Sensory Receptors, Signal Transduction, and Endocrine System Overview

Which tastes use 'sweet' receptors?

Sugars (glucose, sucrose, fructose), artificial sweeteners, some amino acids.

How are smell receptors similar and different from 'sweet' taste receptors?

Similar: Both are G-protein-coupled receptors (GPCRs), use second-messenger cascades, and detect chemical ligands. Different: Smell receptors detect airborne molecules; sweet receptors detect dissolved molecules.

What are the two mechanisms of signal transduction?

Direct mechanical gating of ion channels and indirect transduction via second messengers.

Which senses use direct gating for signal transduction?

Touch, hearing, and balance.

Which types of mechanoreceptors do not generate action potentials on their own?

Hair cells generate graded receptor potentials; action potentials occur in the connected neuron.

What is the function of the tympanic membrane?

Converts sound waves into mechanical vibrations; common name: eardrum.

What are the three bones of the inner ear?

Malleus, incus, stapes.

What is the cochlea?

A spiral, fluid-filled structure that converts vibrations into neural signals for hearing.

What are the two parts of a photoreceptor and their functions?

Outer segment: contains photopigments and detects light. Inner segment: contains mitochondria and supports cell metabolism.

What are the two types of photoreceptors in vertebrates?

Rods and cones.

Which pigment does each photoreceptor type use?

Rods use rhodopsin for dim light; cones use photopsins for color detection.

How do 'sweet' receptors transduce a stimulus into an electrical signal?

Ligand binds GPCR → G-protein activation → second messenger → ion channels open → depolarization.

How do sour receptors transduce a stimulus?

H⁺ ions enter the cell directly, causing depolarization.

How do salt receptors transduce a stimulus?

Na⁺ ions enter through channels, causing depolarization.

How do smell receptors transduce a stimulus?

Odorant binds GPCR → G-protein → cAMP → ion channels open → depolarization.

What is cephalization?

Concentration of sensory organs and nervous tissue at the front of the body for faster processing and directional movement.

How do the SNS and PSNS interact to control the body?

They act antagonistically but cooperatively; SNS is fight or flight, PSNS is rest and digest.

What would happen if you moved the output neurons around in sensory organs?

The brain interprets signals based on where the neuron projects, not the stimulus itself.

What are the three types of muscle?

Skeletal (striated, voluntary), cardiac (striated, involuntary), smooth (non-striated, involuntary).

What are the four layers of skeletal muscle?

Muscle, fascicle, muscle fiber, myofibril.

What is a motor end plate?

Region of muscle fiber receiving neuron signal; also known as the neuromuscular junction.

Which neurotransmitter signals muscles to contract?

Acetylcholine.

What is the abbreviation for acetylcholine?

ACh

Which structure brings the action potential into the muscle fiber?

T-tubules

Which part(s) move when pulled on in a sarcomere?

Actin (thin filaments)

Which part(s) don't move and do the pulling in a sarcomere?

Myosin (thick filaments)

What is the main function of a skeleton?

Support, protection, movement

What are the pros and cons of a hydrostatic skeleton?

Flexible but weak support

What are the pros and cons of an exoskeleton?

Strong but limits growth

What are the pros and cons of an endoskeleton?

Flexible and grows with body

How does a hydrostatic skeleton work?

Fluid pressure resists muscle contraction

What is the main chemical component of exoskeletons?

Chitin

What is the main chemical component of endoskeletons?

Calcium phosphate

What is the difference between a tendon and a ligament?

Tendon connects muscle to bone; ligament connects bone to bone

What trade-off do joints face?

Mobility vs. stability

What is an example of a joint maximizing mobility/flexibility?

Ball-and-socket joint (shoulder)

What is an example of a joint maximizing strength/durability?

Hinge joint (elbow)

How is the signal from the end plate brought into the muscle fiber?

Via T-tubules

What signal does the sarcoplasmic reticulum sense?

Action potential

What signal does troponin sense?

Ca²⁺

What is the state of the sarcomere at rest?

Partial overlap of actin and myosin

How does the muscle fiber eventually relax?

Ca²⁺ pumped back into SR using ATP

What can muscles not do?

Muscles cannot push; they only contract (pull)

What are hormones?

Chemical signals released into the bloodstream that affect distant targets

What type of hormone has a direct effect?

Non-tropic

What type of hormone stimulates the release of other hormones?

Tropic

What are the two structural classes of hormones?

Protein/peptide and steroid

What are the properties of protein/peptide hormones?

Water-soluble, bind membrane receptors, fast

What are the properties of steroid hormones?

Lipid-soluble, bind intracellular receptors, slow but long-lasting

What is the master gland of the endocrine system?

Pituitary

What three endocrine organs are not on the hypothalamic-pituitary axis?

Pancreas, pineal gland, parathyroid glands

What are the two parts of the pituitary gland derived from?

Anterior: oral ectoderm; Posterior: neural tissue

What signals the anterior pituitary?

Releasing hormones via portal system

What signals the posterior pituitary?

Direct neural signals

What acts as the shared cue for the hypothalamus and pancreas?

Blood glucose level

What is the order of glands/organs in the hunger pathway?

Stomach → Hypothalamus → Pituitary → Pancreas

What is the function of ghrelin?

Stimulates hunger

What releases ghrelin?

Stomach

What is the primary signal for inflammation?

Cytokines released by damaged cells

What is the purpose of inflammation?

Increased blood flow and immune access

What are the general properties of the innate immune system?

Fast, non-specific, no memory

What are the cellular components of the innate immune system?

Phagocytes and NK cells

What are the general properties of the adaptive immune system?

Slow, specific, memory

What are the cellular components of the adaptive immune system?

B and T cells

What is clonal selection?

Activation of matching B cell

What is the result of clonal selection?

Plasma cells and memory cells

What is the primary vs secondary response in the adaptive immune system?

Secondary is faster and stronger

What is the complement system?

Enhances other immune responses

What is positive selection in T-cell training?

Must recognize self-MHC

What is the purpose of negative selection in T-cell training?

Prevent autoimmunity

Why do you need an adaptive immune system?

Innate immunity is too nonspecific; adaptive provides specificity and memory