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
Skin
Stimulus at distal
end of neuron
Spinal cord
(in cross section)
Sensory neuron
Receptor
Motor neuron
(a)
Effector
Integration
center
Interneuron
Figure 7.11a
The Reflex Arc
Skin
Stimulus at distal
end of neuron
Receptor
(a)
Figure 7.11a, step 1
The Reflex Arc
Skin
Stimulus at distal
end of neuron
Sensory neuron
Receptor
(a)
Figure 7.11a, step 2
The Reflex Arc
Skin
Stimulus at distal
end of neuron
Receptor
(a)
Spinal cord
(in cross section)
Sensory neuron
Integration
center
Interneuron
Figure 7.11a, step 3
The Reflex Arc
Skin
Stimulus at distal
end of neuron
Spinal cord
(in cross section)
Sensory neuron
Receptor
Motor neuron
(a)
Integration
center
Interneuron
Figure 7.11a, step 4
The Reflex Arc
Skin
Stimulus at distal
end of neuron
Spinal cord
(in cross section)
Sensory neuron
Receptor
Motor neuron
(a)
Effector
Integration
center
Interneuron
Figure 7.11a, step 5
Simple Reflex Arc
Sensory receptors
(stretch receptors
in the quadriceps
muscle)
Sensory (afferent)
neuron
Spinal cord
Sensory receptors
(pain receptors in
the skin)
Sensory (afferent)
neuron
Synapse in
ventral horn
gray matter
Interneuron
Motor
(efferent)
neuron
Motor
(efferent)
neuron
(b)
Effector
(quadriceps
muscle of
thigh)
Effector
(biceps
brachii
muscle)
(c)
Figure 7.11b–c
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
Sensory receptors
(stretch receptors
in the quadriceps
muscle)
Spinal cord
(b)
Figure 7.11b, step 1
Simple Reflex Arc
Sensory receptors
(stretch receptors
in the quadriceps
muscle)
Sensory (afferent)
neuron
Spinal cord
(b)
Figure 7.11b, step 2
Simple Reflex Arc
Sensory receptors
(stretch receptors
in the quadriceps
muscle)
Sensory (afferent)
neuron
Spinal cord
Synapse in
ventral horn
gray matter
(b)
Figure 7.11b, step 3
Simple Reflex Arc
Sensory receptors
(stretch receptors
in the quadriceps
muscle)
Sensory (afferent)
neuron
Spinal cord
Synapse in
ventral horn
gray matter
Motor
(efferent)
neuron
(b)
Figure 7.11b, step 4
Simple Reflex Arc
Sensory receptors
(stretch receptors
in the quadriceps
muscle)
Sensory (afferent)
neuron
Spinal cord
Synapse in
ventral horn
gray matter
(b)
Motor
(efferent)
neuron
Effector
(quadriceps
muscle of
thigh)
Figure 7.11b, step 5
Simple Reflex Arc
Sensory receptors
(pain receptors in
the skin)
Spinal cord
(c)
Figure 7.11c, step 1
Simple Reflex Arc
Sensory receptors
(pain receptors in
the skin)
Spinal cord
Sensory (afferent)
neuron
(c)
Figure 7.11c, step 2
Simple Reflex Arc
Sensory receptors
(pain receptors in
the skin)
Spinal cord
Sensory (afferent)
neuron
Interneuron
(c)
Figure 7.11c, step 3
Simple Reflex Arc
Sensory receptors
(pain receptors in
the skin)
Spinal cord
Sensory (afferent)
neuron
Interneuron
Motor
(efferent)
neuron
(c)
Figure 7.11c, step 4a
Simple Reflex Arc
Sensory receptors
(pain receptors in
the skin)
Spinal cord
Sensory (afferent)
neuron
Interneuron
Motor
(efferent)
neuron
Effector
(biceps
brachii
muscle)
(c)
Figure 7.11c, step 4b
Simple Reflex Arc
Sensory receptors
(stretch receptors
in the quadriceps
muscle)
Sensory (afferent)
neuron
Spinal cord
Sensory receptors
(pain receptors in
the skin)
Sensory (afferent)
neuron
Synapse in
ventral horn
gray matter
Interneuron
Motor
(efferent)
neuron
Motor
(efferent)
neuron
(b)
Effector
(quadriceps
muscle of
thigh)
Effector
(biceps
brachii
muscle)
(c)
Figure 7.11b–c
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 ReflexSimple stretch reflex
Cord-mediated
Corneal Reflex
Patellar Reflex
Simple stretch reflex
Cord-mediated
Plantar (Babinski) Reflex
Superficial 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 Reflex
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.
The patellar reflex is an example of the
monosynaptic reflex arc. There is no
interneuron in the pathway.
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 regulation
An 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 Reflex
Receptors
Afferent
Nerves
Integration
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 Reflex
Receptors
Afferent
Nerves
Integration
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 postganglionic neuron transmit impulse from CNS
to the effector
• Norepinephrine and epinephrine are
neurotransmitters to the effector organs
PNS: Anatomy of the
Autonomic Nervous System
Figure 7.28
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
Effects of the Sympathetic and
Parasympathetic Divisions of the ANS
Table 7.3 (1 of 2)
Effects of the Sympathetic and
Parasympathetic Divisions of the ANS
Table 7.3 (2 of 2)