Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

What are the major components of a spinal

nerve?

Figure 13–6

Spinal Nerves

Organization of Spinal Nerves

• Every spinal cord segment:– is connected to a pair of spinal nerves

• Every spinal nerve:– is surrounded by 3 connective tissue

layers– that support structures and contain

blood vessels

3 Connective Tissue Layers

• Epineurium:– outermost layer– dense network of collagen

fibers• Perineurium:

– middle layer– divides nerve into

fascicles (axon bundles)• Endoneurium:

– inner layer– surrounds individual

axons

Peripheral Nerves

• Interconnecting branches of spinal nerves

• Surrounded by connective tissue sheaths

How does the distribution pattern of spinal nerves

relate to the regions they innervate?

Peripheral Distribution of Spinal Nerves

• Spinal nerves:– form lateral to intervertebral foramen– where dorsal and ventral roots unite– then branch and form pathways to

destination

Figure 13–9

3D Rotation of Peripheral Nerves and Nerve Plexuses

PLAYPLAY

Nerve Plexuses

• Contain no synapses!• For pre-midterm

(Summarized in tables in text and lab guide):– Know cord roots (“ventral

rami,” actually) that contribute to the plexus

– Know the names of the major peripheral nerves that each plexus gives rise to.

Figure 13–10

The Cervical Plexus

Figure 13–12a, b

3D Rotation of Lumbar and Sacral PlexusesPLAYPLAY

The Lumbar and Sacral Plexuses

• Innervate pelvic girdle and lower limbs

Figure 13–12c, d

The Lumbar and Sacral Plexuses

Medical Example: Shingles

• Post-Viral inflammation of the sensory nerves• Rash follows dermatomes.• Notice it does not cross the midline.

Figure 13–8

Dermatomes

• Bilateral region of skin• Monitored by specific

pair of spinal nerves

Figure 13–7b

Peripheral Distribution of Spinal Nerves

• Sensory fibers

Figure 13–7a

Peripheral Distribution of Spinal NervesPLAYPLAY

Peripheral Distribution of Spinal Nerves

• Motor fibers

Functional Organization of Neurons

• Sensory neurons:– about 10 million– deliver information to CNS

• Motor neurons:– about 1/2 million– deliver commands to peripheral

effectors

Functional Organization of Neurons

• Interneurons:– about 20 billion– interpret, plan, and coordinate signals

in and out– often organized into functional

“neuronal pools”

Figure 13–13a

5 Patterns of Neural Circuits in Neuronal Pools

1. Divergence:– spreads stimulation to

many neurons or neuronal pools in CNS

Figure 13–13b

5 Patterns of Neural Circuits in Neuronal Pools

2. Convergence:– brings input from many

sources to single neuron

Figure 13–13c

5 Patterns of Neural Circuits in Neuronal Pools

3. Serial processing:– moves information in

single line

Figure 13–13d

5 Patterns of Neural Circuits in Neuronal Pools

4. Parallel processing:– moves same

information along several paths simultaneously

Figure 13–13e

5 Patterns of Neural Circuits in Neuronal Pools

5. Reverberation:– positive feedback

mechanism– functions until inhibited

Reflexes

Development of Reflexes A reflex is a rapid, predictable

motor response to a stimulus. Innate reflexes are unlearned

and involuntary Acquired reflexes are complex,

learned motor patterns

Nature of Reflex Responses

Somatic: Reflexes involving skeletal muscles and somatic motor neurons.

Autonomic (visceral) Reflexes controlled by autonomic neurons Heart rate, respiration, digestion, urination,

etc Spinal reflexes are integrated within the

spinal cord gray matter while cranial reflexes are integrated in the brain.

Reflexes may be monosynaptic or polysynaptic

Components of a Reflex Arc

1. Activation of a Receptor: site of stimulus 2. Activation of a Sensory Neuron: transmits

the afferent impulse to spinal cord (CNS) 3. Information processing at the Integration

center: synapses (monosynaptic reflexes) or interneurons (polysynaptic) between the sensory and motor neurons. In CNS

Spinal reflexes or cranial reflexes

Components of a Reflex Arc 4. Activation of a Motor Neuron:

transmits the efferent impulse to effector organ

5. Response of a peripheral Effector: Muscle or gland that responds

Interneuron

Spinal Reflexes

4 important somatic spinal reflexes Stretch Tendon Flexor(withdrawal) Crossed extensor reflexes

Stretch Reflexes 1. Stretching of the muscle activates a

muscle spindle (receptor) 2. An impulse is transmitted by afferent

fibers to the spinal cord 3. Motor neurons in the spinal cord

cause the stretched muscle to contract 4. The integration area in the spinal cord

Polysynaptic reflex arc to antagonist muscle causing it to to relax (reciprocal innervation)

Stretch Reflex

Notice hammer

Stretch Reflex ExamplePatellar Reflex

Tap the patellar tendon muscle spindle signals stretch of muscle motor neuron activated & muscle contracts

Quadriceps muscle contracts Hamstring muscle is inhibited (relaxes)

Reciprocal innervation (polysynaptic- interneuron) antagonistic muscles relax as part of reflex

Lower leg kicks forward Demonstrates sensory and motor connections

between muscle and spinal cord are intact.

Tendon Reflexes Monitors external tension produced during

muscular contraction to prevent tendon damage Controls muscle tension by causing muscle relaxation

Golgi tendon organs in tendon (sensory receptor) activated by stretching of tendon inhibitory neuron is stimulated motor neuron is hyperpolarized and muscle relaxes

Both tendon & muscle are protected Reciprocal innervation (polysynaptic)

causes contraction

Martini pg 443 states the receptor is unidentified; this is incorrect.

Tendon Reflex

Notice no hammer

Flexor Reflex Withdrawal reflex

When pain receptors are activated it causes automatic withdrawal of the threatened body part.

Flexor (Withdrawal) Reflex

Is this a monosynaptic or a polysynaptic reflex?

Is this an ipsilateral or a contralateral reflex?

Crossed Extensor Reflex Complex reflex that consists of an

ipsilateral withdrawal reflex and a contralateral extensor reflex

This keeps you from falling over, for example if you step on something painful. When you pull your foot back, the other leg responds to hold you up.

Crossed Extensor Reflex

Superficial Reflexes Elicited by gentle cutaneous

stimulation

Important because they involve upper motor pathways (brain) in addition to spinal cord neurons

Superficial ReflexesPlantar Reflex

Tests spinal cord from L4 to S2 Indirectly determines if the corticospinal

tracts of the brain are working Draw a blunt object downward along the

lateral aspect of the plantar surface (sole of foot)

Normal: Downward flexion (curling) of toes

Normal

Abnormal(Babinski’s)

Plantar Reflex

Abnormal Plantar Reflex: Babinski’s Sign

Great toe dorsiflexes (points up) and the smaller toes fan laterally

Happens if the primary motor cortex or corticospinal tract is damaged

Normal in infants up to one year old because their nervous system is not completely myelinated.

Preview of the ANS

Figure 16–2

Organization Similarities of SNS and ANS

Visceral Reflexes

• Provide automatic motor responses

• Can be modified, facilitated, or inhibited by higher centers, especially hypothalamus

Figure 16–11

Visceral Reflexes

Case of the Woman with HT• Name the two parts of the ANS• Describe the two major groups of receptors

and their subtypes (and their usual ligands.)• Distinguish between receptor stimulation

and cell stimulation.• Explain what “specificity” means when we

are referring to a ligand’s specificity for receptors.

• Provide a background for studying examples of somatic and autonomic reflexes.

• review

Nerve Plexuses

• Complex, interwoven networks of nerve fibers

• Formed from blended fibers of ventral rami of adjacent spinal nerves

• Control skeletal muscles of the neck and limbs

The 4 Major Plexuses of Ventral Rami

1. Cervical plexus2. Brachial plexus3. Lumbar plexus4. Sacral plexus

Dorsal and Ventral Rami

• Dorsal ramus:– contains somatic and visceral motor

fibers– innervates the back

• Ventral ramus:– larger branch– innervates ventrolateral structures and

limbs– contribute to plexuses

Table 13-1

Summary: Cervical Plexus

Figure 13–10

The Cervical Plexus

Table 13–2 (1 of 2)

Summary: Brachial Plexus

Table 13–2 (2 of 2)

Summary: Brachial Plexus

Major Nerves of Brachial Plexus

• Musculocutaneous nerve (lateral cord)

• Median nerve (lateral and medial cords)

• Ulnar nerve (medial cord)• Axillary nerve (posterior cord)• Radial nerve (posterior cord)

Figure 13–12a, b

3D Rotation of Lumbar and Sacral PlexusesPLAYPLAY

The Lumbar and Sacral Plexuses

• Innervate pelvic girdle and lower limbs

Figure 13–12c, d

The Lumbar and Sacral Plexuses

The Lumbar Plexus

• Includes ventral rami of spinal nerves T12–L4

• Major nerves:– genitofemoral nerve– lateral femoral cutaneous nerve– femoral nerve

The Sacral Plexus

• Includes ventral rami of spinal nerves L4–S4

• Major nerves:– pudendal nerve – sciatic nerve

• Branches of sciatic nerve:– fibular nerve – tibial nerve

Table 13-3 (1 of 2)

Summary: Lumbar and Sacral Plexuses

Table 13-3 (2 of 2)

Summary: Lumbar and Sacral Plexuses

Medical Example: Poliomyelitis

• Polio means gray matter

• Virus causes inflammation of the gray matter in the anterior horn motor neurons.

• Results in paralysis which could kill a patient if it reaches the respiratory muscles

• Patients who recover have permanent weakness or paralysis in parts of the body (usually the legs)

Lou Gehrig’s Disease Amyotrophic Lateral Sclerosis

• ALS is a genetic disease that causes progressive destruction of anterior horn motor neurons.

• Leads to paralysis and death within 5 years.

• Stephen Hawking has this disease.

Medical Example: Shingles

• Post-Viral inflammation of the sensory nerves• Rash follows dermatomes.• Notice it does not cross the midline.

Figure 13–19

The Babinski Reflexes

• Normal in infants• May indicate CNS damage in adults

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