Neuroscience: Sensory Neurons, Receptors, and Pain Pathways

What is the function of a sensory neuron?

It responds to stimuli such as light, odor, sound, or touch.

What is the function of a motor neuron?

It contacts muscles or glands to produce an output.

What is the main role of an interneuron?

Integration: it receives input from and sends output to other neurons; most CNS neurons are interneurons.

What is a sensory receptor cell?

A cell or specialized neuron that responds to a particular form of stimulus energy and converts it into a graded potential.

What happens if a receptor potential reaches threshold?

The sensory neuron can generate action potentials.

What is a labeled line in sensory processing?

A receptor activates a specific neural pathway, and the brain region receiving that pathway interprets the signal as a particular sense.

How can one sensory neuron encode a stronger stimulus if action potentials are all-or-none?

By increasing its action-potential firing frequency.

How does recruitment encode stimulus intensity?

As stimulus strength increases, more sensory neurons are activated in parallel.

What is range fractionation?

Different sensory neurons have different firing thresholds and therefore respond over different stimulus-intensity ranges.

What four broad skin sensations are emphasized in the somatosensory lecture?

Pain, touch, vibration, and stretch.

What does a Merkel's disc primarily detect?

Edges and points; it is a slowly adapting/tonic touch receptor.

What does a Meissner corpuscle primarily detect?

Touch and movement of objects across the skin; it adapts rapidly.

What do Pacinian (lamellated) corpuscles detect?

Vibration and pressure.

What do Ruffini corpuscles detect?

Skin stretch.

What do free nerve endings detect in this lecture?

Pain and temperature.

How does mechanical stimulation produce a receptor potential in a touch receptor?

Membrane stretch opens mechanically gated Na+ channels, producing a graded depolarization that can trigger an action potential.

What is a receptive field?

The part of the sensory world to which a particular sensory neuron responds.

How do receptive-field size and receptor density affect two-point discrimination?

Small receptive fields and high receptor density produce better two-point discrimination.

Which body region would have better two-point discrimination: the thumb or thigh?

The thumb, because it has smaller receptive fields and greater cortical representation.

What is a tonic receptor?

A slowly adapting receptor that shows little or no decline in firing during a sustained stimulus.

What is a phasic receptor?

A rapidly adapting receptor whose firing decreases quickly during a sustained stimulus.

How does an on-center, off-surround somatosensory receptive field respond?

Touch in the center produces maximal firing; touch in the surround decreases firing; distant touch has little or no effect.

What is the pathway for discriminative touch from skin to cortex?

Touch receptor → dorsal root ganglion → dorsal columns → synapse and cross in the medulla → thalamus → contralateral primary somatosensory cortex.

Where is primary somatosensory cortex (S1)?

The postcentral gyrus.

Which side of the body does S1 primarily represent?

The contralateral side.

What is the sensory homunculus?

A distorted body map in S1 in which more sensitive body regions occupy more cortical space.

Why are the hands and lips large on the sensory homunculus?

They have high sensory receptor density and require fine sensory discrimination, so they receive large cortical representation.

How does the lecture define pain?

An unpleasant sensory and emotional experience associated with actual or potential tissue damage.

Why is pain considered adaptive?

It warns of damaging conditions and promotes behaviors that prevent or limit injury.

What are nociceptors?

Pain-responsive free nerve endings with receptors that respond to tissue injury, chemicals, and temperature changes.

What kind of pain is carried by A-delta fibers?

Fast, sharp pain; A-delta fibers are relatively large and myelinated.

What kind of pain is carried by C fibers?

Slow, dull pain; C fibers are small and unmyelinated.

Which pain receptor is activated by capsaicin?

TRPV1 on C fibers.

Which receptor is associated with cold sensation in the lecture?

TRPM8.

What neurotransmitter is emphasized in peripheral pain signaling?

Substance P.

What pathway carries touch, vibration, two-point discrimination, and proprioception?

The dorsal column-medial lemniscal pathway.

What pathway carries pain and temperature?

The spinothalamic pathway.

After a spinal cord hemisection, where is discriminative touch lost below the lesion?

Ipsilateral to the lesion, because dorsal-column fibers cross in the medulla.

After a spinal cord hemisection, where are pain and temperature lost below the lesion?

Contralateral to the lesion, because spinothalamic fibers cross near their spinal entry level.

What role does the periaqueductal gray (PAG) play in pain?

It participates in pain perception/modulation; descending PAG output can reduce or increase pain and involves opioid receptors.

What is the gate control theory of pain?

Activity in touch-related A-beta fibers and descending cognitive/emotional influences can inhibit pain transmission in the spinal cord.

Why can rubbing your skin after stubbing a toe reduce pain?

Touch activates A-beta fibers that help close the spinal pain gate.

What is chronic pain?

Pain that persists after an injury has healed, with pain signaling remaining active in the CNS for months to years.

How can chronic neurogenic pain alter the spinal cord?

Substance P-related remodeling and hyperexcitability of dorsal horn neurons can make light touch painful; inhibitory signaling can also become abnormal.

Which cortical region is associated with the unpleasant emotional component of pain?

The anterior cingulate cortex (ACC); its activity increases as pain unpleasantness increases.

What is a movement?

A single relocation of a body part, usually produced by a brief muscle contraction.

What is a reflex?

A simple, highly stereotyped, unlearned response to a particular stimulus.

What is an act in motor control?

A complex, sequential behavior.

What is a motor plan?

A set of muscle commands established before an action occurs.

What is the role of primary motor cortex (M1)?

It initiates voluntary action.

What is the role of non-primary motor cortex?

It sends motor commands involved in planning and organizing movement.

What role does prefrontal cortex have in movement?

It contributes to conscious behavioral plans.

What do the basal ganglia and cerebellum do in the motor system?

They modulate actions rather than directly innervating skeletal muscle.

What is the motor role of the brainstem?

It integrates motor commands from higher levels and sends signals to the spinal cord.

What are antagonistic muscles?

Muscles with opposing actions; for example, biceps flex while triceps extend.

What are synergistic muscles?

Muscles that act together to produce a movement.

What is tremor?

Alternating contraction of flexor and extensor muscles; normally small but debilitating when poorly regulated.

What are the thick and thin filaments of skeletal muscle?

Thick = myosin; thin = actin.

What happens to muscle-fiber length during contraction?

The fiber shortens as actin and myosin filaments slide relative to each other.

Compare slow-twitch and fast-twitch muscle fibers.

Slow-twitch contract slowly, resist fatigue, and are recruited first; fast-twitch contract rapidly, fatigue easily, and are recruited later.

What is a motor unit?

One motor neuron and all the muscle fibers it innervates.

What is the neuromuscular junction (NMJ)?

The synapse where a motor-neuron terminal communicates with a skeletal muscle fiber.

Which neurotransmitter is released at the skeletal NMJ?

Acetylcholine (ACh).

Which ACh receptor is found on skeletal muscle at the NMJ?

The nicotinic ACh receptor, an ionotropic excitatory receptor.

Compare nicotinic and muscarinic ACh receptors as taught in the lecture.

Nicotinic receptors are mostly ionotropic, excitatory, and peripheral; muscarinic receptors are mostly metabotropic and can be excitatory or inhibitory.

Why do muscles controlling fine movements have small motor units?

Each motor neuron innervates only a few fibers, allowing precise control.

What is proprioception?

The sense of body position and movement.

What do muscle spindles detect?

Muscle stretch and muscle length.

What do Golgi tendon organs detect?

Muscle tension in tendons.

What is the protective role of Golgi tendon organs?

Excessive tension can trigger reflexive responses that reduce force and help prevent muscle/tendon damage.

Summarize the stretch reflex.

Muscle stretch activates spindle afferents → excites motor neurons to the agonist → agonist contracts to oppose stretch while the antagonist is inhibited.

What is the two-neuron chain of the pyramidal motor pathway?

Upper motor neuron from M1 crosses in the medulla and descends to an anterior horn motor neuron; the lower motor neuron then projects to muscle.

Where do pyramidal/corticospinal motor fibers decussate according to the lecture?

In the medulla.

What is the supplementary motor area (SMA) important for?

Initiation and encoding of preplanned movement sequences.

What is the premotor cortex important for?

Planning motor sequences guided by external events; neurons fire just before an activity.

What are mirror neurons?

Premotor neurons that fire both when performing an action and when observing another individual perform that action.

What are the main basal ganglia nuclei listed in the lecture?

Caudate nucleus, putamen, and globus pallidus, with inputs from substantia nigra and subthalamic nucleus.

What motor functions are associated with the basal ganglia?

They help control movement amplitude and direction and are important for movement initiation and memory-guided movements.

What type of output do cerebellar Purkinje cells send?

Inhibitory output.

What is the major motor role of the cerebellum?

Coordination and fine-tuning of skilled movement, including feedback-based correction.

Why are postural adjustments often activated before a voluntary limb movement?

The extrapyramidal system predicts the postural consequences of the planned movement and acts to preserve balance.

What is a central pattern generator?

A spinal neural circuit that can generate rhythmic behaviors such as walking without requiring continuous sensory feedback or descending cortical input.

In the movement-control cascade, what asks "Should I move?"

The supplementary motor area.

In the movement-control cascade, what selects the motor plan?

The premotor cortex.

In the movement-control cascade, what provides feedback and correction?

The cerebellum.

What is the main consequence of pyramidal-system damage?

Weakness or paralysis of voluntary movement.

What causes myasthenia gravis in this lecture?

Autoantibodies against the patient's own ACh receptors at the NMJ, causing fatigable skeletal-muscle weakness.

What is ALS and what early sign is emphasized?

Degeneration of motor neurons with loss of target muscles; an early sign can be fasciculations (brief spontaneous muscle twitches).

What does cerebellar damage commonly cause?

Ataxia and impaired motor coordination; vermis damage can disturb balance.

Contrast Parkinson disease and Huntington disease in the lecture.

Parkinson: loss of dopaminergic substantia nigra neurons projecting to basal ganglia; Huntington: progressive basal ganglia damage, especially caudate and putamen, producing chorea.

What is sound physically?

Vibrations of air molecules that propagate as waves.

What does Fourier transformation do to a complex sound?

Breaks it down into its component simple sine waves.

What is the approximate human hearing range?

20 Hz to 20,000 Hz.

At what frequencies is human hearing most sensitive according to the lecture?

About 1,000-4,000 Hz.

Distinguish sensation from perception in hearing.

Sensation is acquisition of sound information; perception is interpretation of that information.

What structures make up the external ear in this lecture?

The pinna and ear/auditory canal.

What is the role of the pinna?

It collects sound waves and its shape modifies the sound frequencies reaching the middle ear.

What is the main function of the middle ear?

To concentrate sound energy and transmit vibration from the tympanic membrane to the oval window.

Name the three ossicles in order.

Malleus, incus, stapes.

What does the stapes contact?

The oval window, which leads into the cochlea.