ACh release from motor neuron terminals, effects are flaccid muscle paralysis

Muscle Fiber

An individual skeletal muscle cell. It is elongated, multinucleated, and surrounded by sarcolemma

Myofibril

A cylindrical contractile structure inside a muscle fiber, composed of thick and thin myofilaments arranged into repeating sarcomeres

Muscle Organization

Muscle → fascicle → muscle fiber → myofibril → sarcomere → myofilaments

Thick Filaments

Primarily myosin, myosin heads contain actin binding sites, myosin heads have ATPase activity, produces force through cross-bridge cycling

Thin Filaments

Actin, troponin, tropomyosin. Contains myosin-binding sites, troponin binds Ca2+, tropomyosin regulates access to binding sites

Contractile Proteins

Actin, myosin — interact to generate muscle tension

Regulatory Proteins

Troponin, tropomyosin — control whether myosin can bind to actin

Calcium/Troponin/Skeletal Muscle Contraction

In a resting muscle, tropomyosin covers the myosin-binding sites on actin.

When Ca2+ is released from the SR:

1. Ca2+ binds to troponin

2. Troponin changes shape

3. Tropomyosin shifts away from the binding sites on actin

4. Myosin heads attach to actin

5. Cross-bridge cycling produces contraction

When Ca2+ is pumped back into the SR, troponin releases Ca2+, tropomyosin blocks the binding sites again, and the muscle relaxes

Calcium permits actin-myosin interaction; ATP powers cross-bridge cycling

Motor Unit

Consists of one motor neuron and all the muscle fibers it innervates. Small ones contain fewer muscle fibers and allow precise movements, such as eye movements. Larger ones contain many muscle fibers and produce greater force, such as gastrocnemius

Motor Unit Recruitment & Size Principle

The activation of additional motor units to increase muscle tension.

Small motor neurons are recruited first, larger motor neurons are recruited as greater force is required, increasing recruitment increases total muscle tension

Fiber Recruitment Order

Type I → Type IIa → Type IIx

Spatial Summation

Increases the number of motor units recruited, more muscle fibers contract, also called motor unit recruitment, increases force by activating additional fibers

Temporal Summation

Increases the frequency of stimulation, repeated contractions overlap, also called wave summation, increases force by limiting relaxation between stimuli

Z disc

Boundary of sarcomere, during contraction it moves closer to opposite Z disc

A band

Entire length of thick filaments, during contraction it stays the same

I band

Region containing thin filaments without thick filament overlap, during contraction it shortens

H zone

Central region containing only thick filaments, during contraction it shortens

M line

Center of sarcomere, during contraction it remains central

Creatine for ATP generation

Creatine phosphate + ADP →← creatine + ATP

• the phosphate group is transferred to ADP to regenerate ATP

3 ATP sources:

1. creatine phosphate: rapid ATP regeneration during the first few seconds

2. anaerobic glycolysis: does not require oxygen; produces 2 ATP per glucose and lactate

3. aerobic metabolism: requires oxygen; occurs in mitochondria; approximately 34 ATP per glucose — provides most of the ATP required by resting muscle

Power Stroke

Myosin binds to actin, release of inorganic phosphate helps trigger movement of the myosin head, the myosin head pivots pulling actin toward the center of the sarcomere, ADP is released

Recovery Stroke

A new ATP molecule binds to myosin causing detachment from actin, ATP is hydrolyzed into ADP and inorganic phosphate, the released energy repositions (re-cocks) the myosin head is ready for another cycle

Rigor Mortis

Muscle stiffness that develops after death — ATP is required for myosin to detach from actin

1. ATP production stops

2. Ca2+ is no longer effectively pumped back into the SR

3. Ca2+ remains available to permit actin-myosin binding

4. Myosin heads cannot detach from actin because ATP is unavailable

5. Persistent cross-bridges cause muscle stiffness

Optimal Length-Tension Relationship to Muscle Contraction

The amount of tension a muscle produces depends on its initial sarcomere length

• too short: excessive filament overlap and structural interference reduce force

• optimal length: actin and myosin have the best arrangement for cross-bridge formation, producing maximum active tension

• too stretched: reduced filament overlap results in fewer cross bridges and less force

Type 1 Fiber Type

Speed - slow

Fiber size - small

Metabolism - oxidative

Fatigue resistance - high

Force production - low

Mitochondria - many

Myoglobin - high

Activities - marathon running, posture

Type IIa

Speed - fast

Fiber size - intermediate

Metabolism - oxidative and glycolytic

Fatigue resistance - moderate to high

Force production - moderate to high

Mitochondria - moderate to many

Myoglobin - intermediate

Activities - middle-distance running, repeated bursts

Type IIx

Speed - fastest

Fiber size - large

Metabolism - primarily glycolytic

Fatigue resistance - low

Force production - high

Mitochondria - fewer

Myoglobin - low

Activities - sprinting, jumping, powerlifting

Skeletal Muscle Calcium Source During Excitation-Contraction Coupling

Ca2+ primarily comes from the sarcoplasmic reticulum, DHPR detects T-tubule depolarization and mechanically activates RYR, SR releases Ca2+ through RYR, does not require extracellular Ca2+ entry for each each normal contraction

Cardiac Muscle Calcium Source During Excitation-Contraction Coupling

Ca2+ comes from extracellular fluid and the SR, Ca2+ enters through DHPR/L-type Ca2+ channels, incoming Ca2+ triggers additional SR Ca2+ release, extracellular Ca2+ entry is essential fro normal excitation-contraction coupling (calcium induced calcium release)