Chapter 3: Basic Reflexes & CPGs · Sensory Receptors

Voluntary motor function

Motor activity controlled cortically (from the brain); purposeful movement.

Involuntary motor function

•Reflexive motor activity, automatic responses to stimuli.

Central Pattern Generators (CPGs):

• Combinations of reflexes that generate complex rhythmic activities (e.g., swallowing, walking).

3 Features

• Autonomous rhythm — it can keep generating its pattern even without sensory feedback or commands from higher brain centers.

• Lives in the spinal cord and brainstem — where repetitive motor behaviors are coordinated.

• Modulated by input — it runs on its own but is shaped by sensory feedback and higher-level commands to adapt to the situation.

Alpha Lower Motor Neurons (α LMN)

• Action: contracts the big extrafusal muscle

• Cell body in the spinal cord; axons exit into the PNS to innervate skeletal muscles; activate extrafusal fibers for movement.

Gamma Lower Motor Neurons (γ LMN)

• Gauge: tunes the spindle’s sensitivity so it can still sense stretch

• Cell body in the spinal cord; regulate the sensitivity of muscle spindles by contracting intrafusal fibers to keep them taut during movement; help modulate tone and reflexes.

Spinal Reflex Arc – Components

•Receptor organ: Detects change in environment (e.g., stretch).

•Afferent neuron: Carries sensory information to the spinal cord

•Interneuron(s): Relay and integrate signals within the spinal cord

•Efferent neuron: Sends motor command to muscle or gland.

•Effector: Muscle fiber or gland that carries out the response.

Spinal Reflex Arc – Receptor organ

•Detects change in environment (e.g., stretch).

Spinal Reflex Arc – Afferent neuron

•Carries sensory information to the spinal cord

Spinal Reflex Arc – Interneuron(s)

•Relay and integrate signals within the spinal cord

Spinal Reflex Arc – Efferent neuron

•Sends motor command to muscle or gland.

Spinal Reflex Arc – Effector

•Muscle fiber or gland that carries out the response.

Intrafusal muscle fibers

Specialized fibers inside the muscle spindle that sense stretch and muscle length.

Extrafusal muscle fibers

•Main contractile fibers of muscle; responsible for movement.

Muscle Spindle

• Stretch receptor that detects passive stretch and triggers contraction to restore muscle length

• Maintains posture.

• Senses LENGTH/STRETCH (and triggers contraction to restore length)

Golgi Tendon Organ (GTO)

• Sensor located in tendons that detects muscle tension

• Prevents excessive contraction and fine-tunes movement by balancing agonist and antagonist activity.

• Senses TENSION (and protects against over-contraction)

Dorsal root fibers

•Afferent fibers carrying sensory input into the spinal cord.

Dorsal root ganglion

Cluster of sensory neuron cell bodies outside the spinal cord.

Alpha motor neuron

Efferent neuron that activates extrafusal muscle fibers to contract.

Ia afferent fibers (pseudo-unipolar neurons)

• Sensory neurons from muscle spindles with one process to the muscle and another to the spinal cord.

• Its cell body sits in the dorsal root ganglion outside the cord, with one long process to the muscle spindle and one short process into the spinal cord.

Knee-jerk reflex (patellar reflex)

•Stretch of the patellar tendon activates muscle spindle → afferent signal to spinal cord → α motor neuron fires → quadriceps contract, restoring length.

• A tap on the patellar tendon stretches the quadriceps.

• The stretch activates muscle spindles (the receptor).

• The afferent neuron carries the signal into the spinal cord.

• It synapses (via the cord) onto the α motor neuron.

• The α motor neuron fires the quadriceps, which contracts — the lower leg extends — until the spindle is no longer stretched.

Reciprocal inhibition (via GTO)

•Agonist contraction triggers inhibition of antagonist to allow smooth and controlled movement.

Reflex vs. CPG

A reflex is a simple response to a stimulus; CPGs combine reflexes into complex rhythmic activities (e.g., walking, swallowing).

2 SLP relevant examples of CPGs

• Spinal locomotor CPG that drives the alternating flexor/extensor activity of walking

• Brainstem CPGs for breathing, and for swallowing/chewing

  • CPGs in the medulla that coordinate sucking, swallowing, and breathing mature at different rates and must work together for safe feeding. Preterm birth can disrupt this coordination

•Transduction:

The process of changing a signal from one form of energy to another (e.g., acoustic energy → electrochemical energy in hair cells).

•Threshold of stimulation:

The point at which a sensory receptor is depolarized and fires.

•Adequate stimulus:

A stimulus strong enough to depolarize a sensor.

•Somatosensors:

Receptors for body sensations (e.g., touch, thermal).

•Special sensors:

Receptors for vision, taste, hearing, and olfaction.

•Exteroceptors:

Detect external stimuli (e.g., pressure from contact).

•Interoceptors:

Detect internal body states (e.g., CO₂ in blood).

•Mechanoreceptors:

Sense mechanical deformation (e.g., pressure on skin).

•Thermoreceptors:

Sense changes in temperature (heat, cold).

•Chemoreceptors:

Detect chemical molecules (e.g., food, smell).

•Encapsulated receptors:

Surrounded by a capsule (e.g., Meissner’s corpuscles, Merkel disks).

•Unencapsulated receptors:

No capsule (e.g., Pacinian corpuscles, Ruffini endings).

•Meissner’s corpuscles:

Superficial, sensitive to force and vibration (~100 Hz), rapidly adapting.

•Ruffini endings:

Deep, sensitive to skin stretch.

•Merkel disk receptors:

Superficial, sensitive to pressure.

•Pacinian corpuscles:

Deep, detect pressure and high-frequency vibration (~400 Hz), rapidly adapting.

•Nociception:

Pain perception; mediated by free nerve endings with high thresholds.

•Silent nociceptors:

Pain receptors linked to homeostasis and illness.

•Thermal sense:

Warm, hot, cool, and cold perception. Pain is sensed at temperature extremes.

•Proprioception:

Subconscious awareness of body position and movement.

Joint sense:

Perception of joint position, orientation, and movement, including ligament stretch.

•Muscle spindle:

Sensor detecting changes in muscle length.

•Golgi Tendon Organ (GTO):

Sensor detecting tension in tendons.

•Dermatome:

An area of skin served by a single spinal nerve

•Afferent fibers:

Carry sensory information to the spinal cord or brainstem

•Efferent fibers:

Carry motor signals from the spinal cord to muscles

•Visual sensation:

Light detection by rods and cones in the retina.

Olfaction:

Sense of smell mediated by the olfactory nerve (CN I); chemoreceptors in nasal cavity.

•Gustation:

Sense of taste; chemoreceptors in taste buds.

•Audition:

Hearing via inner and outer hair cells in the cochlea.

•Vestibular sense:

Balance and head movement detection via semicircular canals, utricle, and saccule.

•Papillae:

Structures containing taste buds.

Filiform:

Most common; mechanoreceptors only (not taste).

Fungiform:

On tip and sides of tongue

Foliate:

On posterior tongue and fauces

•Vallate (circumvallate):

Posterior tongue dorsum in a V-shape.

•Basic tastes:

Sweet, salty, sour, bitter, umami.

•Hair cells:

Sensory cells in the cochlea detect vibration

•Stereocilia:

Rigid cilia on hair cells; deflection leads to depolarization and neurotransmitter release.

•Crista ampullaris:

Sensory organ in semicircular canals detecting head movement.

•Macula (utricle and saccule):

Detects acceleration; contains hair cells in otolithic membrane.