Chapter 13 - Spinal Cord & Spinal Nerves Flashcards
What is the spinal cord?
Part of the central nervous system that extends from the brain
What are the three layers of protection for the central nervous system?
- Bony skull and vertebral column
- Meninges - three membranes
- Cerebrospinal fluid
Where is the spinal cord located?
Within the vertebral canal of the vertebral column
- in the vertebral foramina of all vertebrae
What are the meninges?
Three protective, connective tissue coverings that encircle the spinal cord and brain
What are the three layers of the meninges?
- Dura mater
- Arachnoid mater
- Pia mater
What are the cranial meninges?
Continuous with spinal meninges
Encircle the brain
What is the epidural space?
Cushion of fat and connective tissue that protects the spinal cord
- between dura mater and wall of the vertebral canal
What is the dura mater?
Most superficial
Thick, strong layer of dense, irregular connective tissue
Runs from Foramen magnum in the occipital bone to second sacral vertebrae
What is the arachoind mater?
Middle meningeal layer
Thin, avascular cells and thin, loosely arranged collagen and elastic fibers
What is the subdural space?
Between the dura mater and arachnoid mater
Contains interstitial fluid
What is the pia mater?
Innermost layer
Thin transparent connective tissue layer
Adheres to surface of brain and spinal cord
Thin squamous->cuboidal cells
Many blood vessels that supply oxygen and nutrients to the spinal cord
What is the sub-arachnoid space?
Between the arachnoid mater and pia mater
Contain shock-absorbing cerebrospinal fluid
Where does the spinal cord extend to and from?
From medulla oblongata (inferior part of the brain)
To the superior border of L2 (second lumbar vertebrae)
When does elongation of the spinal cord stop?
Around age 4 or 5
What is the length of an adult spinal cord?
Diameter?
42-45 cm in length
2 cm in the mid-thoracic region (diameter)
- somewhat larger in the lower cervical and mid-lumbar regions
What is the cervical enlargement?
The superior enlargement
Extends from 4th cervical vertebrae (C4) to the 1st thoracic vertebrae (T1)
- nerves to and from the upper limbs arise here
What is the lumbar enlargement?
The inferior enlargement
Extends from 9th to 12th thoracic vertebrae (T9-T12)
- nerves to and from the lower limbs arise here
What is the conus medullaris?
Inferior to the lumbar enlargement
- where the spinal cord terminates as a tapering, conical structure
- ends between L1 and L2
What is the filum terminale?
Arises from conus medullaris
- an extension of the pia mater that extends inferiorly and fuses with the arachnoid mater and dura mater and anchors the spinal cord to the coccyx
What are spinal nerves?
Paths of communication between the spinal cord and specific regions of the body
Why does the spinal cord appear to be segmented?
B/c of 31 pairs of spinal nerves emerge at regular intervals from intervertebral foramina
How are the spinal nerves named?
Based on where they are located:
- 8 pairs of cervical nerves
- 12 pairs of thoracic nerves
- 5 pairs of lumbar nerves
- 5 pairs of sacral nerves
- 1 pair of coccygeal nerves
What is a root?
Bundle of axons that connect each spinal nerve to a segment of the cord
What are rootlets?
Smaller bundles of axons, branches of a root
- connect to the spinal cord
What is the posterior (dorsal) root?
Contains only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and interal organs into the CNS
What is the posterior (dorsal) ganglion?
Each posterior root has a swelling that contains the cell bodies of sensory neurons
What is the anterior (ventral) root?
Contains axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands)
Why do the nerves that leave the lumbar, sacral and coccygeal regions of the spinal cord do not leave the vertebral column at the same level they exit the cord?
B/c the spinal cord is shorter than the vertebral column
- the roots of these lower spinal nerves angle inferiorly alongside the filum terminale
What is the cauda equina?
“Horse’s tail”
Roots of nerves from the lumbar, sacral and coccygeal regions
Describe the internal anatomy of the spinal cord.
White matter that surround an inner core of gray matter
Two grooves penetrate the white matter of the spinal cord and divide it into left and right sides
Gray matter is shaped like an “H”
What is the anterior median fissure?
Wide groove on the anterior (ventral) side
What is the posterior median sulcus?
Narrow furrow on the posterior (dorsal) side
What does the gray matter of the spinal cord consist of?
Dendrites and cell bodies of neurons, unmyelinated axons and neuroglia
What does the white matter of the spinal cord consist of?
Bundles of myelinated axons of neurons
What is the gray commissure?
Forms the crossbar of the H, in the gray matter
What is the central canal?
A small space in the center of the gray commissure
- extends the entire length of the spinal cord and is filled with cerebrospinal fluid
What is the anterior (ventral) white commissure?
Connects the white matter of the right and left sides of the spinal cord
- anterior to the gray commissure
What are the nuclei of the spinal cord?
Clusters of neuronal cell bodies that form functional groups in the gray matter of the spinal cord and brain
What do sensory nuclei do?
Receive input from receptors via sensory neurons
What do motor nuclei do?
Provide output to effector tissues via motor neurons
What are the horns of the spinal cord?
Gray matter on each side of the spinal cord is subdivided into regions called horns
What does the posterior (dorsal) gray horns contain?
Cell bodies and axons of interneurons as well as axons of incoming sensory neurons
Where are the cell bodies of sensory neurons located?
Posterior (dorsal) root ganglion of the spinal nerve
What does the anterior (ventral) gray horns contain?
Somatic motor nuclei
- clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles
What are lateral gray horns?
Between posterior and anterior gray horns
- present only in thoracic and upper lumbar segments of the spinal cord
What do lateral gray horns contain?
Contain autonomic motor nuclei - clusters of cell bodies of autonomic motor neurons that regulate that activity of cardiac muscle, smooth muscle and glands
What are the three broad areas of white matter called?
- Anterior (ventral) white columns
- Posterior (dorsal) white columns
- Lateral white columns
What is a tract?
Bundles of axons, which may extend long distances up or down the spinal cord
- in the white columns of the spinal cord
What is the difference between nerves and tracts?
Nerve = bundles of axons in the PNS Tracts = bundles of axons in the CNS
What is a sensory (ascending) tract?
Consists of axons that conduct nerve impulses toward the brain
What is a motor (decsending) tract?
Consists of axons that carry nerve impulses from the brain
True or False:
Sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain
True
Why does the amount of white matter decrease from cervical to sacral segments of the spinal cord?
- As the spinal cord ascends from sacral to cervical segments, more ascending axons are added to the spinal cord white matter to form more sensory tracts
- As the spinal cord descends from cervical to sacral segments, the motor tracts decrease in thickness as more descending axons leave the motor tracts to synapse with neurons in the gray matter of the spinal cord.
Where do the posterior and anterior roots unite to form a spinal nerve?
Intervertebral foramen
What is a mixed nerve?
B/c posterior root contains sensory axons &
anterior root contains motor axons
What are the three connective tissue layers that wrap around axons?
- Endoneurium
- Perineurium
- Epineurium
What is the endoneurium?
The innermost layer of connective tissue that wraps around axons (myelinated AND unmyelinated)
What is a fascicle?
Groups of axons with their endoneurium are held together in bundles
What is the perineurium?
Middle layer of connective tissue, wraps around each bundle of fascicles of axons
- thickest layer
What is the epineurium?
Outermost covering of connective tissue, over the entire nerve
- extensions of the epineurium also fill the spaces between fascicles
What are rami?
Branches that the spinal nerves divides into after passing through the intervertebral foramen
What does the posterior (dorsal) ramus serve?
The deep muscles and skin of the posterior surface of the trunk
What does the anterior (ventral) ramus serve?
The muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk
What is the meningeal branch?
Branch that re-enters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord and meninges
What is a plexus?
Network of axons on both the left and right sides of the body by joining with various axons from anterior rami of adjacent nerves
What are the principal plexuses?
Cervical plexus Brachial plexus Lumbar plexus Sacral plexus -Coccygeal plexus is also present
What is the cervical plexus?
Formed by the roots (anterior rami) of the first 4 cercival nerves (C1-C4)
- supplies the skin and muscles of the head, neck, and superior part of the shoulders and chest and diaphragm
What is the brachial plexus?
Formed by the roots (anterior rami) of spinal nerves C5-C8 and T1
- extends inferiorly and laterally
What does the brachial plexus supply?
Entire nerve supply of the shoulders and upper limbs
What are the five important nerves that arise from the brachial plexus?
- Axillary nerve
- Musculocutaneous nerve
- Radial nerve
- Median nerve
- Ulnar nerve
What is the lumbar plexus?
Formed by the root (anterior rami) of spinal nerves L1-L4
- minimal intermingling of fibers
- supplies the anterolateral abdominal wall, external genitals and part of lower limbs
What is the saccral plexus?
Formed by the roots (anterior rami) of spinal nerves L4-L5 and S1-S4
- situated largely anterior to the sacrum
- supplies buttocks, perineum, and lower limbs
What is the coccygeal plexus?
Formed by the roots (anterior rami) of spinal nerves S4-S5
- supplies a small area of skin in the coccygeal region
What are the intercostal or thoracic nerves?
Anterior rami of spinal nerves T2-T12
- do not enter into the formation of plexuses
How are the intercostal nerves different?
These nerves connect directly to the structures they supply in the intercostal spaces
- intercostal muscles, skin, abdominal muscles, deep back muscles
What is a dermatone?
The area of skin that provides sensory input to the CNS via one pair of spinal nerves
What is the importance of dermatones?
Knowing which spinal cord segments supply each dermatone make it possible to locate damaged regions of the spinal cord
What are the two principal functions does the spinal cord do to maintain homeostatis?
Nerve impulse propagation
Integration of information
What are white matter tracts?
Spinal cord highways for nerve impulse propagation
- sensory input travels towards brain
- motor output travels away from brain
What is the spinothalamic tract?
Conveys nerve impulses for sensing pain, warmth, coolness, itching, tickling, deep pressure and crude touch
- sensory up to the brain
What is the posterior column?
Consists of two tracts:
- Gracile fasciculus
- Cuneate fasciculus
- light pressure, vibration, awareness of positions and movements of muscles/tendons
What does the cerebral cortex do?
Controls precise voluntary muscular movements
What are the two descending pathways of motor output?
- Direct pathways
2. Indirect pathways
What are direct pathways?
Convey nerve impulses that originate in the cerebral cortex and are destined to cause VOLUNTARY movements of skeletal muscles
What are the three direct pathways?
- Lateral corticospinal
- Anterior corticospinal
- Corticobulbar
What are indirect pathways?
Convey nerve impulses from the brain stem to cause AUTOMATIC movements and help coordinate body movements with visual stimuli
What are the three indirect pathways?
- Rubrospinal
- Tectospinal
- Vestibulospinal
- Lateral reticulospinal
What is a reflex?
A fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus
What is a spinal reflex?
When integration occurs in the spinal cord gray matter
knee jerk
What is a cranial reflex?
When integration occurs in the brain stem
tracking movements of your eyes as you read
What is a somatic reflex?
Contraction of skeletal muscles
What are autonomic (visceral) reflexes?
Generally not consciously perceived
- involve responses of smooth muscle, cardiac muscle and glands
What is a reflex arc (reflex circuit)?
Pathway followed by nerve impulses that produce a reflex
What are the five components of a reflex arc?
- Sensory receptor
- Sensory neuron
- Integrating center
- Motor neuron
- Effector
What does the sensory receptor do in a reflex arc?
Distal end of a sensory neuron
Responds to a stimulus by producing a generator or receptor potential
What is a stimulus?
Change in the environment (internal or external)
What does the sensory neuron do in a reflex arc?
Axon conducts impulses from receptor to integrating center
What does the integrating center do in a reflex arc?
One or more regions within the CNS that relay impulses from sensory to motor neurons
What does the motor neuron do in a reflex arc?
Axon conducts impulses from integrating center to effector
What does the effector do in a reflex arc?
Muscle or gland that responds to motor nerve impulses
What is a monosynaptic reflex arc?
Reflex pathway that has only one synapse in the CNS
- single synapse between sensory neuron and motor neuron
What is a polysynaptic reflex arc?
Reflex pathway that has more than two types of neurons and more than one CNS synapse
How do reflexes provide information about the state of health of the nervous system?
Damage or disease anywhere along a reflex arc can cause a reflex to be absent or abnormal
- useful in diagnosis
What is a stretch reflex?
Causes contraction of a skeletal muscle in response to stretching of the muscle
- monosynaptic reflex arc
- knee jerk example
Describe how a stretch reflex operates.
- Stretching stimulates sensory receptor
- Sensory neuron - conduct impulse to integration center
- Integration center (spinal cord) - activates motor neuron
- Motor neuron - conducts impulse to effector
- Effectory - same muscle contracts and relieves the stretching
What are muscle spindles?
Sensory receptors in muscles
- they monitor changes in the length of the muscle
What is an ipsilateral reflex?
Sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave
True or False:
All polysynaptic reflexes are ipsilateral.
False.
All monosynaptic reflexes are ipsilateral
What is muscle tone?
Small degree of contraction present while the muscle is at rest
What is reciprocal innervation?
Relaxation of the antagonistic muscle that opposes a stretch reflex
- prevents conflict between opposing muscles
- polysynaptic
What is a tendon reflex?
Operates as a feedback mechanism to control muscle tension
- causes muscle relaxation before muscle force becomes so great that tendons might be torn
- polysynaptic
True or False:
Tendon reflex is ipsilateral.
True
What are the sensory receptors of tendon reflexes called?
Tendon (Golgi tendon) organs
- detect changes in muscle tension that are caused by passive stretch or muscular contraction
Where are tendon (Golgi tendon) organs located?
Lie within a tendon near its junction with a muscle
Describe how a tendon reflex operates.
- Increased tension stimulates tendon organ
- Sensory neuron excited
- Integrating center (spinal cord) - sensory neuron activates inhibitory interneuron
- Motor neuron - becomes inhibited
- Effector - muscle attached to same tendon relaxes and relieves excess tension
What is a flexor (withdrawl) reflex?
Polysynpatic reflex, ipsilateral
- if you step on a tack, you withdraw your leg
Describe how a flexor reflex operates.
- Stepping on tack stimulates sensory receptors (dendrites of pain-sensitive neuron)
- Sensory neuron is excited
- Integration center (spinal cord) - sensory neuron activates interneurons in several spinal cord segments
- Motor neuron(S) are excited
- Effector (flexor muscles) - contract and withdraw leg
What is an intersegmental reflex arc?
Nerve impulses from one sensory neuron ascend and descend in the spinal cord and activate interneurons in several segments of the spinal cord
What is a crossed extensor reflex?
Help to maintain your balance
- contralateral reflex
Describe how a crossed extensor reflex operates.
- Stepping on tack stimulates sensory receptors (pain) in the RIGHT foot
- Sensory neuron is excited
- Integration center (spinal cord) - sensory neuron activates several interneurons (up and down)
- Motor neurons are excited
- Effectors (extensor muscles) - contract and extend LEFT leg for balance
What is a contralateral reflex?
Sensory impulses enter one side of the spinal cord and motor impulses exit on the opposite side
- syncronizes the extension of the contralateral limb with the withdrawl (flexion) of the stimulated limb
What is the patellar reflex?
Knee jerk!
- stretch reflex
- involves extension of the leg at the knee joint by contraction of quadsriceps femoris muscle in response to tapping the patellar ligament
- absent in ppl with diabetes
- exaggerated in ppl with disease/injury involving certain motor tracts
What is the Babinski reflex?
Results from gentle stroking of the lateral outer margin of the sole, great toe extends
- due to incomplete myelination of fibers in the corticospinal tract
- occurs in children under 1.5 years old
- normal response after 1.5 years old is plantar flexion reflex
What is shingles?
Acute infection of the PNS
- cause by herpes zoster (chicken pox)
- results in pain, discoloration of the skin, skin blisters
What is poliomyelitis?
Polio
- onset marked by fever, severe headaches, stiff neck & back, deep muscle pain & weakness, loss of somatic reflexes
- can produce paralysis by destroying cell bodies of motor neurons
- can cause death by heart or respiratory failure
What is meningitis?
Inflammation of the meninges due to an infection
- fever, headache, stiff neck, vomiting, confusion, lethargy, drowsiness
- bacterial meningitis may be fatal if not treated promptly
