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 Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Effector Sensory neuron Motor neuron Integration center (a) Figure 7.11a

The Reflex Arc Figure 7.11a, step 1 Skin Stimulus at distal end of neuron Skin Receptor (a) Figure 7.11a, step 1

The Reflex Arc Figure 7.11a, step 2 Skin Stimulus at distal end of neuron Skin Receptor Sensory neuron (a) Figure 7.11a, step 2

The Reflex Arc Figure 7.11a, step 3 Skin Spinal cord Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Sensory neuron Integration center (a) Figure 7.11a, step 3

The Reflex Arc Figure 7.11a, step 4 Skin Spinal cord Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Sensory neuron Motor neuron Integration center (a) Figure 7.11a, step 4

The Reflex Arc Figure 7.11a, step 5 Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Effector Sensory neuron Motor neuron Integration center (a) Figure 7.11a, step 5

Simple Reflex Arc Figure 7.11b–c Sensory receptors (stretch receptors in the quadriceps muscle) Sensory (afferent) neuron Sensory receptors (pain receptors in the skin) Spinal cord Sensory (afferent) neuron Synapse in ventral horn gray matter Inter- neuron Motor (efferent) neuron Motor (efferent) neuron Effector (quadriceps muscle of thigh) Effector (biceps brachii muscle) (b) (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 Figure 7.11b, step 1 Sensory receptors (stretch receptors in the quadriceps muscle) Spinal cord (b) Figure 7.11b, step 1

Simple Reflex Arc Figure 7.11b, step 2 Sensory receptors (stretch receptors in the quadriceps muscle) Sensory (afferent) neuron Spinal cord (b) Figure 7.11b, step 2

Simple Reflex Arc Figure 7.11b, step 3 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 Figure 7.11b, step 4 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 Figure 7.11b, step 5 Sensory receptors (stretch receptors in the quadriceps muscle) Sensory (afferent) neuron Spinal cord Synapse in ventral horn gray matter Motor (efferent) neuron Effector (quadriceps muscle of thigh) (b) Figure 7.11b, step 5

Simple Reflex Arc Figure 7.11c, step 1 Sensory receptors (pain receptors in the skin) Spinal cord (c) Figure 7.11c, step 1

Simple Reflex Arc Figure 7.11c, step 2 Sensory receptors (pain receptors in the skin) Spinal cord Sensory (afferent) neuron (c) Figure 7.11c, step 2

Simple Reflex Arc Figure 7.11c, step 3 Sensory receptors (pain receptors in the skin) Spinal cord Sensory (afferent) neuron Inter- neuron (c) Figure 7.11c, step 3

Simple Reflex Arc Figure 7.11c, step 4a Sensory receptors (pain receptors in the skin) Spinal cord Sensory (afferent) neuron Inter- neuron Motor (efferent) neuron (c) Figure 7.11c, step 4a

Simple Reflex Arc Figure 7.11c, step 4b Sensory receptors (pain receptors in the skin) Spinal cord Sensory (afferent) neuron Inter- neuron Motor (efferent) neuron Effector (biceps brachii muscle) (c) Figure 7.11c, step 4b

Simple Reflex Arc Figure 7.11b–c Sensory receptors (stretch receptors in the quadriceps muscle) Sensory (afferent) neuron Sensory receptors (pain receptors in the skin) Spinal cord Sensory (afferent) neuron Synapse in ventral horn gray matter Inter- neuron Motor (efferent) neuron Motor (efferent) neuron Effector (quadriceps muscle of thigh) Effector (biceps brachii muscle) (b) (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 Reflex- Simple 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 post-ganglionic 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)