Spinal Cord Flashcards
Basic Terminology
A. White Matter
- Named because of the abundance of myelin - whitish color.
- Contains axons of nerve cells and neuroglial cells which surround and support the axons.
B. Gray Matter
- Tends to contain more nerve cell bodies - less myelin.
- Also contains neuroglia and some unmyelinated axons and dendrites.
C. Nerve
- A bundle of nerve fibers (axons) located outside of the CNS.
- Make up most of the PNS.
D. Ganglia - groups of nerve cell bodies not in the CNS - could be considered “Gray matter of PNS.”
E. Tract or Fasiculus
- Transmit impulses from one portion of the CNS to another.
- Those transmitting sensory information in the spinal cord - usually called ascending tracts.
- These transmitting motor information back toward muscles - descending tracts.
F. Nucleus - accumulations of nerve cell bodies within the CNS
1. Specific regions of gray matter - often associated with a single nervous system function.
Spinal Cord - Morphology (Gross Anatomy of)
A. An elongated cord-like structure which receives impulses from, and transmits impulses to the PNS.
B. Limited superiorly by foramen magnum of the skull and a junction with the medulla oblongata of the brainstem.
C. In the adult - approximately 16-18 inches in length.
D. Shows 2 enlargements:
1. Cervical enlargement - cervical region - corresponds with innervation of upper extremity
2. Lumbar enlargement - lumbar region
- corresponds with innervation of lower extremities
E. Cord tapers off into a cone-shaped structure of about the level of “L1” called the conus medullaris and is anchored inferiorly to the coccyx by a connective tissue ligament-like strand.
F. From about the level of L1 and below the solid portion of the cord is no longer present and one sees only roots of nerves coursing to their points of exit from the vertebral column. This is called the cauda equina (horse’s tail).
G. Divided into right and left halves by 2 grooves
1. anterior median fissure
2. posterior median sulcus
Spinal Cord - Coverings
A. Vertebral Canal
1. Formed by the foramina of all of the vertebrae.
2. Totally surrounds spinal cord in bone - important for protection.
B. Meninges (singular - Meninx) - tough connective tissue coverings that surround the entire CNS.
1. Dura Mater - outermost meninx
a. means “tough mother”
b. dense fibrous C.T. - toughest of the layers
2. Arachnoid - layer inside of the Dura Mater - Middle Meninx
a. means “spider” because of its similarity to a spider web in appearance.
b. very thin, easily torn.
3. Pia Mater - innermost meninx
a. means “tender mother” or “delicate mother”
b. adherent to the nervous tissue itself.
c. this layer also sends ribbon-like extensions out laterally to secure the spinal cord in place - these extensions pass through the arachnoid layer to the dura mater. They are called denticulate ligaments.
C. Subarachnoid Space
1. Area between the Arachnoid and Pia Mater
2. This is the space in which a fluid called Cerebrospinal Fluid (CSF) circulates and constantly bathes the CNS.
D. Meningitis - inflammation of the meninges
Lumbar Puncture
A. Involves removal of CSF - also called a “spinal tap”.
B. Usually done between the third and fourth or fourth and fifth lumbar vertebrae - (spinous process of 4th lumbar vertebra at level of superior portion of iliac crest).
C. Patient lies on one side and draws knees and chest together - this usually produces a slight parting of the vertebrae posteriorly.
D. Purposes:
1. to relieve intracranial pressure.
2. for diagnostic purposes (e.g. blood? - disease?).
3. for anesthesia (i.e. lumbar block during labor).
Internal Structure of the Spinal Cord Gray Matter
A. Dorsal Horn of Spinal Cord - cell bodies synapsed by afferent neurons (sensory).
B. Gray Commisure - Gray matter connecting 2 lateral halves of Gray matter.
C. Central Canal - Continuous thru spinal cord (this can be debated) - CSF circulates thru it.
D. Anterior Commisure - White matter connecting lateral regions of white matter - Axons are crossing.
E. Lateral Horn - associated with autonomic nervous system - nerve cell bodies, in thoracic and upper lumbar regions only.
F. Ventral Horn - cell bodies of efferent motor neurons.
Note: Ipsilateral = Same Side / Contralateral = Opposite Side
White Matter
A. Dorsal Columns - all ascending tracts - Sensory info. to brain.
B. Lateral Columns - contains some ascending and some descending tracts.
D. Ventral Columns - both ascending and descending tracts
Spinal Roots
A. A spinal nerve enters and exits the spinal cord via spinal roots.
B. Cells of the ventral horn give rise to axons which form the ventral root (efferent fibers) (motor).
C. The afferent fibers enter the dorsal horn of the spinal cord via the dorsal root (Sensory).
D. Dorsal Root Ganglion - swelling within the dorsal root which contains the nerve cell bodies of the “primary afferents”.
E. Primary Afferent - the nerve which comes to the spinal cord from the receptor. Usually a pseudounipolar cell with its cell body in the dorsal root ganglion.
Reflexes
A. A “reflex” is an automatic response of our body designed in a way to best enable us to avoid injury.
B. Reflexes don’t require the brain to integrate them and decide on an appropriate response. Instead the impulse need only go as far as the spinal cord and back. Reflexes involving only the spinal cord and its afferents and efferents are called Spinal Reflexes.
C. Stretch Reflex
1. monosynaptic reflex - this means that it only involves 2 neurons (1 synapse)
2. muscle contains tiny organs called neuromuscular spindles (“spindle fibers”) whose only function is to monitor stretch within a muscle.
3. an afferent neuron passes from the spindle to the spinal cord where it synapses with a motor neuron in the ventral horn.
4. the motor neuron sends an efferent stimulus back to the muscle causing it to contract, thus counteracting the stretch - example - “knee jerk” or patellar reflex.
D. Flexor Reflex and Crossed Extensor Reflex
1. polysynaptic reflex - involves more than one synapse and several neurons.
2. Flexor or Crossed Extensor Reflexes are an example of a polysynaptic reflex - they are also known as the “withdrawal reflex”. They involve withdrawing an extremity from a painful stimulus.
3. Like the simple monosynaptic stretch reflex, an afferent stimulus travels into the spinal cord via the dorsal root. However, instead of synapsing on just one motor neuron, it may have several synapses on “association neurons” (interneurons).
4. Ultimately one will see the response of several muscles to the stimulus. For example - withdrawing the foot from a sharp tack may involve several muscles (flexors) on the stimulated side, as well as extensors on the opposite side to shift the body’s weight.
E. Spastic Paralysis
1. “Upper Motor Neurons” = Neurons in motor cortex sending axons down to Lower Motor Neurons in spinal cord.
2. “Lower Motor Neurons”= Neurons in spinal cord sending axons to muscles
3. When Upper Motor neuron input to movement is damaged (Brain injury or stroke or spinal cord injury) patient presents like a “rag doll” with limp paralysis of affected muscles = “Flaccid Paralysis”.
4. As days pass, and recovery progresses, reflexes become hyper sensitive. Stretch reflex become hyper reactive. Health care worker is often frustrated thinking patient is fighting against them as they try to dress or undress the patient.
5. Patient is not purposefully causing this reaction, but health care worker is causing this reaction as moving the extremity stimulates stretch receptors that cause the muscle to contract and pull away from the worker trying to dress the patient.
6. Solution – Health care worker need to very gently an slowly move the extremity to prevent reflex contraction of extremity.
Spinal Nerves
A. The body has 31 pairs of spinal nerves.
B. They enter and exit the spinal cord via the dorsal and ventral roots - and pass as mixed nerves (both afferent and efferent) thru the intervertebral foramina.
C. A spinal nerve contains thousands of nerve fibers (axons).
D. Surrounding each fiber (whether it is myelinated or not) is a connective tissue sheath called the endoneurium.
E. These then are arranged in bundles called fascicles and each bundle is ensheathed in another connective tissue sheath called the perineurium.
F. Many of these fascicles then travel together as a bundle and are enclosed within an outermost layer of connective tissue surrounding the nerve called the epineurium.
G. Spinal nerves are identified by the vertebra above the intervertebral foramen from which they exist.
1. Example: Spinal nerve T6 is the nerve exiting from the intervertebral foramen at the base of vertebra T6.
2. The exception to this rule is with the cervical spinal nerves - there are only 7 cervical vertebrae, but there are 8 spinal nerves - this is because the first 7 exit from above the vertebra for which they are named, and the 8th from between the 7th cervical and the 1st thoracic vertebra.
3. There are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, and 5 sacral - thus there are 8 cervical spinal nerves, 12 thoracic spinal nerves, 5 lumbar spinal nerves, 5 sacral spinal nerves and 1 coccygeal pair.
H. Lateral to the vertebral column itself, a typical spinal nerve is divided into:
1. a dorsal ramus which innervates the deep muscles and skin on the posterior (dorsal) side of the body.
2. a ventral ramus which innervates all the structures of the extremities (this would include superficial muscles of the back) and all the structures of the lateral and anterior (ventral) portion of the body.
3. All ventral rami - with the exception of those from T2 to T12 - form interchanging networks with adjacent nerves that are called Plexuses. Peripheral nerves then flow out from (and into) these plexuses of nerves.
Pathology
Spina Bifida = Developmental condition where neural tube formed in the womb does not form completely along the whole length of the CNS. Can be a minor as an abnormal tuft of hair over the vertebral column at the caudal extent. Can, in some cases, result in newborn with spinal tissue protruding from the sacral region of the vertebral column. In its most severe form can be as serious as no development of the rostral portion of the nervous system, resulting in a child born with an incomplete skull and no brain (anacephaly).
