Physiology Laboratory: Reflexes and Sensory Systems

Quiz Review and Terminology

• Sclera: The white outer layer of the eyeball.
• Refraction: The bending of light as it travels through a denser medium than air.
• Transduction (Signal Transduction): The conversion of any type of energy into action potentials.
• Greater Resolution of Point Discrimination: Fingers can distinguish a greater resolution of points with calipers than the lower back due to a greater proportion, density, or more receptors per unit area in the fingers.
• Cones: Photoreceptors that require stimulation of bright light and can detect millions of colors.
• Color Blindness Genotypes:
  • A male with a $X^{a}Y$ configuration regarding photoreceptors would exhibit red-green color blindness.
  • A female with a $X^A X^a$ configuration would have normal color vision.
• Olfactory Masking: An example provided where an individual smells vanilla for approximately $15 \, \text{seconds}$ and subsequently cannot detect a new smell brought to the nose.
• Conduction Deafness: Concluded from a Rhine test when a patient hears a softer tone after the tuning fork dies away when placed on the mastoid process.
• Homeostasis: The fundamental principle upon which physiology is based.
• Hyperopia: The clinical term for a condition in which an individual can focus on far objects but not on near objects (farsightedness).
• Astigmatism: A condition resulting from an irregularly shaped cornea, characterized by fuzziness.

Fundamentals of Reflexes

• Definition of a Reflex: A rapid, unconscious, and stereotyped response to a stimulus. It happens quickly, does not require conscious thought, and the reaction is always the same.
• Types of Reflex Arcs:
  • Disynaptic (Polysynaptic) Reflex Arc: Involves multiple synapses.
  • Monosynaptic Reflex Arc: Involves only one synapse.

The Disynaptic (Polysynaptic) Reflex Arc

• Example: The pain withdrawal reflex (e.g., touching a hot pan or stepping on something sharp).
• Five Components of Every Disynaptic Reflex Arc:
  1. Receptor: Receives an adequate stimulus to start the process.
  2. Sensory Neuron: Also known as the afferent neuron; carries transduced information (nerve impulses) toward the Central Nervous System (CNS).
  3. Interneuron: Also known as an association neuron; found within the integrating center (CNS).
  4. Motor Neuron: Also known as the efferent neuron; carries the signal from the CNS to the effector.
  5. Effector: The structure that carries out the response, which can only be muscles (contract) or glands (secrete).
• Structural Details:
  • Dendrites: Located at the receptor to receive signals.
  • Axon: The pathway for the electrical signal.
  • Terminal Boutons (Synaptic Bulbs/Termini): The ends of the neuron where neurotransmitters are released.
  • Synapse: Literally means "gap"; the interface where the electrical signal is converted to a chemical signal (neurotransmitter) to communicate with the next cell.
• Excitatory Postsynaptic Potential (EPSP):
  • Symbolized by a positive sign ($+$).
  • Represents stimulatory or excitatory action.
  • A disynaptic arc has two synapses, both resulting in EPSPs in the stimulatory pathway.
• Inhibition: Disynaptic reflexes often work in pairs; for instance, when the biceps brachii is stimulated to flex the arm, the triceps must be inhibited to allow movement.

The Monosynaptic Reflex Arc (Stretch Reflex)

• Definition: A reflex arc involving only one synapse.
• Muscle Fiber Types:
  • Extrafusal Muscle Fibers: Make up $98-99\%$ of muscle cells; these are the typical contracting fibers used for movement.
  • Intrafusal Muscle Fibers: Also known as stretch fibers or spindle fibers; make up about $1-2\%$ of the muscle. They are spindle-shaped (fat in the middle, tapered at the ends).
  • Contractile Region: The ends of the spindle fibers that can contract to maintain tension.
• Neuronal Components:
  • Type 1a Sensory Neuron: Wraps around the intrafusal fibers and detects stretch.
  • Alpha ($\alpha$) Motor Neuron: Innervates every extrafusal fiber in the body to cause contraction.
  • Gamma ($\gamma$) Motor Neuron: Innervates the contractile regions of the intrafusal fibers.
• Alpha-Gamma Coactivation: The process by which gamma motor neurons increase the sensitivity of intrafusal fibers during muscle contraction to prevent them from becoming flaccid. This maintains the stretch signal throughout the entire range of motion.
• Clinical Significance: In spinal cord injuries, gamma motor neurons may fail to work, preventing the individual from maintaining posture against gravity.

Laboratory Experiments: Disynaptic Eye Reflexes

• Eye Blink Reflex:
  • Response: Blinking.
  • Inhibition: Can be consciously inhibited if the stimulus is anticipated.
  • Unconscious Individual: The response would NOT be found in an unconscious person.
  • Consensual Response: Stimulating one eye results in both eyes reacting.
• Corneal Reflex (Puff of Air):
  • Procedure: A puff of air is directed near the eye from about $6 \, \text{inches}$ away.
  • Consensual Response: Both eyes blink.
  • Unconscious Individual: The response IS found in an unconscious person.
  • Cross-Wiring: The sensory nerves join up in a way that allows a direct response in the stimulated eye and a consensual response in the non-stimulated eye. This is a protective mechanism.
• Pupillary Reflex:
  • Response: Pupils constrict in bright light and dilate when light is removed.
  • Inhibition: Cannot be inhibited.
  • Unconscious Individual: Response is present (used to check for brain swelling or trauma).
  • Terms:
    • Ipsilateral: Same side.
    • Contralateral: Opposite side.
  • Decussation: The crossing over of neurons in the spinal cord or brain (e.g., at the optic chiasma).

Laboratory Experiments: Stretch Reflexes

• Testing Areas: Patellar (knee-jerk), Achilles (base of foot/heel), Biceps (elbow), Triceps (back of arm), and Forearm Extensor (upper third of forearm to move the middle finger).
• Facilitation (Pre-stretching):
  • By creates tension in other muscle groups (e.g., pulling hands apart), one "pre-stretches" all spindle fibers in the body.
  • This is a Feed Forward Mechanism that fine-tunes the body for emergencies or stress, increasing the firing rate of gamma motor neurons.
• Babinski Reflex:
  • Procedure: Firmly stroking the bottom of the foot from heel to toe in an "S" shape toward the big toe.
  • Positive Babinski (Present): Toes spread out and extend; normal in infants because their nervous systems are not fully developed.
  • Negative Babinski (Absent): Toes curl and flex; normal for adults.
  • Clinical Use: Used to test for spinal cord injury, stroke, or brain injury in adults.

Vestibular Reflexes and the Cerebellum

• Cerebellum: Coordinates gross muscle movements (walking, running) and interacts with the vestibular apparatus.
• Vestibular Apparatus Components:
  1. Utricle and Saccule: Detect head position and linear acceleration (e.g., a car suddenly stopping). Contain endolymph and hair cells.
  2. Semicircular Canals: There are three canals oriented in the $x$, $y$, and $z$ planes to detect rotation in any direction.
• Sensory Inputs for Balance (Equilibrium):
  • Vision: Humans rely heavily on visual cues for stability.
  • Proprioception: The brain's knowledge of the position and movement of body parts in space.
  • Vestibular Input: Balance and posture (telling the brain where the body is in 3D space).
• Experiments in Balance:
  • Stability is greater with two feet than one, and eyes open than closed.
  • Head position change (tilting back) disrupts the orientation of semicircular canals, making balance harder.
  • Cerebellar Override: When trying to rise on toes very slowly with eyes closed and head back, the cerebellum overrides the motor cortex to prevent a fall. This happens via Inhibitory Postsynaptic Potentials (IPSPs).
  • IPSPs vs. EPSPs: IPSPs inhibit action, while EPSPs excite action.

Vestibulo-Ocular Reflexes (V.O.R.)

• Rapid Head Impulse Test: Maintaining focus on a target (nose) while the head is moved quickly. Tracking back (staccato movement) can indicate brain injury or intoxication.
• Post-Rotatory Nystagmus:
  • Procedure: Spin a subject $10 \, \text{times}$ in a chair and stop them.
  • Mechanism: When the chair stops, the eyes tell the brain movement has ceased, but moving endolymph in the semicircular canals deforms the cupula (a dome-shaped structure over the hair cells), telling the brain the body is still spinning.
  • Depolarization vs. Hyperpolarization: These are "on/off" switches. Depolarization reaches threshold to fire the neuron; hyperpolarization moves the potential away from threshold to inhibit the signal.
  • Time Measurement: Measured in milliseconds ($1/1000 \, \text{second}$).

The Mammalian Dive Reflex

• Experimental Design: Measuring heart rate (EKG) during face immersion in tap water ($\approx 21 \, ^{\circ}\text{C}$) and ice water ($< 10 \, ^{\circ}\text{C}$, ideally $7-10 \, ^{\circ}\text{C}$).
• Results: Heart rate decreases slightly in tap water and significantly in cold water.
• Adaptive Value:
  • Conserves oxygen and slows metabolism.
  • Brownian Motion Link: Cold environments slow chemical reactions, decreasing the demand for ATP and oxygen.
• Autonomic Nervous System Divisions:
  • Parasympathetic: "Rest and Digest"; homeostatic, brings body back to normal.
  • Sympathetic: "Fight or Flight"; takes body away from homeostasis (e.g., adrenaline rush, white coat hypertension).
• Clinical Application: Victims of drowning in cold water have a much higher chance of survival than those in warm water due to the immediate slowing of metabolism via the dive reflex.