In-Depth Notes on Animal Sensory Systems and Muscle Function

Chemical Senses in Animals

• Animals utilize chemical senses for various purposes:
  • Finding mates
  • Recognizing territories
  • Navigating during migration
• Social insects (e.g., ants, bees) rely on chemical communication.
• Chemical senses play a crucial role in feeding behaviors.

Taste and Smell

• Definitions:
  • Gustation (Taste): Detection of tastants (chemical substances in solution).
  • Olfaction (Smell): Detection of odorants (chemical signals in the air).
• Aquatic vs. Terrestrial Animals:
  • No distinction in taste and smell for aquatic animals.
• Insects:
  • Taste receptors located on sensory hairs on feet and mouthparts to select food.
  • Olfactory receptors located on antennae.
• Chemical DEET:
  • Repels mosquitoes by blocking receptors that detect human scent.

Taste Buds in Mammals

• Receptor cells for taste are organized into taste buds.
• Tongue Structure:
  • Taste buds are associated with papillae (nipple-shaped projections).
  • Taste maps of the tongue are inaccurate; any region with taste buds can detect all five types of taste.
• Taste Receptors:
  • Sweet, umami, and bitter tastes require GPCR proteins.
  • Humans have:
  • 1 type of sweet receptor.
  • 1 type of umami receptor.
  • Over 30 bitter taste receptors (each recognizing multiple bitter tastants).
  • Sour receptor belongs to the TRP family (similar to capsaicin receptor).
  • Salt receptor specifically detects sodium channels.

Smell in Humans

Olfactory System

• Structure:
  • Olfactory receptor cells are neurons lining the nasal cavity.
  • Cilia on the cells extend into mucus to detect odorants.
• Process of Detection:
  1. Odorant binds to olfactory receptor (GPCR).
  2. Triggers signal transduction, leading to cyclic AMP production.
  3. Opens channels for Na+ and Ca2+, causing depolarization and action potentials.

Gene Family for Olfactory Reception

• Humans have about 380 OR genes, significantly fewer than mice (1,200 OR genes).
• Each olfactory receptor cell expresses one OR gene and transmits signals to the same olfactory bulb region.
• The brain integrates signals from different receptors, allowing odor discrimination and contributing to the perception of flavor.

Muscle Function and Structure

Basics of Muscle Contraction

• Muscle contraction relies on the interaction of thin (actin) and thick (myosin) filaments.
• Thin Filaments:
  • Composed mostly of actin; two strands polymerize to form a double helix.
• Thick Filaments:
  • Arrays of myosin molecules; responsible for muscle contraction.

Skeletal Muscle Structure

• Organized into hierarchical units: fibers → myofibrils → sarcomeres.
• Sarcomeres:
  • Basic contractile units of skeletal muscle; exhibit striations.
  • Composed of overlapping thin and thick filaments (Z lines, M line).

Sliding-Filament Model of Muscle Contraction

• Thin and thick filaments slide past each other, powered by ATP.
• Myosin Heads:
  • Bind to actin, forming cross-bridges and pulling thin filaments.
• ATP’s Role:
  • Hydrolysis of ATP provides energy for contraction and release.

Energy Supply in Muscle Contraction

• Muscle fibers utilize:
  • Creatine Phosphate: Regenerates ATP quickly for about 15 seconds.
  • Glycogen: Breaks down to glucose for ATP during aerobic respiration.
  • Lactic Acid Fermentation: Supplies ATP during intense activity but is less efficient.

Regulatory Proteins in Muscle Contraction

• Tropomyosin and Troponin:
  • Regulatory proteins that cover myosin-binding sites on actin strands when muscle is at rest.
• Calcium Role:
  • Triggered release from sarcoplasmic reticulum exposes binding sites, initiating contraction.

Control of Muscle Contraction

Motor Neurons and Muscle Tension

• Motor Units:
  • Consist of a motor neuron and the muscle fibers it controls.
• Graded Contraction:
  • Varies based on the number of activated motor fibers and stimulation frequency.
• Tetanus:
  • Smooth, sustained contraction resulting from rapid successive stimuli.

Types of Skeletal Muscle Fibers

Classification

• Oxidative Fibers:
  • Use aerobic respiration; rich in myoglobin, mitochondria.
• Glycolytic Fibers:
  • Larger, use glycolysis; fatigue quickly.
• Fast-Twitch vs. Slow-Twitch Fibers:
  • Fast-twitch fibers contract quickly; slow-twitch fibers contract slowly and sustain longer.

Muscle Types in Vertebrates

• Cardiac Muscle:
  • Striated, found in the heart, involuntary contraction.
• Smooth Muscle:
  • Non-striated, found in hollow organs, involuntary contraction, slower to contract.

Skeletal Systems in Animals

Types of Skeletons

• Hydrostatic Skeletons:
  • Fluid-filled compartments (e.g., cnidarians, annelids) control form and movement.
• Exoskeletons:
  • Hard outer covering found in arthropods and mollusks.
• Endoskeletons:
  • Internal skeleton found in vertebrates, includes cartilage and bone.

Function of Skeletons

• Support, movement, and protection of internal organs.
• Muscles attached to skeleton enable movement through contraction.
• Animal body weight and posture are critical factors in the design of skeletal structures.