Neural Pathways, Reflex Mechanisms, Biological Clocks, and Sensory-Motor Integration

Vocabulary flashcards covering neural circuits, spinal reflex pathways, biological pacemakers, basal ganglia pathways, and sensory-motor integration systems from the lecture material.

Central Pattern Generators (Pacemakers)

Neuronal circuits that produce rhythmic motor output and possess clock-like capabilities, whose activity can be started, stopped, or modulated by sensory or central pathways.

Suprachiasmatic Nucleus (SCN)

A pair of round, bilaterally occurring neuroanatomical structures located in the anterior hypothalamus dorsal to the optic chiasm and on either side of the third ventricle that serve as the master biological clock.

Endogenous SCN Clock Period

An intrinsically generated timing cycle of approximately $24.5\,\text{hours}$ that drifts daily unless reset or entrained by environmental light cues.

Retinorecipient Ventral SCN

The ventral subdivision of the suprachiasmatic nucleus that receives direct photic input from melanopsin-containing cells in the retina via the retinohypothalamic tract.

Free-Running Biological Clock

A circadian rhythm state occurring in the absence of external light cues, resulting in the sleep-wake schedule shifting approximately $30\,\text{minutes}$ later each day.

Mammoth Cave Experiment (1938)

A study conducted by Nathaniel Kleitman and Bruce Richardson $140\,\text{feet}$ underground at constant $54^\circ\text{F}$ to evaluate the body's endogenous circadian rhythms during a forced $28\text{-hour}$ day.

Feed-Forward Inhibition

A circuit mechanism utilizing inhibitory interneurons to suppress activity in neighboring cells, limiting the spatial spread of information and duration of response.

Feed-Back Inhibition

A circuit mechanism where an activated output neuron sends an inhibitory signal back to preceding neurons to report output and halt continuous excitation.

Lateral Inhibition

An inhibitory interneuron mechanism that sharpens spatial mapping and contrast by suppressing signal transmission in neurons adjacent to the center of excitation.

Dorsal Root Ganglion

A bulge containing a cluster of primary sensory neuron cell bodies located in the peripheral nervous system along the dorsal root of a spinal nerve.

Myotatic Stretch Reflex

A monosynaptic reflex wherein passive stretching of muscle spindles triggers afferent signals that directly excite alpha motor neurons to contract the same muscle while polysynaptically inhibiting antagonistic muscles.

Golgi Tendon Organ Reflex

A polysynaptic reflex mediated by Ib afferents sensitive to muscle tension and force that inhibits alpha motor neurons of the contracting muscle to control generated force.

Alpha Motor Neurons

Anterior horn motor neurons that innervate extrafusal muscle fibers to drive muscle contraction and force development.

Gamma Motor Neurons

Anterior horn motor neurons that innervate intrafusal muscle fibers within muscle spindles to maintain spindle sensitivity during muscle shortening.

Flexor-Withdrawal Reflex

A protective polysynaptic reflex triggered by nociceptive pain fibers (groups II, III, IV) that excites ipsilateral flexor muscles and inhibits ipsilateral extensor muscles to retract the limb.

Crossed-Extensor Reflex

A contralateral reflex component accompanying the flexor-withdrawal reflex that excites extensor muscles and inhibits flexor muscles in the opposite limb to maintain balance.

Foot Slapping

A compensatory gait modification in individuals with loss of spinal sensory input, where stomping the foot provides auditory and mechanical cues confirming foot placement.

Purkinje Cells

Large inhibitory output neurons of the cerebellar cortex that receive sensory and motor inputs and project to deep cerebellar nuclei to regulate fine motor coordination.

Parkinson's Disease

A neurodegenerative movement disorder caused by a loss of approximately $80\%$ of dopaminergic neurons in the substantia nigra pars compacta, leading to a slow shuffling gait, rigidity, and freezing.

Huntington's Disease

A genetic motor planning disorder caused by the loss of striatal neurons within the basal ganglia.

Lateral Geniculate Nucleus (LGN)

A layered thalamic relay structure receiving monocular visual inputs from both retinas and projecting to ocular dominance columns in layer IV of the primary visual cortex.

Ocular Dominance Columns

Vertical bands of neurons in layer IV of the primary visual cortex (V1) that process input arriving selectively from either the ipsilateral or contralateral eye.

Cytochrome Oxidase Blobs

Pillar-like structures rich in cytochrome oxidase located in layers II and III of area V1 that selectively process color information.

Orientation Columns

Columns of neurons in the primary visual cortex that fire preferentially to elongated visual stimuli presented at specific angles.

Superior Colliculus

A midbrain structure containing superimposed topographic maps of visual, auditory, somatosensory, and motor space to guide orienting responses toward environmental stimuli.

Dorsal Column System

An ascending somatosensory pathway conveying fine touch, pressure, and proprioception via the nucleus gracilis (lower body) and nucleus cuneatus (upper body) to the thalamus and cortex.

Anterolateral System

An ascending somatosensory pathway conveying pain, temperature, and light touch that synaptically decussates in the spinal cord before ascending to the thalamus.

Whisker Barrels

Anatomical clusters of neurons in layer IV of the rodent somatosensory cortex where each barrel represents tactile input from a single facial whisker.

Thalamus

A nuclear complex bordering the third ventricle that acts as a neuroanatomical crossroad, processing and segregating somatic sensory signals before relaying them to designated cerebral cortical regions.

Cortical Homunculus

A distorted topographical representation of the human body along the primary motor and somatosensory gyri, reflecting the proportion of cortex dedicated to sensory or motor control.

Interaural Intensity Difference

A mechanism for localizing high-frequency sounds in the horizontal plane by detecting sound loudness differences between ears caused by the head's acoustic shadow.

Interaural Time Difference

A mechanism for localizing low-frequency sounds utilizing delay lines and coincidence detectors in the Medial Superior Olive (MSO) to detect arrival time differences ($\sim 0.6\,\text{ms}$) between ears.

Pinna

The external ear structure that produces direct and reflected sound path variations into the auditory canal to allow monaural vertical sound localization.