Sensory and Motor Mechanisms: Mechanisms of Biological Mechanisms

Sensory Information and the Linking of Stimuli to Animal Activity

All stimuli represent various forms of energy which must be processed by an organism.
Sensory receptors are responsible for converting stimulus energy into a change in the membrane potential of the cell.
The interaction between the nervous system and received stimuli results in a motor response, which may range from a simple reflex to more elaborate, complex processing.
A specific example involves a mole foraging in a tunnel:
- The mole encounters an object (Sensory input).
- The brain or nervous system determines if food is detected (Integration).
- If food is not detected, the mole moves on; if food is detected, the mole bites (Motor output).

Functions of Sensory Pathways

All sensory pathways share four fundamental functions: reception, transduction, transmission, and perception.
Sensory Reception: This is the detection of stimuli by sensory receptors. These receptors can be located internally (sampling the internal environment) or externally (sampling the outside world).
Transduction: This is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor. This change in potential varies directly with the strength of the stimulus.
Transmission: Sensory information is conveyed through the nervous system via action potentials. The intensity of a signal is communicated by the frequency of the action potentials, not their amplitude.
Perception: This is the brain’s interpretation of stimuli. The brain distinguishes different stimuli based on which specific neural pathways deliver the action potentials.
While most pathways involve Brain circuits, reflex pathways are unique because they do not require processing in the brain to produce a response.

Signal Modification: Amplification and Adaptation

Signal transduction is not a static process and can be modified through amplification or adaptation.
Amplification: This refers to the strengthening of a sensory signal during the transduction process. For example, the energy from just a few photons of light can trigger an action potential that possesses approximately $100,000 \times$ more energy than the original photons by the time the signal reaches the brain.
Sensory Adaptation: This is a physiological decrease in responsiveness to continued, constant stimulation. An example is walking into a room and being overwhelmed by the smell of garlic and tomatoes, only to find that the scent seemingly dissipates after a few minutes until it is barely noticeable.

Categories of Sensory Receptors

Sensory receptors are categorized based on the type of energy they transduce:

Mechanoreceptors: These respond to physical deformation caused by stimuli such as touch, pressure, vibration, and stretch or movement. They are found in the skin, ears, and arteries.
Chemoreceptors: These respond to specific chemical stimuli, including those related to smell, taste, and changes in blood chemistry. They are located in the tongue, nose, and various internal tissues.
- Specific instance: The antennae of the male silkworm moth possess highly sensitive and specific chemoreceptors for detecting pheromones.
Electromagnetic Receptors: These detect electromagnetic energy, including light, electricity, and magnetism.
- Photoreceptors in the human eye are a type of electromagnetic receptor.
- Many animals utilize Earth’s magnetic field for orientation during migration.
Thermoreceptors: These respond to changes in temperature, both heat and cold. They are located in the skin and the hypothalamus.
- Specific instance: Certain snakes possess specialized thermoreceptors to detect infrared radiation emitted by warm-blooded prey.
Pain Receptors (Nociceptors): These respond to potentially harmful stimuli, such as extreme pressure, extreme temperature, or chemicals released by damaged or inflamed tissues.

Structure and Hierarchy of Vertebrate Skeletal Muscle

Vertebrate skeletal muscle (also known as striated muscle) is organized in a hierarchy of smaller units:
1. Skeletal Muscle: A bundle of long fibers.
2. Muscle Fiber: A single muscle cell within the bundle.
3. Myofibrils: These are bundles that make up the individual muscle fiber.
4. Sarcomere: The functional unit of the muscle, which is a specific section of a myofibril.
5. Thick and Thin Filaments: The protein structures within the sarcomere.
Thin Filaments: Composed primarily of the protein actin.
Thick Filaments: Composed primarily of the protein myosin.
Muscle contraction is a response to nervous system input and relies entirely on the interaction and longitudinal movement of these filaments.

The Sliding-Filament Model of Muscle Contraction

The sarcomere is bordered by structures called Z lines, which serve as the attachment points for thin filaments.
According to the Sliding-Filament Model, thin and thick filaments do not shorten, but rather ratchet past each other longitudinally.
Mechanism of the Myosin Molecule:
- Each myosin molecule has a long "tail" and a globular "head."
- The myosin head (in a high-energy configuration) binds to an actin filament to form a cross-bridge.
- The head then pulls the thin filament toward the center of the sarcomere.
- Muscle contraction requires repeated cycles of this binding, pulling, and releasing.
- This process is powered by ATP.

Regulation of Muscle Contraction: Calcium and Regulatory Proteins

At Rest: The regulatory protein tropomyosin and a set of proteins called the troponin complex bind to the actin strands on the thin filaments. This physical blockage prevents myosin and actin from interacting.
Role of Calcium ( $Ca^{2+}$ ): For contraction to occur, myosin-binding sites must be exposed.
- When the concentration of $Ca^{2+}$ is high, the ions bind to the troponin complex.
- This binding causes a conformational change that shifts the tropomyosin, exposing the myosin-binding sites on the actin.
Contraction occurs when $Ca^{2+}+$ levels are high and ceases when the concentration of $Ca^{2+}$ returns to a low level.

Physiological Triggers of Muscle Contraction

The process begins with an action potential in a motor neuron that synapses with a muscle fiber.
The synaptic terminal of the motor neuron releases the neurotransmitter acetylcholine (ACh).
Acetylcholine induces depolarization in the muscle cell, triggering an action potential in the muscle fiber.
This action potential travels deep into the muscle fiber along transverse (T) tubules, which are infoldings of the plasma membrane.
The action potential along the T tubules triggers the sarcoplasmic reticulum (SR)—a specialized endoplasmic reticulum—to release stored $Ca^{2+}$ .
The released $Ca^{2+}$ binds to the troponin complex, initiating the sliding-filament mechanism.

Questions & Discussion

Question: Compared to a weak signal stimulating a sensory neuron, what does a strong signal trigger?
- Response: It triggers action potentials of similar amplitude but greater frequency. A strong signal does not change the amplitude of the action potential.
Question: Which elements of the sliding-filament model are correctly or incorrectly defined?
- Response: Tropomyosin is a regulatory protein. The troponin complex consists of additional regulatory proteins. Transverse tubules are infoldings of the plasma membrane. The sarcoplasmic reticulum is a specialized endoplasmic reticulum. However, it is incorrect to say calcium ions are bound to the myosin protein to play a role in contraction; they actually bind to the troponin complex on the actin filament.
Question: What initiates the contraction of myofibrils in skeletal muscle?
- Response: Contraction is initiated specifically by the release of calcium from the sarcoplasmic reticulum, not its uptake, and not by the T tubules or mitochondria.