Spinal Cord Chapter 12 - continued. The Spinal Cord n The spinal cord extends from the foramen...

Spinal Cord

Chapter 12 - continued

The Spinal Cord The spinal cord

extends from the foramen magnum to the level of the 1st or 2nd lumbar vertebrae

It is enclosed within the vertebral column


is a provides a two way conduction pathway to and from the brain

It is a major reflex center

The Spinal Cord

The spinal cord is protected by bone, cerebrospinal fluid, and meninges– Dura mater, arachnoid, pia mater

The Spinal Cord

Between the bony vertebrae and the dural sheath is a large epidural space filled with a soft padding of fat and a network of veins

Cerebrospinal fluid fills the subarachnoid space

The Spinal Cord Inferiorly, the

dural and subarachnoid membranes extend to the level of S2 while the spinal cord ends at L1

Subarachnoid space beyond L1 is an ideal site for a spinal tap


terminates in a tapering cone shaped structure called the conus medullaris

The Spinal Cord A fiberous

extension of the pia mater, the filum terminale extends inferiorly from the conus medullaris to attach to the posterior surface of the coccyx

The Spinal Cord There are 31 pairs of spinal nerves that

arise from the cord by paired roots and exit from the vertebral column via the intervertebral formina

Each segment of the spinal cord is defined by a pair of spinal nerves that lie just superior to their corresponding vertebra


has obvious enlargements where the nerves serving the upper and lower limb arise– Cervical

enlargement

– Lumbar enlargement

The Spinal Cord Because the cord does not reach the end

of the vertebral column, the lumbar and sacral spinal nerve roots angle sharply downward and travel inferiorly before reaching their intervertebral foramina

This collection of nerve roots at the inferior end of the vertebral canal is called the cauda equina

The arrangement reflects the fact that during vertebral column grows more rapidly than does the spinal cord

Embryonic Development

The spinal cord develops from the caudal portion of the embryonic neural tube

By the end of the 6th week each side of the developing cord has two clusters of neuroblasts that have migrated outwarded from the neural tube


The two clusters are the dorsal alar plate and a ventral basal plate

Alar plate neurons become interneurons

The basal plate neurons become motor neurons that sprout axons that grow out to the effector organs

Embryonic Development Axons that emerge

from alar plate cells form the external white matter of the cord by growing outward along the length of the CNS

The alar plates expand dorsally and the basal plates expand vertically to become the H-shaped mass of gray matter


Neural crest cells that come to lie alongside the cord form the dorsal root ganglia containing sensory nerve cell bodies, which send their axons to the dorsal aspect of the brain

Neural crest cells

Cross Section of Spinal Cord

The spinal cord has two grooves that mark its surface – Anterior median fissure / Posterior medial sulcus

Gray Matter and Spinal Roots

These grooves run the length of the cord and partially divide it into right and left halves

Gray matter inside, the white matter outside


The gray matter consists of a mixture of neuron cell bodies, their unmyelinated processes, and neuroglia (support cells)


The white matter is composed of myelinated and unmyelinated nerve fibers that represent ascending, descending and transverse pathways


The gray matter consists of mirror-image lateral gray masses connected by a cross-bar of gray matter called the gray commissure that encloses the central canal


The two posterior projections of gray matter are the posterior (dorsal) horns; the anterior pair are the anterior (ventral) horns with lateral horns in the lumbar and thoracic portions of the cord


The anterior horns house nerve cell bodies of the somatic motor neurons

These send their axons out via ventral roots of the spinal cord to the skeletal muscles

Gray Matter and Spinal Roots The amount of ventral gray matter

present at a given level of the spinal cord reflects the amount of skeletal muscle innervated at that particular level

Thus, the anterior horns are the largest in the areas where the innervation for limbs is present– Cervical enlargement / arms– Lumbar enlargement / legs


The lateral horn neurons are autonomic (sympathetic) motor neurons that serve the visceral organs

Their axons also leave the cord via the ventral root


Afferent fibers carrying impulses from peripheral sensory receptors form the dorsal roots of the spinal cord


The cell bodies of the associated sensory neurons are found in an enlarged region of the dorsal root called the dorsal root ganglion or spinal ganglion


After entering the cord, the axons take a number of routes

Some enter the posterior white matter of the cord or brain, others synapse with interneurons


The dorsal and ventral roots are very short and fuse laterally to form the spinal nerves which are then considered part of the peripheral nervous system (PNS)


The spinal gray matter can be divided further according to its neurons relative involvement in the innervation of the somatic and visceral regions of the body

The four zones are; somantic sensory (ss); visceral sensory (vs); visceral motor (vm); somatic motor (sm)

White Matter

The white matter of the spinal cord is composed of myelinated and unmyelinated nerve fibers that allow communication between different parts of the spinal cord and between the cord and the brain

White Matter

Nerve fibers run in three directions– Ascending / up to higher centers (sensory inputs)

– Descending / down to the cord from the brain or from within the cord to lower levels (motor outputs)

– Transversely / across from one side of the cord to the other (commissural fibers)

White Matter

The ascending and descending tracts make up most of the white matter of the spinal cord– Ascending tracts are shown in blue and labeled at left

– Descending tracts are shown in red and labeled at right

White Matter

The white matter on each side of the column is divided into three white columns or funiculi and labeled according to their position (posterior, lateral, anterior)

Each funiculi contains several fiber tracts, and each tract is made up of axons with similar destinations and functions

White Matter

All major spinal tracts are actually part of multi- neuron pathways that connect the brain to the body

These ascending and descending pathways contain not only spinal cord neurons but also parts of peripheral neurons and neurons in the brain

White Matter Generalizations about spinal pathways

– Most pathways cross over from one side of the CNS to the other at some point

– Most consist of a chain of two or three neurons that contribute to successive tracts

– Most exhibit somatotopy, a precise spatial relationship among the tract fibers that reflects the orderly mapping of the body

– All pathways and tracts are paired (right and left) with a member of the pair on each side of the spinal cord or brain

Ascending (Sensory) Tracts The ascending pathways conduct sensory

impulses upward, typically through chains of three successive neurons (first-, second, and third-order neurons) to various regions of the brain

Most of the incoming information results from stimulation of – General sensory receptors

• Touch / pressure / temperature / pain

– Stimulation of proprioceptors• Muscle stretch / tendon / joint

Ascending (Sensory) Tracts In general, sensory information is conveyed

along six main pathways on each side of the spinal cord– Four transmit impulses to the sensory cortex for

conscious interpretation• Fasciculi cuneatus

• Fasciculi gracilis

• Lateral spinothalamic tract

• Anterior spinothalamic tract

– Two transmit impulses to the cerebellum to coordinate muscle activity

• Anterior spinocerebellur tract

• Posterior spinocerebellur tract

Ascending (Sensory) Tracts

Posterior funiculi (dorsal white column)– Fasciculi cuneatus

– Fasciculi gracilis Transmit information from the fine touch and pressure

receptors and joint proprioceptors– These tracts comprize what is referred to as discriminative

touch and conscious proprioception


Lateral and anterior funiculi– Lateral spinothalamic tract

– Anterior spinothalamic tract Convey information on pain, temperature, deep

pressure and course touch (undiscriminated)


Anterior and posterior funiculi– Anterior spinocerebellar tract

– Posterior spinocerebellar tract Convey information from proprioceptors (muscle and

tendon stretch) to the cerebellum which uses this information to coordinate skeletal muscle activity

Ascending (Sensory) Tracts Since the spinocerebellar tracts do not

terminate in the cortex, these pathways do not contribute to conscious sensation

The spinocerebellar tracts do not decussate and thus contribute to ipsilateral innervation

Descending (Motor) Tracts

The descending motor tracts that deliver impulses from the brain to the spinal cord are divided into two groups– Pyramidal tracts

– All others

Descending (Motor) Tracts Motor pathways involve two neurons,

referred to as upper and lower motor neurons

The pyramidal cells of the motor cortex, as well as the neurons in subcortical motor nuclei that give rise to other descending motor pathways, are called upper motor neurons

The anterior horn motor neurons, which actually innervate the skeletal muscles are called lower motor neurons


The lateral (pyramdial) and anterior corticospinal tracts are the major motor pathways concerned with voluntary movement, particularly precise or skilled movement


The pyramdial tracts are also called the direct pathways because their axons descend without synapsing from the pyramidal cells of the primary motor cortex all the way to the spinal cord

Descending (Motor) Tracts Pyramidal tracts synapse primarily with

interneurons, but also directly with anterior horn motor neurons, principally those controlling limb muscles

The anterior horn motor neurons activate the skeletal muscles with which they are associated


The remaining descending tracts include:– Rubrospinal

– Anterior reticulospinal

– Lateral reticulospinal

– Vestibulospinal

– Tectospinal


The remaining tracts originate in different subcortical motor nuclei of the brain stem

These tracts were formerly lumped together as the extrapyramidal tracts

The current term is to label them indirect pathways or just the names of the individual pathway

Descending (Motor) Tracts Although the cerebellum coordinates

voluntary muscle activity, no motor efferents descend directly from the cerebellum to the spinal cord

The cerebellum influences motor activity by acting through relays on the motor cortex

Spinal Cord Trauma Damage to the spinal cord is associated

with some form of loss of function– Paralysis / loss of function– Paresthesis / sensory loss– Flaccid paralysis / motor loss– Spastic paralysis / upper motor neuron loss

Body regions below lesion– Quadriplegia / spinal cord injury - 4 limbs– Paraplegia / spinal cord injury - 2 limbs– Hemiplegia / brain injury - one side of body

Developmental Aspects of CNS Fetal alcohol syndrome Cerebral palsy Anencephaly (without brain) Spina bifida (forked spine)

Spinal Cord Chapter 12 - continued. The Spinal Cord n The spinal cord extends from the foramen...

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Transcript of Spinal Cord Chapter 12 - continued. The Spinal Cord n The spinal cord extends from the foramen...