3.5 Sensory & Motor Mechanisms

Sensory Reception and Perception

  • Sensations are action potentials that reach the brain via sensory neurons. Perception is the awareness and interpretation of the sensation. Example: taste, smell, color, etc.

  • Sensory reception begins with the detection of stimulus energy by sensory receptors.

    • Exteroreceptors detect stimuli originating outside the body.

    • Interoreceptors detect stimuli originating inside the body.

    • Sensory receptors convey the energy of stimuli into membrane potentials and transmit signals to the nervous system.

  • Steps of sensory processing: sensory transduction, amplification, transmission, and integration.

    • Sensory transduction: conversion of stimulus energy into a change in membrane potential; receptor potential is the receptor’s version of a graded potential.

    • Amplification: strengthening of stimulus energy that can be detected by the nervous system.

    • Integration: processing of sensory information (begins at the sensory receptor).

    • Transmission: conduction of sensory impulses to the CNS; some receptors transmit signals to sensory neurons, while others are themselves sensory neurons.

  • Type of sensory receptors corresponds to the energy transduced (examples):

    • Temperature

    • Touch, pressure

    • Pain

    • Light

    • Sound

    • Smell

    • Taste

    • etc.

  • Mechanoreceptors respond to mechanical energy.

    • Example: muscle spindles are interoceptors that respond to stretch of skeletal muscle.

    • Hair cells detect motion.

  • Thermoreceptors respond to heat or cold (surface and body core temperature).

  • Pain receptors = nociceptors.

    • Different nociceptors respond to different types of pain.

    • Prostaglandins increase pain by decreasing a pain receptor threshold. (i.e., they lower the threshold for activation)

  • Chemoreceptors respond to chemical stimuli.

    • General chemoreceptors transmit information about total solute concentration.

    • Specific chemoreceptors respond to specific molecules.

    • Internal chemoreceptors respond to glucose, O₂, CO₂, amino acids, etc.

    • External chemoreceptors are gustatory receptors and olfactory receptors.

  • Electromagnetic receptors respond to electromagnetic energy.

    • Photoreceptors respond to visible light.

    • Electroreceptors are used by some fish to locate objects via electric currents.

Vision

  • Major components often labeled in diagrams: sclera, cornea, iris, pupil, lens, aqueous humor, vitreous humor, retina, choroid, fovea (center of visual field), optic disc (blind spot), optic nerve, central retinal artery and vein.

  • Supporting structures in the diagram may include ciliary body and suspensory ligaments that position the lens.

  • The eye structure supports light entry, focuses imagery on the retina, and transduces light into neural signals via photoreceptors in the retina (rods and cones).

Gustatory System (Taste)

  • In mammals, taste receptors are located in taste buds on the surface of the tongue.

  • Basic tastes: salty, sweet, sour, bitter.

  • Umami is a newly recognized basic taste (Japanese: umai = delicious; mi = essence).

  • Taste receptor responses depend on patterns of receptor activation rather than a single receptor; the flavor is determined by the pattern of nerve signals.

  • Umami is associated with amino acids, notably glutamate.

  • Data illustrating glutamate content (approximate values in foods; patterns shown):

    • Umami sources include diverse foods and their glutamate content; the table lists foods such as avocado, champignon (button mushroom), kombu (kelp), lotus root, fish sauce, onion, snow crab, cheddar, green mussel, green pea, white shrimp, nori, soy sauce, tomato, corn, cheddar cheeses, beef, pork, chicken, potato, dried shiitake, parmesan, milk varieties, and others.

    • Example entries (glutamate content as reported in the source):

    • Avogadro (การอธิบาย): 18 mg/100 g? [Note: table contains mixed Thai/English terms; numbers represent mg per 100 g in many entries.]

    • Kombu (Kombu/Kelp): 2,240 mg/100 g

    • Shiitake dried: 1,060 mg/100 g

    • Parmesan cheese: 1,680 mg/100 g

    • Soy sauce: 1,090 mg/100 g

    • Tomato: 246 mg/100 g

    • Cheese (Cheddar): 182 mg/100 g

    • Milk (cow): 2 mg/100 g

    • The overall message: foods vary widely in free glutamate content, contributing to umami flavor; the “Umami” tabulated data is presented in a milligram-per-100-gram style in the source.

  • Etymology and explanation: UMAMI = Umai (Delicious) and Mi (Essence).

Olfactory Receptor (Smell)

  • Receptors line the upper portion of the nasal cavity.

  • Binding of odor molecules to olfactory receptors initiates signal transduction pathways.

  • The signal is transmitted through sensory neurons to the brain, ending in the olfactory bulb.

Movement and Locomotion

  • Skeletons provide support and protect the body; essential to movement.

  • Joints allow movement and are categorized by types such as ball-and-socket, hinge, and pivot.

  • The human skeleton includes:

    • Shoulder girdle: clavicle, scapula

    • Thoracic elements: sternum, ribs

    • Upper limb elements: humerus, radius, ulna, carpals, metacarpals, phalanges

    • Pelvic girdle and lower limb elements: pelvis, femur, patella, tibia, fibula, tarsals, metatarsals, phalanges

  • Examples of joints:

    • Ball-and-socket joint (e.g., shoulder and hip)

    • Hinge joint (e.g., elbow, knee)

    • Pivot joint (e.g., between radius and ulna)

Muscles and Locomotion

  • Muscles move skeletal parts by contracting.

  • Antagonistic pairs: one muscle (flexor) contracts to bend a joint, while the opposite muscle (extensor) contracts to straighten it.

  • Examples from the diagrams:

    • Triceps = extensor

    • Biceps = flexor

  • Types of skeletal support systems:

    • Endoskeleton

    • Exoskeleton

  • The musculoskeletal arrangement includes muscle groups arranged in opposing pairs to enable movement in multiple directions.

Structure and Function of Vertebrate Skeletal Muscle

  • The sarcomere is the functional unit of muscle contraction.

  • Thin filaments consist of two strands of actin and one tropomyosin coiled about each other.

  • Thick filaments consist of myosin molecules.

Sliding Filament Model

  • Key components: thick filaments (myosin) and thin filaments (actin) arranged in sarcomeres.

  • Myosin configuration and ATP involvement drive the cycle.

  • The cycle can be summarized in stages (cross-bridge cycle) with energy changes:
    1) Resting state: Tropomyosin blocks the myosin-binding sites on actin.
    2) Activation: Ca2+Ca^{2+} binds to the troponin complex, causing a conformational change that moves the tropomyosin-troponin complex away from actin’s binding sites, exposing them for myosin binding.
    3) Cross-bridge formation: Myosin head binds to actin forming a cross-bridge; ATP is bound to myosin in the pre-attachment state.
    4) ATP hydrolysis: The myosin head hydrolyzes ATP to ADP and inorganic phosphate, which causes the head to rotate into a high-energy (cocked) configuration.
    5) Power stroke: Release of ${P_i}$ triggers the power stroke; actin moves toward the center of the sarcomere.
    6) ADP release: After the power stroke, ADP is released.
    7) Detachment: A new ATP binds to myosin, causing the myosin head to detach from actin.
    8) Re-cocking: Hydrolysis of the new ATP molecule re-sets the myosin head to the high-energy configuration, ready for another cycle if Ca2+ remains elevated.

  • The cycle repeats as long as Ca2+ remains bound to troponin and ATP is available.

  • Visual representation phases include cross-bridge formation, the power stroke, cross-bridge detachment, and re-cocking of the myosin head.

  • The sliding of thin filaments past thick filaments shortens the sarcomere, producing contraction.

Calcium Regulation and Regulatory Proteins

  • At rest, tropomyosin blocks myosin-binding sites on actin.

  • When Ca2+Ca^{2+} binds to the troponin complex, a conformational change moves tropomyosin off actin’s binding sites, exposing them for myosin interaction.

  • This regulatory mechanism allows muscle contraction to proceed only when Ca2+ is present.

Nerve-Muscle Relationship and Neuromuscular Transmission

  • An action potential travels through a motor neuron to the neuromuscular junction (NMJ).

  • The axon terminal releases acetylcholine (Ach).

  • The sarcoplasmic reticulum (SR) within muscle cells releases Ca2+Ca^{2+} from storage.

  • The increased intracellular Ca2+ enables the sliding filament mechanism to proceed.

Summary of Skeletal Muscle Contraction (Table 10-1 style)

  • Steps that initiate a contraction:

    • Motor end plate receives acetylcholine (ACh) from the motor neuron

    • ACh causes depolarization at the motor end plate and propagates along the T-tubules

    • The T-tubules trigger the sarcoplasmic reticulum to release Ca2+ (Ca2+ release)

    • Active sites on actin are exposed, enabling cross-bridge formation

  • Steps that end a contraction and lead to relaxation:

    • ACh is removed by acetylcholinesterase (AChE)

    • Ca2+ is pumped back into the sarcoplasmic reticulum (reuptake)

    • Cross-bridge cycling ceases as myosin binding sites become blocked again by tropomyosin-troponin complex

    • Active sites on actin are covered; muscle relaxes and returns to resting length

  • The sequence can be summarized in a more compact form (as presented in the source):

    • 1) ACh released, binding to receptors

    • 2) Action potential reaches T-tubule

    • 3) ACh removed by AChE

    • 4) Active site exposure

    • 5) Contraction begins

    • 6) ACh released, binding to receptors (repeats) – actually part of the synaptic cycle; see below for synapse details

    • 7) SR releases Ca2+

    • 8) Cross-bridge formation

    • 9) Contraction proceeds

    • 10) Relaxation occurs

  • The above is complemented by a labeled, stepwise depiction of cross-bridge cycling and Ca2+-mediated regulation.

Acetylcholine at the Synapse: Synaptic Activity (Cholinergic Synapse)

  • The sequence of events at a typical cholinergic synapse: 1) An arriving action potential depolarizes the synaptic knob. 2) Calcium ions enter the cytoplasm of the synaptic knob.

    • ACh is released via exocytosis of neurotransmitter vesicles.
      3) ACh diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.

    • Chemically gated sodium channels on the postsynaptic surface are activated, producing a graded depolarization.
      4) ACh release ceases because Ca2+ is removed from the cytoplasm of the synaptic knob.
      5) The depolarization ends due to breakdown of ACh by acetylcholinesterase (AChE), producing acetate and choline.
      6) The synaptic knob reabsorbs choline from the synaptic cleft and uses it to resynthesize ACh.

  • Diagrams in the source include components: acetyl-CoA, choline, acetylcholine, ACh receptor, Na+ channels, vesicles, mitochondria, synaptic knob, synaptic cleft, and acetylcholinesterase.

Acetylcholine Recycling at the Synapse (Process Overview)

  • The neuron recycles choline to synthesize new ACh for subsequent release.

  • Ach breakdown products (acetate and choline) are cleared from the synapse, ensuring transient signal transmission.

Ethical and Practical Implications: Nerve Agents and Inhibition of AChE

  • The slide notes the existence and hazard of enzyme inhibitors used as chemical weapons (e.g., VX nerve agent).

  • VX nerve agent is an acetylcholinesterase (AChE) inhibitor, illustrating the severe consequences of disrupting cholinergic signaling.

  • Important cautionary notes:

    • AChE inhibitors cause uncontrolled ACh accumulation, leading to excessive stimulation of muscles and glands, potentially causing paralysis or death.

    • The topic raises ethical and safety considerations regarding chemical weapons and the importance of strict regulation and safety protocols.

Key Concepts and Terms

  • Sensory receptor: a cell or neuron specialized to respond to a specific stimulus.

  • Receptor potential: a graded potential produced by a sensory receptor in response to a stimulus.

  • Transduction: conversion of one energy form into another (e.g., stimulus energy into an electrical signal).

  • Amplification: strengthening of a stimulus to a level detectable by the nervous system.

  • Integration: processing of sensory information within the CNS, beginning at the sensory receptor.

  • Exteroceptors vs interoceptors: external vs internal stimulus origin.

  • Mechanoreceptors, thermoreceptors, nociceptors (pain), chemoreceptors, photoreceptors, electroreceptors.

  • Photoreceptors, olfactory receptors, gustatory receptors.

  • Troponin-tropomyosin complex regulation of actin-myosin interaction.

  • Role of Ca2+ in muscle contraction.

  • Sliding filament model: coordinated interaction of actin and myosin leading to sarcomere shortening.

  • Sarcomere: functional unit of a muscle.

  • ATP hydrolysis and cross-bridge cycling: energy source for muscle contraction.

  • Neuromuscular junction: synapse between motor neuron and muscle fiber; ACh as neurotransmitter.

  • AChE: enzyme that degrades acetylcholine in the synaptic cleft.

  • VX nerve agent: AChE inhibitor illustrating chemical weapon hazard.

Key Equations and Symbols

  • Calcium activation and regulation in contraction: Ca2++Troponin<br>ightarrowConformational<br>d<br>eformation<br>ofTropomyosinCa^{2+} + Troponin <br>ightarrow Conformational<br>d<br>eformation<br>of Tropomyosin

  • ATP hydrolysis driving myosin head cocking: extATP<br>ightarrowextADP+extPiext{ATP} <br>ightarrow ext{ADP} + ext{P}_i

  • Muscle cross-bridge cycle (simplified):
    1) Resting state with blocked binding sites on actin;
    2) Ca2+Ca^{2+}-bound troponin exposes sites;
    3) Myosin binds to actin forming a cross-bridge;
    4) ATP hydrolysis re-cocks the myosin head;
    5) Pi release triggers power stroke;
    6) ADP release and detachment upon new ATP binding.

  • Neurotransmitter release steps (ACh):

    • Exocytosis triggered by Ca2+ influx; ACh release into synaptic cleft; receptor-mediated depolarization; degradation by AChE and choline reuptake for resynthesis.

Connections to Foundations and Real-World Relevance

  • Sensory transduction links physical stimuli to neural signals, enabling perception and behavior.

  • The senses of vision, taste, and smell illustrate how specialized receptors convert environmental energy into neural codes
    and how patterns of receptor activation yield complex percepts like flavor and odor.

  • The muscular system exemplifies basic principles of biology: energy transduction, regulation, and intricate coordination between nervous and muscular systems.

  • Understanding synaptic transmission informs neuroscience, pharmacology, and medical approaches to neuromuscular disorders.

  • Ethical considerations around nerve agents highlight the societal responsibility in science, safety, and policy.