Chapter 13: The Spinal Cord and Spinal Nerves Flashcards
describe the protective structures of the spinal cord
spinal cord anatomy – very delicate, does not respond well to injury or damage. Layers of protection: 1) Vertebral column (bones), (2) Meninges (3) Epidural space w/ cerebrospinal fluid
- *Meninges (**singular is meninx) -3 protective, connective tissue covers that encircle the spinal cord and brain.
a. Superficial to deep: dura mater, arachnoid mater, pia mater
b. Continuous with the cranial meninges which encircle the brain
c. All 3 meninges cover the spinal nerves up to the point where they exit the spinal column through the intervertebral foramina.
Meningeal Layers (Superficial to Deep)
Dura Mater (tough mother)
- most superficial of the three spinal meninges.
- A thick, strong layer of dense irregular connective tissue
- Forms a sac from the level of the foramen magnum in the
occipital bone, (where it is continuous with the meningeal dura
mater of the brain), to the S2 vertebrae. - Also continuous with the epineurium – the outermost covering of
spinal and cranial nerves.
Arachnoid Mater (Spider Like)
- middle meningeal layer.
- Thin, avascular covering of cells and thin, loosely arranged collagen and elastic fibers.
- Continuous through the foramen magnum with the arachnoid
mater of the brain. - Between the dura mater and arachnoid mater is a thin space:
subdural space – thin space between dura mater and arachnoid mater that contains interstitial fluid.
Pia Mater (Delicate)
- innermost meninx
- a thin transparent connective tissue layer that adheres to the
surface of the spinal cord and brain - consists of thin squamous to cuboidal cells within interlacing
bundles of collagen fibers and some fine elastic fibers. - Contains many blood vessels that supply O2 and nutrients to
spinal cord -
Denticulate ligaments – triangular-shaped membranous extensions of the pia mater that suspend the spinal cord in the middle of its dural sheath.
-Thickenings of the pia mater
-Project laterally and fuse with the arachnoid mater and inner surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side.
-Extend along the entire length of the spinal cord.
-Protect the spinal cord against sudden displacement/shock
subarachnoid space – between the arachnoid mater and pia mater-contains shock absorbing cerebrospinal fluid.
- *Epidural Space** – a space between the dura mater and the wall of the vertebral canal.
a. Contains a cushion of fat and connective tissue
Describe the external anatomy of the spinal cord
roughly oval in shape, flattened slightly anteriorly and posteriorly.
In adults, extends from medulla oblongata to superior border of
L2. Extends to L3 or L4 in infants.
cervical enlargement – from C4 to T1. Nerves to and from arms arise
from the cervical enlargement conspicuous enlargement
lumbar enlargement – T9 to T12. Nerves to and from the legs arise here. nerves that supply the lower limb emerge
conus medullaris – inferior to the lumbar enlargement.
A tapering, conical structure where the spinal cord terminates.
Ends at the level of the intervertebral disc between L1-L2
filum terminale – arises from the conus medullaris
- an extension of the pia mater
- extends inferiorly, fuses with arachnoid mater and dura mater
- anchors spinal cord to the coccyx.
Describe the Internal Anatomy of the spinal cord
a. anterior median fissure – wide groove on the anterior (ventral) side of the spinal cord. With the posterior median sulcus, separates cord into right and left sides.
b. posterior median sulcus – a narrow furrow on the posterior (dorsal) side of the spinal cord.
c. gray commissure – forms the crossbar of the ‘H’ of grey matter in the spinal cord. A narrow strip of gray matter connecting the two lateral grey masses with the spinal cord.
d. central canal – at the center of the gray commissure; a microscopic tube running the length of the spinal cord; filled with cerebrospinal fluid. At superior end, the central canal is continuous with the fourth ventricle in the medulla oblongata of the brain
e. anterior (ventral) white commissure – anterior to the gray commissure; connects the white matter of the left and right sides of the spinal cord.
f. Nuclei – functional groups of clusters of neuronal bodies within the gray matter of the spinal cord and brain.
g. posterior (dorsal) gray horn – subdivided region of gray matter on each side of the spinal cord. Contains cell bodies and axons of interneurons as well as axons of incoming sensory neurons. (cell bodies of sensory neurons are located in the posterior root ganglion of a spinal nerve)
h. anterior (ventral) gray horn – another subdivided region of gray matter on each side of the spinal cord. Contain somatic motor nuclei (clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles)
i. lateral gray horns – present only in the thoracic and upper lumbar
segments of the spinal cord. Between the posterior and anterior gray
horns. Contain autonomic motor nuclei (clusters of cell bodies of
autonomic motor neurons that regulate the activity of cardiac muscle,
smooth muscle, and glands.
**Are not really a horn, just a small rounded part that sticks out
from the letter H
j. Columns
Group of white matter tracts in the spinal cord. 3 columns on each side, divided by the anterior and posterior gray horns.
Each column contains distinct bundles of axons having a
common origin or destination and carrying similar information.
- *anterior (ventral) white column – front * carries nerve impulses for proprioception**
- *posterior (dorsal) white column – rear**
- *lateral white column – out to the side.**
k. Tracts
a bundle of nerve axons in the CNS, each contained within a specific column of white matter. Recall that nerves are bundles of axons in the PNS. Tracts are bundles of axons in the CNS.
sensory (ascending) tract – consist of axons that conduct impulses
toward the brain
motor (descending) tract – consist of axons that carry nerve impulses
from the brain
Sensory and motor tracts of the spinal cord are continuous with
sensory and motor tracts in the brain.
explain how spinal nerves are connected to the spinal cord.
*Typical spinal nerve has 2 connections to spinal cord
the paths of communication between the spinal cord
and specific regions of the body.
Spinal cord appears segmented because the 31 pairs of spinal
nerves emerge at regular intervals from the intervertebral
foramina.
Each pair of spinal nerves emerges from a spinal segment
No obvious segmentation in spinal cord but for convenience,
naming of spinal nerves is based on the segment in which they are located:
- 8 pairs of cervical nerves C1-C8
- 12 pairs of thoracic nerves T1-T12
- 5 pairs lumbar nerves L1-L5
- 5 pairs sacral nerves, S1-S5
- 1 pair of coccygeal nerves Co1
cauda equine – “horse’s tail”
where the nerves of the lumbar, sacral, and coccygeal regions of
the spinal cord exit the vertebral column at levels different than
where they leave the spinal cord. (Because the cord stops at L1-
The roots of these lower spinal nerves angle inferiorly alongside
the filum terminale in the vertebral canal like wisps of hair.
- *Roots** – two bundles of axons that connect each spinal nerve to a segment of the cord by even small bundles of axons called rootlets
- *Rootlets** – smaller bundles of axons that connect the spinal cord to the roots
posterior or dorsal root of spinal nerve – contain only sensory axons (which conduct nerve impulses from sensory receptors in the skin, muscles, internal organs into the CNS. exclusively sensory information towards the spinal cord
posterior or dorsal root ganglion - a swelling of each posterior root which contains the cell bodies of sensory neurons
anterior or ventral root of spinal nerve– contain axons of motor neurons (which conduct nerve impulses from the CNS to effectors *exclusively motor information away from the spinal cord
describe the components, connective tissue coverings, and branching of a spinal nerve.
spinal nerves - associated with the spinal cord
Are parallel bundles of axons and their associated neuroglial cells,
wrapped in several layers of connective tissue (like all nerves of the PNS)
b. Connect the CNS to sensory receptors, muscles, and glands in all parts of
the body.
II. mixed nerve – classification of a spinal nerve, because it contains sensory axons
and motor axons.
III. connective tissue coverings - layers of protective connective tissue coverings
a. Endoneurium – innermost layer. Covers all individual axons within a
nerve, whether myelinated or unmyelinated. Consists of a mesh of
collagen fibers, fibroblasts, and macrophages.
Fascicle – a group of axons and their endoneurium held together by the
perineurium
c. Perineurium – the middle layer. A thicker layer of connective tissue.
Consists of up to 15 layers of fibroblasts within a network of collagen
fibers.
d. Epineurium – outermost covering over the entire nerve. Consists of
fibroblasts and thick collagen fibers. Extensions of the epineurium also fill
spaces between fascicles.
1. The dura mater of the spinal meninges fuses with the
epineurium as the nerve passes through the intervertebral
foramen.
2. Blood vessels are present between fascicles within the
epineurium.
distribution of spinal nerves
Spinal nerve divides into several branches shortly after passing through the intervertebral foramen.
- *ramus (plural is rami) -** the branches of a spinal nerve
a. Posterior (dorsal) ramus – serves the deep muscles and skin of the posterior surface of the trunk
b. Anterior (ventral) ramus – serves the muscels and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk.
c. meningeal branch – this branch reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges.
d. Rami communicantes – other branches of a spinal nerve; components of the autonomic nervous system, discussed in chapter 15.
define plexus and identify the principal plexuses of spinal nerves.
plexus - a network of nerves, formed on both the left and right sides of the body by
jointing with various numbers of axons from anterior rami of adjacent nerves.
a. There are several principal plexuses:
cervical plexus – a network formed by nerve axons from the ventral rami of the
first four cervical nerves and receiving gray rami communicantes from the superior
cervical ganglion
Ibrachial plexus – network of nerve axons of ventral rami of spinal nerves C5-T1.
The nerves that emerge from the brachial plexus supply the upper limb
lumbar plexus – a network formed by the anterior (ventral) branches of spinal
nerves L1-L4
sacral plexus – network formed by the ventral branches of spinal nerves L4-S3
coccygeal plexus – smaller plexus.
describe the clinical significance of dermatomes.
Dermatome - the cutaneous area developed from one embryonic spinal cord segment and receiving most of its sensory innervation from one spinal cord.
I. the area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal (V) nerve.
II. The nerve supply in adjacent dermatomes overlaps somewhat.
III. Can locate damaged regions of the spinal cord by knowing which spinal cord segments supply each dermatome.
IV. If the skin in a particular region is stimulated the sensation is not perceived, the nerves supplying that dermatome are probably damaged. (Little loss of sensation may occur in regions where overlap is significant.)
V. Therapeutic use: cutting or blocking nerves can provide permanent or temporary pain relief.
describe the origin and distribution of the cervical plexus.
plexus - a network of nerves, formed on both the left and right sides of the body by jointing with various numbers of axons from anterior rami of adjacent nerves.
cervical plexus – a network formed by nerve axons from the ventral rami of the first four cervical nerves and receiving gray rami communicantes from the superior cervical ganglion
describe the origin and distribution of the brachial plexus.
brachial plexus – network of nerve axons of ventral rami of spinal nerves C5-T1. The nerves that emerge from the brachial plexus supply the upper limb
describe the origin and distribution of the lumbar plexus
lumbar plexus – a network formed by the anterior (ventral) branches of spinal nerves L1-L4
describe the origin and distribution of the sacral and coccygeal plexuses.
- *sacral plexus** – network formed by the ventral branches of spinal nerves L4-S3
- *coccygeal plexus** – smaller plexus.
describe the functions of the major sensory and motor tracts of the spinal cord.
White matter tracts – highways for nerve impulse propagation.
1. Sensory input travels along these tracts toward the brain
2. Motor output travels from the brain along these tracts toward
skeletal muscles and other effector tissues
Gray matter – receives and integrates incoming and outgoing
information.
Lateral and anterior spinothalamic tracts - bottom left border of the anterior
facing H of the spinal cord image.
Sensory (ascending) tract that conveys information up the spinal cord to the thalamus for sensations of pain, temperature, crude touch, and deep pressure.
(Spinothalamic – spine to thalamus = ascending)
direct motor pathways - collections of upper motor neurons with cell bodies in the motor cortex that project axons into the spinal cord, where they synapse with lower motor neurons or interneurons in the anterior horns. AKA pyramidal
pathways.
-Include lateral corticospinal, anterior corticospinal, and corticobulbar tracts
-Convey nerve impulses that originate in the cerebral cortex and are destined to cause voluntary movements of skeletal muscles
IV. indirect motor pathways - motor tracts that convey information from the brain down the spinal cord for automatic movements, coordination of body movements with visual stimuli, skeletal muscle tones and posture, and balance. AKA
extrapyramidal pathways.
-Include rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal,
and medial reticulospinal tracts
-Convey nerve impulses from the brain stem to cause automatic
movements and help coordinate body movements with visual stimuli.
-Also maintain skeletal muscle tone, sustain contraction of postural
muscles, and play a major role in equilibrium by regulating muscle tone in response to movements of the head.
Intercostal Nerves
intercostal or thoracic nerves – a nerve supplying a muscle located between the ribs. The anterior rami of spinal nerves T2-T12 connect directly to the structures they supply in the intercostal spaces. Do not enter plexuses.
a. T2 – intercostal muscles of second intercostal space, skin of the axilla and posteromedial aspect of the arm
b. T3-T6 – extend along the costal grooves of the ribs and then to the intercostal muscles and skin of the anterior and lateral chest wall
c. T7-T12 – supply the intercostal muscles and abdominal muscles along with overlying skin.
d. The posterior rami of the intercostal nerves supply the deep back muscles and skin of the posterior aspect of the thorax.
describe the functional components of a reflex arc
reflex arc – the pathway followed by nerve impulses that produce a reflex.
Contains 5 functional components:
-
sensory receptor - the distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor.
-It responds to a specific stimulus by producing a graded potential called a generator (or receptor) potential.
-If a generator potential reaches the threshold level of
depolarization, it will trigger one or more nerve impulses in the sensory neuron. - Stimulus - a change in the internal or external environment that excites a sensory receptor, a neuron, or a muscle fiber; any stress that changes a controlled condition -
sensory neuron - the nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem.*a cell body in the posterior root ganglion
-From here, relay neurons send nerve impulses to the area of the brain that allows conscious awareness that the reflex has occurred. -
integrating center - one or more regions of gray matter within the CNS acts as an integrating center.
-In the simplest type of reflex, the integrating center is a single synapse between a sensory neuron and a motor neuron. receives sensory information and decides how to respond to a change in the body’s condition
-monosynaptic reflex arc - a reflex pathway having only one synapse in the CNS – Less common type
-polysynaptic reflex arc - involves more than two types of neurons and more than one CNS synapse. The integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron
- motor neuron - impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond.
-
effector - the part of the body that responds to the motor nerve
impulse, such as a muscle or gland, is the effector. - Its action is called a reflex.
- If the effector is skeletal muscle, the reflex is a somatic reflex
- If the effector is smoother muscle, cardiac muscle, or a gland,
the reflex is an autonomic reflex.
Describe the ways reflexes maintain homeostasis.
*NOT A FUNCTION of spinal reflex that uses muscles spindles and tendon organs as sensors - Protection of spinal nerves
stretch reflex –
causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle
a. Occurs via a monosynaptic reflex arc
b. The reflex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron.
c. Can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints.
1. Ex. Patellar reflex (knee jerk)
muscle spindle – an encapsulated proprioceptor in a skeletal muscle, consisting of specialized intrafusal muscle fibers and nerve endings. Stimulated by change in length or tension of muscle fibers.
Steps in the stretch reflex:
1. Stretching of muscle stimulates muscle spindles (sensory receptors) which monitor changes in the muscle length
2. Muscle spindle responds to the stretch by generating one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord.
3. In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with, and thereby activates, a motor neuron in the anterior gray horn.
- ACh released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector) and the muscle contracts. Thus, muscle stretching is followed by muscle contraction which relieves the stretching. *contraction of the antagonist, relaxation of the agonist*
ipsilateral reflex arc a reflex arc in which sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it. (All monosynaptic reflexes are ipsilateral.)
- *muscle tone** – the degree of muscle contraction present while the muscle is at rest.
a. The brain sets muscle tone by adjusting how vigorously a muscle spindle responds to stretching.
b. Helps avoid injury by preventing overstretching of muscles - *reciprocal innervation** – the type of arrangement in which components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonists.
a. Prevents conflict between opposing muscles
b. Vital to coordinating body movements - *tendon reflex** – feedback mechanism that controls muscle tension by causing muscle relaxation before force becomes so great that tendons might tear.
a. Less sensitive than the stretch reflex, but can override the stretch reflex when tension is great - Ex. Dropping a very heavy weight.
- Ipsilateral.
tendon organ or Golgi tendon organ – the sensory receptors for the tendon reflex. Located within a tendon near its junction with a muscle.
- Detect and respond to changes in muscle tension that are caused by passive stretch or muscular contraction.
Steps in the tendon reflex:
- As tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold)
- Nerve impulses arise and propagate into the spinal cord along a sensory neuron
- Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron
- The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses
- The muscle relaxes and relieves excess tension *contraction of the antagonist, relaxation of the agonist*
- *flexor or withdrawal reflex** – another reflex involving a polysynaptic reflex arc
a. Ipsilateral, incoming and outgoing impulses propagate to and from same side of spinal cord.
b. This reflex is protective, because contraction of flexor muscles moves a limb away from the source of possibly damaging stimulus.
c. Ex. Step on a tack and withdraw leg in response to the painful stimuli - *intersegmental reflex arc** – where nerve impulses from one sensory neuron ascend and descend in the spinal cord and activate interneurons in several segments of the spinal cord.
a. Ex. In flexor reflex, several segments of spinal cord are activated by interneurons, resulting in whole muscles of entire leg to react and withdraw from painful stimuli.
crossed extensor reflex – where the side of the body opposite the painful stimuli reacts to offset or balance the body part reacting from painful stimuli.
- *contralateral reflex arc** – where motor impulses leave the spinal cord on thesideo pposite the entry of sensory impulses
a. Ex. The crossed extensor reflex.
