Laboratory 7.2 Reflexes

Reflex — definition

Instantaneous, automatic, and involuntary motor response within the nervous system

Somatic reflexes

Reflexes that exert control over skeletal muscles

Autonomic reflexes (visceral reflexes)

Reflexes whose effectors are smooth muscle and glands; control processes such as heart rate, blood pressure, digestion, salivation, pupillary dilation, and micturition

Reflex arc

The neural pathway that a nerve impulse travels during a reflex; passes through the CNS, allowing clinicians to test nervous system function

5 components of a reflex arc

Receptor → sensory neuron → integration center (CNS) → motor neuron → effector

Dorsal root ganglion (DRG)

Contains the cell bodies of sensory neurons; sensory information enters the CNS via the dorsal root

Dorsal horn

Region of spinal cord gray matter where sensory neurons synapse on interneurons

Anterior horn

Region of spinal cord gray matter where motor neuron cell bodies are located

Ventral root

Motor information exits the spinal cord through this root to reach the effector

Role of interneurons in a reflex

Located only in the CNS; relay sensation to the brain via axon collaterals, but the brain does not initiate the motor response — the reflex arc acts before brain processing

Monosynaptic reflex arc

Involves only 2 neurons (sensory + motor); no interneuron; allows very rapid response due to direct synapse and large myelinated fibers; example: patellar (stretch) reflex

Polysynaptic reflex arc

Involves 3+ neurons (sensory + interneuron + motor); allows more complex processing, coordination of multiple muscle groups, and integration with higher brain centers; slower than monosynaptic

Stretch reflex

Monosynaptic spinal reflex; muscle spindle proprioceptors detect stretch → sensory neuron synapses directly on motor neuron → muscle contracts; example: patellar (knee-jerk) reflex

Tendon reflex

Polysynaptic reflex that prevents damage from excessive muscle tension; Golgi tendon organs detect overstretched collagen fibers → inhibitory interneurons → decreased muscle contraction

Withdrawal (flexor) reflex

Polysynaptic reflex that retracts a limb from a painful stimulus; flexor muscles contract and extensor muscles are simultaneously inhibited (reciprocal inhibition)

Reciprocal inhibition

The simultaneous inhibition of antagonist muscles by interneurons during a withdrawal reflex, allowing smooth flexor movement

Crossed-extensor reflex

Polysynaptic contralateral reflex that straightens the opposite limb to support body weight just before the stimulated limb is withdrawn; works alongside the flexor reflex

Ipsilateral reflex arc

Reflex arc that activates muscles on the same side of the body as the stimulus; examples: stretch, tendon, and withdrawal reflexes

Contralateral reflex arc

Reflex arc that activates muscles on the opposite side of the body from the stimulus; example: crossed-extensor reflex

Plantar reflex (adult)

Stroking the sole of the foot causes the toes to curl downward; indicates normal spinal nerve and corticospinal tract function

Babinski reflex

Stroking the sole of the foot causes upward fanning of the toes; normal in infants under 2 (immature neural pathways); pathological in adults — indicates corticospinal tract abnormality (e.g., brain/spinal cord injury, MS, ALS)

Jendrassik maneuver

Technique to reinforce (amplify) the patellar reflex; subject interlocks flexed fingers and pulls apart strongly while clenching teeth, redirecting descending inhibition away from the lower limb

Effect of muscle fatigue on the patellar reflex

Fatigued muscle spindles and motor units produce a reduced or absent reflex response

Innate reflexes

Genetically hardwired, involve fewer neurons/synapses, processed in spinal cord, very fast; example: knee-jerk, eye blink

Acquired reflexes

Learned through experience, involve more complex brain processing and more synapses, slower than innate reflexes; example: braking for a red light, musician's hand movements

Factors affecting reaction time

Age (slows with age), sensitivity of sensory receptors, velocity of nerve conduction, complexity of neural pathway involved

Autonomic reflex pathway (neuron relay)

Uses a 2-neuron relay: myelinated preganglionic neuron synapses in a ganglion on an unmyelinated postganglionic neuron, which then innervates the smooth muscle or gland effector

Somatic motor pathway vs. autonomic motor pathway

Somatic: 1 highly myelinated motor neuron → skeletal muscle; Autonomic: 2-neuron relay (myelinated preganglionic + unmyelinated postganglionic) → smooth muscle or gland

Postural heart rate change — mechanism

Standing causes blood to pool in the lower body; baroreceptors detect the drop in blood pressure → ANS (sympathetic division) increases heart rate to restore perfusion; after 2 minutes, parasympathetic activity returns heart rate toward baseline

Baroreceptors

Interoceptors that detect changes in blood pressure; trigger autonomic cardiovascular reflexes

Pupillary light reflex (pupillary reflex)

Involuntary autonomic reflex that constricts the pupil (miosis) in response to light; protects the retina from excess brightness and aids vision adaptation

Pupillary reflex — afferent and efferent nerves

Afferent: optic nerve (CN II) sends signal to the midbrain pretectal area; Efferent: oculomotor nerve (CN III) innervates the iris sphincter, causing pupil constriction

Consensual reflex (pupil)

When light is shone in one eye, both pupils constrict; the contralateral response occurs because the pretectal midbrain signal is sent to both sides of the brain; clinically tests integrity of CN II and CN III

Miosis

Pupil constriction; produced by parasympathetic stimulation via CN III

Mydriasis

Pupil dilation; produced by sympathetic stimulation (e.g., ciliospinal reflex, sympathomimetic drugs)

ANS division responsible for pupil constriction

Parasympathetic nervous system (via oculomotor nerve, CN III)

Salivary reflex — pathway

Taste/smell/thought of food → chemoreceptors in oral mucosa detect stimulus → afferent signal via CN VII (anterior 2/3 tongue) and CN IX (posterior 1/3) → salivary nuclei in brainstem → efferent via CN VII and CN IX → salivary glands produce saliva

Why saliva increases in acidic conditions

Acidic substances lower oral pH → chemoreceptors trigger stronger parasympathetic activation → increased salivary output; saliva is alkaline and neutralizes acid, protecting the oral mucosa

ANS branch controlling salivation

Parasympathetic nervous system; neurotransmitter is acetylcholine (ACh) binding muscarinic receptors on salivary glands

Drug causing dry mouth (xerostomia)

Parasympatholytic (anticholinergic) drug — blocks muscarinic ACh receptors, inhibiting salivary gland secretion

Ciliospinal reflex

Autonomic sympathetic reflex causing pupil dilation (mydriasis) in response to a startling or painful stimulus to the skin of the neck, upper chest, or face; afferent via trigeminal nerve → ciliospinal center → sympathetic efferent via superior cervical ganglion → iris dilator muscle

Neurotransmitter causing iris dilation in ciliospinal reflex

Norepinephrine (NE) released by sympathetic postganglionic fibers; binds adrenergic receptors on the iris dilator muscle

Drug to dilate pupil for ophthalmologic exam

Sympathomimetic or parasympatholytic (anticholinergic) drug — either mimics sympathetic dilation or blocks parasympathetic constriction

Drug causing excessive miosis (pupil constriction)

Parasympathomimetic drug — mimics parasympathetic activity, causing excessive iris sphincter contraction