Spinal Cord Organization and Neural Systems Flashcards
How many pairs of spinal nerves are there?
There are 31 pairs of spinal nerves
8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal
Where does the spinal cord end? Where do the meninges end?
The spinal cord ends at the level of the L2 vertebra at the conus medullaris. The cauda equina contains the subsequent spinal nerves that exit at each vertebrae. The meninges extend to the level of the S2 vertebra.
Describe the basic anatomy of the spinal cord: what is grey matter and white matter? What is in each horn? Where do spinal nerves enter and exit?
Grey matter contains cell bodies of neurons while white matter contains axonal projections organized into tracts.
The grey matter has a pair of dorsal horns (alar plate), which contain sensory neurons, and ventral horns, which contain the cell bodies of motor neurons.
Spinal nerves have mixed motor and sensory fibers–sensory fibers are pseudounipolar cells that have their cell bodies in the dorsal root ganglion (neural crest) and enter into the doral horn. Motor neurons have their cell bodies in the ventral horn and project to the periphery.
Describe Ia and Ib fibers. What division are they a part of, what type of sensory information do they carry, and where do they synapse?
Ia and Ib fibers: heavily myelinated fibers which carry proprioceptive information from muscle spindles and Golgi tendon organs (respectively) into the dorsal horn via the medial division. They project collaterals into the dorsal column and into the ventral horn for reflex contraction.
Ia fibers wrap around both static and dynamic intrafusal fibers and convey proprioceptive information about ongoing changes in movement and maintained muscle strecthes–they fire fastest when the muscle is actively stretching.
Ib fibers innervate Golgi tendon organs and fire when the muscle contracts (i.e., when the tendon is stretched).
Describe type II and A-beta fibers. What division are they a part of, what type of sensory information do they carry, and where do they synapse?
Type II fibers: heavily myelinated fibers which carry proprioceptive information from static intrafusal muscle spindles regarding maintained muscle stretch. It’s firing rate does not depend on rate of change of muscle length, only the immediate length of the muscle. It is part of the medial division and sends fibers into the dorsal column and ventral horn
A-beta fibers: heavily myelinated fibers which carry information about fine touch, pressure, and vibration from cutaneous mechanoreceptors (Meisner’s corpuscles, Pacinian corpuscles, Ruffini endings, and Merkel discs), through the medial divison, and send collaterals into the dorsal column and ventral horn.
Describe A-delta and C fibers. What division are they a part of, what type of sensory information do they carry, and where do they synapse?
A-delta fibers: small-diameter, thinly myelinated fibers with free cutaneous nerve endings that respond to sharp pain and cold sensation.
C-fibers: small-diameter, unmyelinated fibers with free cutaneous nerve endings that respond to dull pain and warm temperature.
A-delta and C fibers are part of the lateral division and synapse in the dorsal horn and the second order neuron decussates in the white anterior commisure and sends fibers up through the spinothalamic tract.
What are the two types of motor neurons that exit the ventral roots? What do they innervate?
Alpha motor neurons: synapse on the muscle (extrafusal fibers) to induce muscle contraction. Alpha motor neurons receive their inputs from descending upper motor neurons in the corticospinal tract.
Gamma motor neurons: innervate the intrafusal muscle fibers in muscle spindles. These neurons cause the tails of the muscle spindles to contract and maintain tension.
Describe the topographic relationship of motor neurons in the ventral horn.
Lower motor neurons that are medially located innervate more proximal muscles, while those that are laterally located innervate more distal muscles.
Will a lesion to a lower motor neuron present on the ipsilateral or contralateral side?
Ipsilateral: cell bodies and axons of lower motor neurons are located on the same side as their target.
Indicate the position of each tract in the spinal cord:
Describe the paths of the neurons involved in the Dorsal Column Medial Lemniscus Pathway. What type of neuron carries the information into the spinal cord and from what type of nerve endings do they originate?
The Dorsal Column Medial Lemniscus pathway conveys fine touch, vibration, two point discrimination, pressure, and proprioception from Ia, Ib, II, and A-beta fibers. This information originates in Meisner’s corpuscles, Pacinian corpuscles, Ruffini endings, Merkel endings, muscle spindles, and Golgi tendon organs.
The sensory information enters the dorsal horn and ascends in either the gracile (lower limb) or cuneate (upper limb) fasciculus and synapses in the gracile or cuneate nucleus in the medulla. The second order neuron decussates in the medulla and synapses on the ventral posterolateral nucleus in the thalamus. The third order neuron conveys this information to the post central gyrus in the parietal lobe.
Describe the paths of the neurons involved in the Spinothalamic tract and indicate the difference between the lateral and anterior spinothalamic tracts. What types of neurons are involved in conveying this information? What is the significance of Lissauer’s fasiculus?
The Spinothalamic tract conveys pain, temperature, and crude touch to the brain. A-delta and C fibers carry this information into the dorsal root via the medial divison and synapse on cell bodies in the dorsal root. These fibers may synapse on cell bodies in the spinal segment in which they entered or they may travel up or down 1-2 segments in Lissauer’s fasciculus before synapsing in the dorsal horn. These second order neurons decussate in the anterior white commissure and ascend in either the anterior spinothalamic tract (crude touch) or the lateral spinothalamic tract (pain and temperature). These neurons then synapse in the ventral posterolateral nucleus of the thalamus and the third order neurons project into the post central gyrus of the brain.
Describe the paths of the neurons involved in the corticospinal tract. Where do their cell bodies reside? Where do their projections synapse and decussate? What is the difference between the anterior and lateral corticospinal tracts?
Upper motor neurons in the primary motor cortex send projections through the brainstem and into the spinal cord. 75% of the axons cross to the contralateral side at the pyramidal decussation and descend in the lateral corticospinal tract; the other 25% descend on the ipsilateral side in the anterior corticospinal tract. The axons in the lateral corticospinal tract synapse in the ventral horn–cell bodies located most laterally project to distal limb muscles while those located more medially project to proximal limb structures. The axons descending in the anterior corticospinal tract decussate at the white anterior commissure and synapse on motor neurons that control axial muscles.
What is the dorsal spinocerebellar system? How many neurons are involved and where do they synapse and decussate? What is the purpose of this pathway?
The dorsal spinocerebellar system conveys proprioceptive information from the lower limb to the cerebellum. The sensory neuron synapses in Clarke’s nucleus and the second neuron carries information to the ipsilateral cerebellum via the inferior cerebellar peduncle. These axons do not decussate. The cerebellum uses this information to coordinate smooth execution of movement generated by skeletal muscles.
What is the cuneocerebellar system? How many neurons are involved and where do they synapse and decussate? What is the purpose of this pathway?
The cuneocerebellar system transmits proprioceptive information from the upper limb to the cerebellum. The sensory neurons project directly into the brainstem and synapse in the external cuneate nucleus which projects into the cerebellum via the inferior cerebellar peduncle. This input is ipsilateral and does not decussate. Its purpose is to coordinate skeletal muscle movement.
What is the rubrospinal tract?
The rubrospinal tract is a descending pathway that originates in the red nucleus of the midbrain, decussates in the midbrain, descends laterally near the corticospinal tract, and terminates in the cervical region. It can function in motor control of the upper limb.
What are the medullary (lateral) and pontine (medial) reticulospinal tracts?
The medullary reticulospinal tract originates in the medullary reticular nucleus and projects into the spinal cord where it synapses on interneurons that inhibit axial extensor muscles. The pontine reticulospinal tract originates in the pontine reticular nucleus and projects into the spinal cord where it synapses on interneurons that excite axial extensor muscles. Together they help coordinate automatic locomotion and posture.
What are the lateral and medial vestibulospinal tracts? Where do they synapse and terminate?
The medial vestibulospinal tract originates in the medial and inferior vestibular nuclei and terminates in the cervical and upper thoracic cord. The lateral vestibulospinal tract originates in the lateral vestibular nucleus and terminates in the entire cord.
The medial vestibulospinal tract functions in positioning of the head and neck while the lateral VST is involved in balance.
What is the tectospinal tract?
The tectospinal tract is perhaps involved in coordination of head and eye movement. It originates in the superior colliculus, decussates at the dorsal tegmental decussation in the midbrain, and terminates in the cervical cord.
What spinal cord injury is likely represented by the image below? What symptoms will the patient present with?
Complete Cord Transection
- Motor: loss of all motor function below the lesion (spastic plegia)
- Sensory: loss of all sensory modalities below the lesion
- Autonomic: bowel and bladder dysfunction (spastic with incontinence)
- Causes: trauma, compression (tumor, hematoma), transverse myelitis
- Outcome: high cervical require ventilatory support, sparing C7 retains ability to independently transfer
Describe the presentation of a central cord lesion. In what order are the fibers affected? What can cause this? What type of lesion is present in the image below?
- The image shows a cervical syrinx–a CSF filled cavity in the cervical spinal cord
- Cord damage starts centrally and spread centrifugally with crossing fibers of spinothalamic tract affected first (capelike loss of pain and temperature), followed by anterior horn cells at the level of the lesion (LMN findings)
- Causes: syringomyelia (with or without Chiari malformation–which is the partial herniation of the cerebellar tonsils through the foramen magnum), hematomyelia, intramedullary tumor
Describe posterior column syndrome. What can cause it? What neurons are lost?
- Posterior column syndrome or tabes dorsalis is caused by late stage syphilis
- Results in destruction of the dorsal column (loss of myelinated axons), causing imapired vibration and position sense (sensory ataxia) that is worse in the dark
- Positive Romberg’s sign
- Pains in legs, areflexia in legs, abnormal pupils, blindness (optic atrophy)
- Penicillin may arrest progression but not reverse deficit
What is posterolateral column syndrom? What symptoms does it present with? What is the most common cause?
Posterolateral column syndrome (subacute combined degeneration)
- Cause: B12 deficiency
- Results in myelin degeneration without inflammation
- Dorsal column dysfunction: loss of proprioception and vibration sense in legs, positive Romberg sign
- Corticospinal tract dysfunction: spasticity, hyperactive reflexes, Babinski sign
What is anterior horn cell disease? What are some causes of it?
- Causes lower motor neuron findings at the affected segment
- Spinal muscular atrophy can be inherited (SMA or Kennedy’s disease, which is bulbospinal muscular atrophy) or aquired (progressive muscular atrophy)
- Also caused by infectious agents: polio, West Nile virus, enterovirus 71, coxsackie A and B, Echoviruses
- Also caused by benign focal forms (monomelic amyotrophy)
What is combined anterior horn cell syndrome? Where is the disease classically located?
Also called pyramidal tract syndrome
- Amyotrophic lateral sclerosis is a form
- Combined UMN and LMN findings without alternative etiologies
What is the result of anterior spinal artery occlusion? What can cause it?
- Corticospinal tract: flaccid weakness initially (spinal shock) followed by spastic paraparesis
- Ventral horn: LMN abnormalities at the level of the lesion
- Spinothalamic tracts: loss of pain and temperature below the lesion
- Dorsal columns: spared (supplied by posterior spinal artery)
- Impaired bowel and bladder function
- Causes: aortic dissection, atherosclerosis, hypotension
What type of lesion is shown below? What are examples of this type of lesion?
Intramedullary lesion
- Ependymoma
- Astrocytoma
- Glioblastoma
- Myelitis
- Abscess
Initial symptoms reflect parenchymal involvement with early segmental sensory and motor findings.
What type of lesion is shown below? What types of tumors cause this?
Intradural extramedullary lesion
- Schwannoma
- Meningioma
Initial symptoms are extraparenchymal and reflect root compression. Myelopathy will develop with increasing tumor size.
What type of lesion is shown below? What can cause it?
Extradural lesion
- Disk disease/herniation
- Epidural metastasis
- Primary bone tumor
- Lymphoma
- Epidural abscess
Initial symptoms are extraparenchymal and may reflect root compression. Myelopathy may develop later. Clinical syndrome is the same as an intradural extramedullary lesion.
Describe the anatomy of a muscle spindle. What are the two types of intrafusal fibers? How are they innervated and what information do they convey?
The muscle spindle contains one or two dynamic intrafusal fibers and 8 or 9 static intrafusal fibers. The Ia sensory neursons wrap around the non-contractile region of all of the intrafusal fibers while the group II fibers innervate only the static fibers. During muscle stretch, the Ia fibers signal ongoing changes in length strongly while the group II fibers signal only maintained stretch. Local membrane stretch activated stretch sensitive ion channels to in sensory neurons which cause depolarization.
The intrafusal fibers are only contractile near the tip and are innervated by gamma motor neurons in order to maintain tension in the muscle spindle.
What is the Golgi tendon organ? How does it work and what does it signal?
The Golgi tendon organ is a cluster of extrafusal muscle fiber endings with collagen fibers bundles that connect them to the tendon. The dendrites of the sensory Ib axon are interspersed in this bundle. When the muscle contracts, the collagen bundles stretch and compress the dendrite branches, thus activating stretch sensitive ion channels and causing depolarization. Because not all extrafusal fibers are recruited in all instances of muscle contraction, the Ib sensory neurons in the Golgi tendon organs are able to signal different levels of contraction.
Describe the concept of reciprocal innervation with respect to motor neurons in spinal reflexes.
Sensory information entering the spinal cord have both excitatory and inhibitory connections with motor neurons (or their respective interneurons). This ensures that one muscle relaxes while the other contracts.
Describe the myotactic reflex.
The myotactic (stretch reflex) opposes muscle stretch and is the fastest somatic reflex, involving only a single synapse in the spinal cord. Muscle stretch stimulation causes sensory neurons (Ia) to stimulate motor neurons in the stretched muscle and inhibit alpha motor neurons going to the functionally anagonist muscle.
Describe the Golgi tendon organ reflex.
The Golgi tendon organ reflex opposes excessive contraction/tension in a muscle (signaled when all GTO’s are activated). It signals back to the spinal cord (Ib sensory fibers) to inhibit contraction of the muscle and stimulate contraction of the antagonist muscle. This also ensures maintenance of the intended joint position or tension level. This pathway receives CNS input.
Describe the withdrawal reflex.
The withdrawal reflex induces ipsilateral flexion and contralateral extension in response to a painful stimulus. Ipsilateral flexion is mediated by stimulating contraction of flexor muscles and inhibiting contraction of antagonist extensors. The same sensory stimulus projects across the spinal cord to stimulate extension and inhibit flexion in the contralateral limb in order to maintain balance. The strength of the stimulus determines the strength of the reflex. The sensory neurons that mediate this reflex are the A-delta and C fibers–more fibers activated, leads to more interneurons activated, which leads to a more forceful withdrawal.
