General Physiology Exam 3

Sensation

the conscious or subconscious awareness of a change to the environment (external or internal)

Perception

the conscious awareness and interpretation of a change to the environment (external or internal)

Explain the events that take place in order for a sensation to occur.

1. Stimulation of sensory receptor

2. Transduction of the stimulus 

3. Generation of action potentials

4. Integration of sensory input in the Central Nervous System (CNS)

Identify the components of a sensory pathway.

1. Sensory Receptors
   Specialized cells or nerve endings that detect specific types of stimuli (e.g., mechanoreceptors, thermoreceptors, photoreceptors).

2. First-Order Neuron (Primary Afferent Neuron)
   The sensory neuron that receives the signal from the receptor and transmits it from the peripheral sensory organ to the central nervous system (CNS), typically entering the spinal cord or brainstem.

3. Second-Order Neuron
   Located in the spinal cord or brainstem, this neuron receives input from the first-order neuron and relays the signal to higher brain centers, often crossing (decussating) to the opposite side of the CNS.

4. Third-Order Neuron
   Found in the thalamus, this neuron transmits the sensory information to the primary sensory cortex of the brain for processing.

5. Sensory Cortex
   The area of the cerebral cortex (usually the primary somatosensory cortex) where sensory input is consciously perceived and interpreted.

Describe the two major somatic sensory pathways to the brain.

Dorsal column pathway- conveys information up the spinal cord to the brain for sensations of touch, pressure, vibration, and proprioception.

Anterolateral pathway-conveys input from nociceptors to the cerebral cortex via relays in the spinal cord and thalamus. This pathway is responsible for conscious awareness of pain sensations.

Explain the functional role of the primary somatosensory cortex.

responsible for the initial processing of sensory information from the body, including, touch, pressure, temperature, pain, and proprioception 

Discuss the importance of the somatosensory association area.

Its role is to interpret the meaning of somatic sensory information. The somatosensory association area stores memories of somatic sensory experiences and compares current sensations with previous ones. This allows you to recognize an object by using somatic sensory cues (for example, touching or feeling the object). So when you reach into your pocket, you are able to recognize items such as coins or keys simply by touching them because of the somatosensory association area.

Mechanoreceptor

sensitive to mechanical stimuli such as the deformation, stretching, and bending of cells and proprioception (muscle and joint position)

Thermoreceptor

detect changes in temperature

Photoreceptor

detects changes in light

Chemoreceptor

detect chemicals in the mouth (taste), nose (smell), and body fluids.

Nociceptor

respond to painful stimuli resulting from physical or chemical damage to tissues.

Compare the receptive fields of different types of sensory neurons.

Tactile receptors
Meissner corpuscles	Sensation at the onset of touch Vibration
Hair root plexuses	Movements that disturb hairs
Merkel discs	Respond to continuous touch Pressure (deeper stimulus)
Ruffini corpuscles	Stretching of the skin Pressure
Pacinian corpuscle	Vibration (rapid repetitive stimulation)
Free nerve endings	Itch (chemical irritant) Tickle
Proprioceptors	Proprioception Kinesthesia
Muscle spindles
Tendon organs
Thermoreceptors
Cold receptors (free nerve endings)	10-35°C
Warm receptors (free nerve endings)	30-45°C
Nociceptors	Pain/tissue damage
Mechanical nociceptors	Pinch Puncture
Thermal nociceptors	Below 10°C or above 45°C
Polymodal nociceptors	Variety of stimuli

Receptive field

The physical area of the body stimulated

Acuity

The sharpness of perception

Two point discrimination

the ability to the distinguish between 2 separate points of touch on the skin

Lateral Inhibition

a process where a stimulated neuron reduces the activity of its neighbors, enhancing contrast in sensory perception

Discuss how the following attributes of a stimulus are encoded: modality, location, intensity, and duration.

Modality- encoded

Location-

Intensity-

Duration-

Explain the types of pain and the sensory pathways for pain.

Fast pain- sharp, prickling, A-delta fibers (thin, myelinated → rapid conduction)

Slow pain- dull aches C fibers (unmyelinated → slow conduction)

Superficial somatic pain- arises on the surface of the skin

Deep somatic pain- arises in muscle

Visceral pain- arises in the organs

Referred pain- pain that is felt in a part of the body that it did not originate from (sensory input from receptor in skin input from organ converge on some 2nd order neuron and is sent to the brain)

Nociceptors can activate two types of pathways: (1) spinal reflex pathways and (2) ascending pathways to the brain. Spinal reflex pathways that are activated by nociceptors provide unconscious protective responses when a noxious stimulus begins to damage the body

Discuss the role of the somatic nervous system.

plays a central role in controlling voluntary movements and transmitting sensory information from the body to the central nervous system.

Describe how the premotor cortex contributes to motor control

contributes to motor control by planning and organizing movements before they are executed.

Describe the functions of the basal nuclei.

• Influences initiation of movements

• Can suppress unwanted movements

Describe how the cerebellum controls body movements.

coordinating voluntary actions, regulating balance, and refining movements to make them smooth and accurate

Explain the role of the primary motor cortex in motor control.

Identify the components of a somatic motor pathway.

1. Upper motor neuron (in brain)

2. Lower motor neuron (in spinal cord or brainstem)

3. Neuromuscular junction

4. Skeletal muscle effector

Describe the roles of the corticospinal and corticobulbar pathways.

Role of the corticospinal pathway

Controls skeletal muscles of limbs and trunk; fibers decussate (cross) to opposite side of CNS.

Role of the corticobulbar pathways 

Controls muscles of the head via cranial nerves without decussation for some fibers 

Explain the role of the lower motor neuron.

• Final common pathway that innervates skeletal muscle fibers.

• Generates action potentials to trigger muscle contraction.

Describe the functional operations of the neuromuscular junction

• Transmission of action potentials from motor neurons to muscle fibers via acetylcholine release and receptor activation.

• Initiation of end plate potentials leading to excitation-contraction coupling.

Explain how chemical agents alter events at the neuromuscular junction.

Agent	Effect	Additional Notes
Botulinum Toxin	Prevents exocytosis of acetylcholine	Used medically in small doses (Botox)
α-latrotoxin	Causes excessive acetylcholine release	Potent neurotoxin from black widow spider
Curare	Prevents opening of nicotinic acetylcholine channels	Used to relax muscles during surgery
Organophosphates	Inhibit acetylcholinesterase enzyme	Found in nerve gases (Sarin) and insecticides; causes prolonged muscle contraction

Describe the components of a muscle fiber.

Sarcolemma=plasma membrane

Sarcoplasm= cytoplasm

Sarcoplasmic reticulum = modified endoplasmic reticulum (stores Ca2+)

Myofibril = contractile elements and actin (thin filament) myosin (thick
filament) are the contractile proteins
Troponin and tropomyosin are
regulatory proteins

Explain the functions of the different types of muscle proteins. 

Actin

• contractile protein.

• It forms the thin filaments in the myofibrils.

• Provides binding sites for myosin heads for contraction.

Myosin

• contractile protein.

• It forms the thick filaments in the myofibrils.

• Motor protein with ATPase activity; generates force via crossbridge cycling.

Tropomyosin

• regulatory protein.

• It regulates muscle contraction by blocking the myosin-binding sites on actin filaments when the muscle is at rest. 

Troponin

• Regulatory protein.

• It binds to Ca2+ ions, causing tropomyosin to move and expose myosin-binding sites on actin, which initiates muscle contraction.

Summarize the events by which a somatic motor neuron causes a skeletal muscle fiber to generate an action potential.

1. Neuromuscular Junction Activation: The somatic motor neuron releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft at the neuromuscular junction.

2. ACh Binding: Acetylcholine binds to nicotinic receptors on the sarcolemma (muscle fiber plasma membrane).

3. Ion Channel Opening: Binding of ACh causes ion channels to open, allowing sodium ions (Na⁺) to flow into the muscle fiber and potassium ions (K⁺) to flow out, leading to a localized depolarization called the end plate potential.

4. Generation of Action Potential: If the end plate potential reaches threshold, voltage-gated sodium channels open, causing a rapid influx of Na⁺ that propagates an action potential along the sarcolemma.

Describe excitation-contraction coupling in a skeletal muscle fiber.

1. Stimulation at the Neuromuscular Junction

2. Propagation of the Action Potential and Role of T-Tubules

3. Calcium Release from the Sarcoplasmic Reticulum

4. Calcium Binding and Exposure of Myosin Binding Sites

Outline the steps of the contraction cycle.

1. Myosin head hydrolyzes ATP and becomes energized and oriented. 

2. Myosin head binds to actin, forming a crossbridge 

3. Myosin bridge pivots, pulling the thin filament past the thick filament toward center of the sarcomere (power stroke) 

4. As myosin head binds ATP, the crossbridge detaches from actin. 

Explain the significance of the sliding filament mechanism.

1. Basis of Muscle Contraction:
   It describes how muscle fibers generate tension and shorten by thin (actin) filaments sliding past thick (myosin) filaments within sarcomeres, the functional units of muscle.

2. Energy Conversion:
   Converts chemical energy stored in ATP into mechanical work, enabling movement.

3. Sarcomere Shortening:
   Though the filaments themselves do not shorten, their sliding reduces sarcomere length, leading to overall muscle shortening and force production.

4. Universality:
   Applies to all types of striated muscle — skeletal and cardiac — showing a conserved mechanism of contraction.

5. Regulated Process:
   Allows precise control of muscle contraction through calcium-regulated exposure of myosin-binding sites on actin.

6. Basis for Motor Function:
   Provides the molecular foundation for all voluntary movements, posture maintenance, and bodily functions requiring muscle activity.

Discuss how a skeletal muscle fiber relaxes after a period of contraction.

Describe the reactions by which skeletal muscle fibers produce ATP.

Explain the factors that contribute to muscle fatigue.

1. Depletion of Energy Stores:

   • Reduced ATP availability limits the energy needed for crossbridge cycling and calcium pumping.

   • Creatine phosphate stores are quickly exhausted during intense activity.

2. Accumulation of Metabolic Byproducts:

   • Lactic acid and hydrogen ions build up during anaerobic glycolysis, lowering pH and impairing enzyme function.

   • Inorganic phosphate (Pi) accumulates from ATP breakdown and may interfere with crossbridge function.

3. Ion Imbalances:

   • Disruption of normal concentrations of potassium, calcium, and sodium ions affects muscle excitability and calcium release from the sarcoplasmic reticulum.

4. Impaired Excitation-Contraction Coupling:

   • Reduced calcium release from the sarcoplasmic reticulum limits contraction strength.

5. Central Nervous System Fatigue:

   • Diminished motor neuron output and neurotransmitter depletion can reduce stimulation to muscles.

6. Oxygen Deficit:

   • Inadequate oxygen supply limits aerobic ATP production, increasing reliance on less efficient anaerobic pathways.

Outline the phases of a twitch.

List and explain the factors that determine muscle tension.

1. Number of Muscle Fibers Stimulated (Recruitment):

   • The more motor units (a motor neuron and the muscle fibers it innervates) that are activated, the greater the muscle tension.

   • Recruitment allows precise control of muscle force by varying how many motor units are engaged.

2. Frequency of Stimulation:

   • The rate at which action potentials arrive at the muscle fibers affects tension.

   • Higher frequency leads to temporal summation, where successive twitches add together, producing greater tension.

   • At very high frequencies, tetanus occurs—a sustained maximal contraction.

3. Muscle Fiber Length at Rest (Length-Tension Relationship):

   • Muscle fibers have an optimal length at which they generate maximum tension.

   • If the fiber is too stretched or too compressed, the overlap between actin and myosin filaments is suboptimal, reducing force.

4. Muscle Fiber Type:

   • Different fiber types (slow-twitch vs. fast-twitch) have varying capacities for tension and fatigue resistance.

   • Fast-twitch fibers produce higher tension but fatigue quickly; slow-twitch fibers produce lower tension but sustain contractions longer.

5. Fatigue Level:

   • Muscle fatigue reduces the ability to generate tension due to factors like depletion of ATP, accumulation of lactic acid, or ionic imbalances.

6. Muscle Cross-Sectional Area:

   • Larger muscles with greater cross-sectional area can generate more tension due to having more contractile proteins.

7. Degree of Muscle Warm-up:

   • Warm muscles contract more efficiently due to increased enzyme activity and muscle elasticity.

8. Type of Contraction:

   • Isometric contractions develop tension without changing muscle length.

   • Isotonic contractions involve muscle shortening or lengthening, with differing tension profiles

Describe the importance of a motor unit.

• Control of muscle force: Small motor units allow fine motor control (few fibers per neuron), large motor units produce greater force.

• Graded muscle response: Increasing the number of active motor units (recruitment) progressively increases muscle tension.

• Coordination during contraction to produce smooth and efficient movement.

Explain the role of the autonomic nervous system.

plays a crucial role in controlling involuntary bodily functions. It regulates activities of smooth muscles, cardiac muscles, and glands, ensuring the maintenance of homeostasis without conscious effort.

Provide examples of autonomic control centers.

Where is the cardiovascular control located? 

in the medulla oblongata

Where is the respiratory control located? 

split in between the medulla and the pons

What important control centers does the spinal cord house ?

Ejaculation 

Erection

Micturition

Defecation

Identify the components of an autonomic motor pathway.

Preganglionic neuron- extends from the spinal cord to the ganglion

Ganglion- a cluster of cell bodies

Postganglionic neuron- extends from the ganglion to the effector 

Neuroeffector junction- the junction between postganglionic neuron and the effector

Varicosities 

swelling at the end of the autonomic post ganglionic neuron 

What are the target cells of the autonomic nervous system? 

Cardiac muscles, smooth muscle, and glands

Discuss the functional operations of a neuroeffector junction.

Describe the neurotransmitters and receptors of the ANS.

Sympathetic

Compare the major physiological responses of the parasympathetic and sympathetic branches of the ANS.

Identify the components of a smooth muscle fiber.

Few sarcoplasmic reticulum

sarcolemma with caveolae- pouch like invaginations in the sarcolemma that contain Ca2+ (no T-tubules)

Sarcoplasm

calmodulin- regulatory protein that

MLCK-activated by the Ca²⁺-calmodulin complex to phosphorylate myosin, enabling cross-bridge cycling.

intermediate filaments- Structural support and force distribution

Dense Bodies

Nucleus

Distinguish between the 2 types of smooth muscle.

Single unit smooth muscle: 

Cells are connected by gap junction

Cells contract as a unit 

Multiunit smooth muscle:

Cells are not connected. 

Cells work independently

Discuss the importance of smooth muscle tone.

Definition: a state of sustained partial contraction 

Allows for of a sustained pressure 

Explain how smooth muscle is innervated by the autonomic nervous
system.

Describe how autorhythmicity occurs in a single unit smooth muscle

Describe how action potentials are generated in contractile smooth muscle fibers

Explain the mechanism of excitation-contraction coupling in smooth
muscle.

List the factors that regulate smooth muscle activity.

1. Neural Regulation: Neurotransmitters from the autonomic nervous system (e.g., acetylcholine, norepinephrine) can either stimulate or inhibit smooth muscle contraction, depending on the receptor type.

2. Hormonal Regulation: Various hormones (e.g., oxytocin, vasopressin, angiotensin II, histamine, serotonin) can influence smooth muscle activity by binding to specific receptors.

3. Local Chemical Factors: Changes in the local chemical environment such as pH, oxygen levels, carbon dioxide (CO2CO2​) concentration, presence of nitric oxide (NO), adenosine, and prostaglandins can regulate smooth muscle tone and activity.

4. Stretch: Mechanical stretch of the smooth muscle can induce contraction, particularly in single-unit smooth muscles (myogenic response).

5. Autorhythmicity: Specialized pacemaker cells in single-unit smooth muscle can spontaneously generate action potentials without external stimuli, leading to rhythmic contractions.

6. Calcium Sensitivity: The sensitivity of the smooth muscle contractile proteins to intracellular Ca2+Ca2+ can be modulated by various mechanisms, affecting the force of contraction.