Exercise 16: Human Reflex Physiology

The Reflex Arc

• 1. Reflex—rapid, predictable, and involuntary response to a stimulus

2. 5 Essential Components of the Reflex Arc

Figure 7.11a

Stimulus at distalend of neuron

Skin Spinal cord(in cross section)

Interneuron

Receptor

Effector

Sensory neuron

Motor neuron

Integrationcenter

(a)

The Reflex Arc

Figure 7.11a, step 1

Stimulus at distalend of neuron

Skin

Receptor

(a)

The Reflex Arc

Figure 7.11a, step 2

Stimulus at distalend of neuron

Skin

ReceptorSensory neuron

(a)

The Reflex Arc

Figure 7.11a, step 3

Stimulus at distalend of neuron

Skin Spinal cord(in cross section)

Interneuron

ReceptorSensory neuron

Integrationcenter

(a)

The Reflex Arc

Figure 7.11a, step 4

Stimulus at distalend of neuron

Skin Spinal cord(in cross section)

Interneuron

ReceptorSensory neuron

Motor neuron

Integrationcenter

(a)

The Reflex Arc

Figure 7.11a, step 5

Stimulus at distalend of neuron

Skin Spinal cord(in cross section)

Interneuron

Receptor

Effector

Sensory neuron

Motor neuron

Integrationcenter

(a)

Simple Reflex Arc

Figure 7.11b–c

Spinal cord

Sensory (afferent)neuron

Inter-neuron

Motor(efferent)neuron

Motor(efferent)neuron

Sensory receptors(stretch receptorsin the quadricepsmuscle)

Sensory (afferent)neuron

Sensory receptors(pain receptors inthe skin)

Effector(quadricepsmuscle ofthigh)

Effector(bicepsbrachiimuscle)

Synapse inventral horngray matter

(c)

(b)

3. Why test your reflexes?

•Reflexes determine the general health of the motor portion of the nervous system.

•Whenever reflexes are exaggerated, distorted, or absent, nervous system disorders are indicated.

•Reflex changes often occur before the pathological condition has become obvious

Simple Reflex Arc

Figure 7.11b, step 1

Spinal cord

Sensory receptors(stretch receptorsin the quadricepsmuscle)

(b)

Simple Reflex Arc

Figure 7.11b, step 2

Spinal cord

Sensory (afferent)neuron

Sensory receptors(stretch receptorsin the quadricepsmuscle)

(b)

Simple Reflex Arc

Figure 7.11b, step 3

Spinal cord

Sensory (afferent)neuron

Sensory receptors(stretch receptorsin the quadricepsmuscle)

Synapse inventral horngray matter

(b)

Simple Reflex Arc

Figure 7.11b, step 4

Spinal cord

Sensory (afferent)neuron

Motor(efferent)neuron

Sensory receptors(stretch receptorsin the quadricepsmuscle)

Synapse inventral horngray matter

(b)

Simple Reflex Arc

Figure 7.11b, step 5

Spinal cord

Sensory (afferent)neuron

Motor(efferent)neuron

Sensory receptors(stretch receptorsin the quadricepsmuscle)

Effector(quadricepsmuscle ofthigh)

Synapse inventral horngray matter

(b)

Simple Reflex Arc

Figure 7.11c, step 1

Spinal cord

Sensory receptors(pain receptors inthe skin)

(c)

Simple Reflex Arc

Figure 7.11c, step 2

Spinal cord

Sensory (afferent)neuron

Sensory receptors(pain receptors inthe skin)

(c)

Simple Reflex Arc

Figure 7.11c, step 3

Spinal cord

Inter-neuron

Sensory (afferent)neuron

Sensory receptors(pain receptors inthe skin)

(c)

Simple Reflex Arc

Figure 7.11c, step 4a

Spinal cord

Inter-neuron

Motor(efferent)neuron

Sensory (afferent)neuron

Sensory receptors(pain receptors inthe skin)

(c)

Simple Reflex Arc

Figure 7.11c, step 4b

Spinal cord

Inter-neuron

Motor(efferent)neuron

Sensory (afferent)neuron

Sensory receptors(pain receptors inthe skin)

Effector(bicepsbrachiimuscle)

(c)

Simple Reflex Arc

Figure 7.11b–c

Spinal cord

Sensory (afferent)neuron

Inter-neuron

Motor(efferent)neuron

Motor(efferent)neuron

Sensory receptors(stretch receptorsin the quadricepsmuscle)

Sensory (afferent)neuron

Sensory receptors(pain receptors inthe skin)

Effector(quadricepsmuscle ofthigh)

Effector(bicepsbrachiimuscle)

Synapse inventral horngray matter

(c)

(b)

Types of Reflexes and Regulation

• Somatic reflexes– Activation of skeletal muscles – Example: When you move your hand away from

a hot stove– A “stretch reflex” is a muscle contraction in

response to stretching within the muscle– “Superficial reflexes” are motor responses to

scraping of the skin.

4. Somatic Reflexes (Activation of skeletal muscles)

Achilles Reflex-Simple stretch reflexCord-mediated

Corneal Reflex

Patellar ReflexSimple stretch reflexCord-mediated

Plantar (Babinski) ReflexSuperficial cord reflex

5. Cord- Mediated Reflexes

• Muscle stretch (myotatic) reflexes are mediated purely at the spinal cord level, with monosynaptic connections between muscle spindle afferents and motor neurons to the muscle that was stretched.

• Examples: Achilles and plantar

5. Somatic reflex involving higher brain centers

• Example: pupillary light reflex

Patellar ReflexThe patellar reflex is an example of the monosynaptic reflex arc. There is no interneuron in the pathway.

Striking the patellar tendon with a tendon hammer just below the patella stretches the quadriceps muscles in the thigh. This stimulates stretch sensory receptors to trigger an afferent impulse in a sensory nerve fiber of the femoral nerve which synapses at the level of L4 in the spinal

From there, an motor neuron conducts an efferent impulse back to the quadriceps femoris muscle, triggering contraction. This contraction causes the leg to kick.

This reflex helps maintain posture and balance, allowing one to walk without consciously thinking about each step.

Superficial reflexes

• Superficial reflexes are motor responses to scraping of the skin.

• Plantar Reflex is a normal reflex that involves plantar flexion of the foot (toes move away from the shin, and curl down.)

• Babinski's reflex occurs when the big toe moves toward the top of the foot and the other toes fan out after the sole of the foot has been firmly stroked.

• This reflex, or sign, is normal in younger children, but abnormal after the age of 2.

Plantar Reflex

• Plantar Reflex: Flexion of the toes in response to stroking of the outer surface of the sole, from heel to little toe.

Plantar Reflex

Corneal Reflex

• Cranial Nerve V “trigeminal nerve”• Protect the eyeball from damage

Autonomic reflexes

• Autonomic reflexes– Smooth muscle regulation– Heart and blood pressure regulation– Regulation of glands– Digestive system regulationAn autonomic reflex is one that involves the response of an organ, such as the peristaltic contraction of the smooth muscle of the intestines, that is not controlled consciously

5. Example of Autonomic Reflex:Pupillary Light Reflex

• Receptor = retina of the eye• Afferent Fibers = in Cranial Nerve II,

“optic nerve”• Efferent impulses = carried by cranial nerve III

“ oculomoter nerve”• Effector = smooth muscles of the iris• Function =protects the retina from excessive

illumination, which is damaging to the photoreceptors

Pupillary Light Reflex

Autonomic Reflexes• May be spinal (e.g.,

urination and defecation) or modified by higher brain structures.

• The thalamus, hypothalamus and brain stem are in charge of multiple reflexes – HR, BP, breathing, eating, osmotic balance, temperature, vomiting, gagging, sneezing.

• All are polysynaptic.

9. Pupillary Response

• Ipsilateral: A reflex in which the response occurs on the side of the body that is stimulated.

• Contralateral: A reflex in which the response occurs on the opposite side of the body that is stimulated.

9. Test For Cranial Nerves II and III

• Normally, both pupils should constrict with light shone into either eye alone. If not working:

• Optic nerve damage?

• Oculomotor nerve damage?

10. Somatic vs Autonomic Reflexes

• Somatic Reflexes are all reflexes that stimulate skeletal muscles.

• Autonomic Reflexes stimulate smooth muscles, cardiac muscle, and glands of the body, and they regulate body functions such as digestion and blood pressure.

Types of Reflexes and Regulation

• Patellar, or knee-jerk, reflex is an example of a two-neuron reflex arc

Figure 7.11d

6. Reflex Arc of Patellar ReflexReceptors Afferent

NervesIntegration Center

Efferent Nerve Effector

Stretch receptors in quadriceps muscle

Afferent neuron

Spinal Cord Efferent neuron (Femoral Nerve)

Quadriceps muscle of the thigh

6. Reflex Arc of Achilles ReflexReceptors Afferent

NervesIntegration Center

Efferent Nerve Effector

Stretch receptors in Achilles Tendon

Afferent neuron

Spinal Cord Efferent neuron (Tibial Nerve, S1-S2)

Gastrocnemius muscle

7. Effect of Muscle Fatigue on Patellar Reflex

• The intensity of the response is less

Differences Between Somatic and Autonomic Nervous Systems

• Nerves– Somatic: one motor neuron– Autonomic: preganglionic and postganglionic

nerves• Effector organs– Somatic: skeletal muscle– Autonomic: smooth muscle, cardiac muscle, and

glands

PNS: Differences Between Somatic and Autonomic Nervous Systems

• Neurotransmitters– Somatic: always use acetylcholine– Autonomic: use acetylcholine, epinephrine, or

norepinephrine

Autonomic Nervous System

• Motor subdivision of the PNS– Consists only of motor nerves

• Also known as the involuntary nervous system– Regulates activities of cardiac and smooth muscles

and glands• Two subdivisions– Sympathetic division– Parasympathetic division

PNS: Comparison of Somatic and Autonomic Nervous Systems

Figure 7.27

Anatomy of the Sympathetic Division

• Originates from T1 through L2

• Ganglia are at the sympathetic trunk (near the spinal cord)

• Short pre-ganglionic neuron and long post-ganglionic neuron transmit impulse from CNS to the effector

• Norepinephrine and epinephrine are neurotransmitters to the effector organs

Figure 7.28

PNS: Anatomy of the Autonomic Nervous System

PNS: Sympathetic Pathways

Figure 7.29

Anatomy of the Parasympathetic Division

• Originates from the brain stem and S1 through S4

• Terminal ganglia are at the effector organs• Always uses acetylcholine as a

neurotransmitter

Autonomic Functioning

• Sympathetic—“fight or flight”– Response to unusual stimulus– Takes over to increase activities– Remember as the “E” division • Exercise, excitement, emergency, and embarrassment

Autonomic Functioning

• Parasympathetic—“housekeeping” activites– Conserves energy– Maintains daily necessary body functions– Remember as the “D” division• digestion, defecation, and diuresis

Table 7.3 (1 of 2)

Effects of the Sympathetic and Parasympathetic Divisions of the ANS

Effects of the Sympathetic and Parasympathetic Divisions of the ANS

Table 7.3 (2 of 2)