CH 12 - Spinal Cord and Spinal Reflexes Flashcards
Describe the spinal cord physically.
18” long, 0.5” wide, ends at L2 vertebra, bilaterally symmetrical, located in vertebral foramen
Two-way conduction pathway to/from brain
Stops growing at age 4-5
Posterior (dorsal) median sulcus
Groove on the posterior/dorsal side of the spinal cord
Anterior (ventral) median fissure
Deeper groove on the anterior/ventral side of the spinal cord
What are spinal cord enlargements caused by?
The amount of grey matter in the segment
Involvement with sensory/motor nerves of limbs
Innervate
To supply or control
Cervical enlargement
Spinal nerves that innervate the shoulder and upper limbs
Lumbar enlargement
Spinal nerves that innervate the pelvis and lower limbs
Conus medularis
The thin, conical spinal cord below the lumbar enlargement
Where the spinal cord ends
Filum terminale
The thin thread of fibrous tissue inferior to the conus medularis that anchors the spinal cord to the coccyx
Fusion of the spinal meninges beyond sacral vertebra S2
Cauda equina
The long dorsal/ventral roots that extend below the conus medularis/filum terminale
“The horse’s tail”
How many spinal cord segments are there?
How many nerves in each?
31 segments, each with 2 nerves
*Each segment is the same length, but the position changes with age
How are cervical spinal nerves named?
Named for their inferior vertebra
*First nerve is above C1 vertebra, and eighth nerve is below C7
How are all spinal nerves besides cervical named?
Named for their superior vertebra
*First nerve is below T1 vertebra
Where do the C01, S5, and the filum terminale nerves enter/leave the spinal column?
Inside the sacral hiatus
What makes up a spinal nerve?
Dorsal and ventral roots join to form a spinal nerve (on each side of the spine)
Contains mixed nerves (sensory and motor axons)
What kind of axons do ventral roots contain?
Axons of motor neurons (motor output)
Where do ventral roots carry information?
Away from the spinal cord to effectors
What kind of axons do dorsal roots contain?
Axons of sensory neurons (sensory input)
Where do dorsal roots carry information?
Into the spinal cord from sensory receptors
What does a dorsal root ganglion contain?
Cell bodies of sensory neurons
*Unipolar neurons
What are spinal meninges?
Name them.
Specialized membranes that isolate the spinal cord from its surroundings
Dura mater, arachnoid mater, pia mater
*Continuous with cranial meninges
What are the functions of spinal meninges?
Protect the spinal cord and provide blood supply
Meningitis
A viral or bacterial infection of the meninges
List the layers from a vertebra to the spinal cord.
Bone Periosteum Epidural space Dura mater Subdural space Arachnoid mater Subarachnoid space Pia Mater Spinal cord
How does the dura mater of the spinal cord differ from that of the brain?
The spinal dura mater has only one layer, whereas the cranial dura mater has two layers
Describe the superior, inferior, and lateral aspects of the spinal dura mater.
Superiorly continuous with cranial dura mater
Inferiorly tapers into filum terminale in the coccygeal ligament
Laterally fuses with the epineurium of the spinal cord
What is found within the epidural space of the spinal cord?
Adipose tissue & blood vessels
Epidural
A procedure that injects a local anesthetic into the epidural space
Lumbar puncture (spinal tap)
A procedure in which a needle is inserted into the subarachnoid space between vertebrae L4 and L5 to withdraw CSF
What is the pia mater of the spinal cord made of?
Collagen and elastic fibers
What are denticulate ligaments and where are they found?
Ligaments that anchor the spinal cord along its length at each side to stabilize side-to-side movements
Fused from the pia mater to the dura mater
Where are the blood vessels of the spinal cord located?
Along the surface of the pia mater within the subarachnoid space
What type of nuclei are found in posterior grey horns?
Somatic and visceral sensory nuclei
What type of nuclei are found in lateral grey horns?
Describe them.
Visceral motor nuclei
*Two neurons- a myelinated preganglionic neuron and an unmyelinated postganglionic neuron
Which segments of the spinal cord have lateral grey horns?
Thoracic and lumbar segments T1-L2
What type of nuclei are found in anterior grey horns?
Somatic motor nuclei
What do commissures (transverse tracts) contain?
Commissural fibers that cross from one side of the spinal cord to the other
*Needed for contralateral control
Name the three connective tissue layers that surround spinal nerves.
Epineurium, perineurium, endoneurium
Describe the epineurium.
The outermost layer, continuous with the dura mater
Made of dense collagenous CT
Describe the perineurium.
The middle layer that divides nerves into fascicles (bundles of axons)
Describe the endoneurium.
The inner layer that surrounds individual axons
Name the layers surrounding a myelinated axon from innermost to outermost.
Axon
Myelin sheath
Neurolemma
Endoneurium
Peripheral nerves
The terminal end of branched spinal nerves that form pathways to innervate organs and tissues
Surrounded by the same CT sheaths as the spinal nerves
What are rami communicantes?
Name them.
“communicating branches”
White ramus communicans & grey ramus communicans
Which spinal cord segments contain white rami communicans?
Spinal cord segments T1-L2
Describe the size of the dorsal (posterior) ramus.
What does it innervate?
Smaller branch
Innervates the posterior region of the body
Describe the size of the ventral (anterior) ramus.
What does it innervate?
Larger branch
Innervates the anterior/lateral regions of the body
Describe the meningeal branch’s location and function.
A small branch off of the dorsal ramus that re-enters the vertebral cavity though the intervertebral foramen
Supplies nerve function to the vertebrae, vertebral ligaments, spinal cord blood vessels, spinal meninges
Spinal nerve plexuses
Complex networks of nerve fibers formed from the ventral rami of spinal nerves
What makes up the cervical plexus?
Ventral rami of spinal nerves C1-C5
What structures does the cervical plexus innervate?
Skin and skeletal muscles of the head, neck, shoulders, chest, and diaphragm
Which major peripheral nerve (terminal branch) belongs to the cervical plexus?
What does it control?
Phrenic nerve
Controls muscles of the diaphragm
What makes up the brachial plexus?
Ventral rami of spinal nerves C5-T1
What are the 5 major groups of the brachial plexus?
Ventral rami (roots) Trunks Divisions Cords Peripheral nerves (terminal branches)
What structures does the brachial plexus innervate?
Skin and skeletal muscles of the pectoral girdle and upper limbs
Which major peripheral nerves (terminal branches) belong to the brachial plexus?
Musculocutaneous nerve Axillary nerve Median nerve Radial nerve Ulnar nerve
What makes up the lumbar plexus?
Ventral rami of spinal nerves T12-L4
What structures does the lumbar plexus innervate?
Anterolateral abdominal wall, external genitalia, and parts of the lower limbs
Which major peripheral nerves (terminal branches) belong to the lumbar plexus?
Genitofemoral nerve
Lateral femoral cutaneous nerve
Femoral nerve
Obturator nerve
What makes up the sacral plexus?
Ventral rami of spinal nerves L4-S4
What structures does the sacral plexus innervate?
Buttocks, perineum, and lower limbs
Which major peripheral nerves (terminal branches) belong to the sacral plexus?
Sciatic nerve (which branches into the tibial nerve and common fibular nerve) and the pudendal nerve
Which spinal nerves make up the intercostal (thoracic) nerves?
Spinal nerves T2-T12
These do not form branches
Describe the ventral (anterior) rami of the intercostal (thoracic) nerves.
Spinal nerves T2-T12 do not form plexuses
Instead the nerves directly innervate the intercostal muscles and the anterior/lateral skin of the thorax (between neck and abdomen)
Describe the dorsal (posterior) rami of the intercostal (thoracic) nerves.
Spinal nerves T2-T12 innervate the deep muscles of the back and posterior skin of the thorax (between neck and abdomen)
Describe the ventral ramus of the T12 spinal nerve.
The ventral ramus splits, with one part forming the intercostal nerve and the other forming part of the lumbar plexus
Peripheral neuropathy
Damage to the spinal nerves due to compression, trauma, or diabetes
Results in the loss of sensation/motor function in the associated body region
Dermatome
An area of skin supplied by peripheral nerve fibers originating from a single dorsal root ganglion
Loss of sensation in a dermatome can indicate the level of spinal cord damage
Hypoesthesia
Partial loss of sensation in a dermatome caused by a severed dorsal root
Dermatomal anesthesia
Complete loss of sensation caused by damage to at least three overlapping dermatomes
Why does spinal nerve C1 not have a dermatome?
It is the only spinal nerve that does not contain a dorsal root
Shingles
A disease caused by Varicella-Zoster Virus (VZV) that remains dormant within the dorsal root ganglia
A type of herpes virus that attacks sensory neurons in the dorsal root of spinal nerves, and sensory ganglia of cranial nerves
Skin eruptions follow the distribution of the dermatomal innervation
Reflex
A rapid, involuntary, automatic response to a stimulus
Which effectors do somatic reflexes travel to?
Skeletal muscle
Which effectors do visceral (autonomic) reflexes travel to?
Smooth/cardiac muscle or glands
Reflex arc
The wiring of a single reflex that begins at a sensory receptor and ends at an effector (with some using interneurons)
Generally opposes the original stimulus (negative feedback)
List and describe the 5 steps involved in a reflex arc.
Step 1: stimulus & activation of sensory receptors
-physical or chemical changes; graded potential
Step 2: activation of sensory (unipolar) neurons
-AP at trigger zone of sensory neuron
Step 3: integration at CNS
-sensory neuron synapses with interneuron
Step 4: activation of motor neurons
-interneurons synapse with motor neurons
Step 5: effector response
-motor neuron synapses with effector
What are the 4 main classes of reflexes?
Describe their subtypes.
Developmental
- Innate (genetic)
- Acquired (learned)
Response
- Somatic (skeletal muscle)
- Visceral/autonomic (smooth/cardiac muscle, glands)
Complexity of circuit
- Monosynaptic (one synapse, no interneurons)
- Polysynaptic (multiple synapses, interneurons, EPSP/IPSP)
Processing site
- Spinal reflexes (spinal cord)
- Cranial reflexes (brain)
Muscle spindles
Proprioceptors involved in the stretch reflex that primarily detect changes in the length of skeletal muscle
Contribute to fine motor control and provide body position information to CNS
What are intrafusal muscle fibers?
What are they innervated by?
Small bundles of specialized skeletal muscle fibers surrounded by dendrites of sensory neurons
Forms muscle spindle
Innervated by somatic sensory/motor neurons (gamma motor neurons)
What are extrafusal muscle fibers?
What are they innervated by?
Contracting muscle fibers that surround intrafusal muscle fibers
Innervated by somatic motor neurons (alpha motor neurons)
Golgi tendon organs
Proprioceptors involved in the tendon reflex
Location- within tendons
Neuron- dendrites of somatic sensory neuron
Function- monitor tendon stretching
Name examples of monosynaptic reflexes.
Stretch reflex: patellar reflex & postural reflex
Name examples of polysynaptic reflexes.
Tendon reflex, withdrawal reflex, crossed extensor reflex
Reciprocal inhibition (reciprocal innervation)
A polysynaptic reflex that prevents conflict between opposing muscles by utilizing EPSP/IPSP’s
Allows agonist to contract and antagonist to relax
Does an AP frequency increase or decrease during stretching of the patellar reflex? Why?
The AP frequency increases
The rubber hammer tap stretches the patellar tendon, causing the muscle spindles to stretch as well
Does an AP frequency increase or decrease during contraction of the patellar reflex? Why?
The AP frequency decreases
If this were not the case, our knee would stay contracted/extended
Describe postural reflexes.
Stretched muscle responds by contracting automatically to maintain normal upright posture and balance
Many stretch reflexes are postural reflexes
Describe the stretch reflex.
Stimulus: stretching of muscle and muscle spindle
Stretch reflex: contraction of agonist
Reciprocal inhibition: relaxation of antagonist
Describe the tendon reflex.
Stimulus: intense/excessive contraction of agonist
Tendon reflex: relaxation of agonist
Reciprocal inhibition: contraction of antagonist
Describe the withdrawal (flexor) reflex.
Stimulus: pain/pressure on a limb
Withdrawal (flexor) reflex: contraction of agonist (flexors)
Reciprocal inhibition: relaxation of antagonist (extensors)
Describe the crossed extensor reflex.
Stimulus: balance or defense
Crossed extensor reflex: contraction of agonist (extensors)
Reciprocal inhibition: relaxation of antagonist (flexors)
*Reflex occurs on opposite body side as stimulus, and occurs simultaneously with the withdrawal reflex
Ipsilateral reflexes
Reflexes that occur on the same side of the body that received the stimulus
*Stretch, tendon, and withdrawal reflexes
Contralateral reflexes
Reflexes that occur on the opposite side of the body to where the stimulus was received
*Crossed extensor reflex
Superficial reflexes
Reflexes elicited by cutaneous stimulation
- Dependent on functional upper motor pathways and spinal reflex arcs
- Plantar (Babinski) reflex
What does the plantar (Babinski) reflex test for?
Tests integrity of spinal cord L4-S2
Determines if the primary motor cortex & corticospinal tracts are functioning properly
*Normal result is negative (no fanned toes)
