The Spinal Cord
The spinal cord is continous with the medulla in the brain stem and runs from the base of the skull to the body of the first vertebra. It contains 31 segments that each have a motor and sensory nerve root, which combine to form 31 pairs of spinal nerves.
- Spinal Cord anatomy
- spinal cord cross sections
- problems with the spinal cord
- development of the Spinal Cord
Spinal Cord Anatomy
about as round as your finger
begins at foramen magnum and extends to L1/L2, at which point the conus medullaris tapers into the filum terminalis, a connective tissue tether. begins and forms the cauda equina.
The spinal cord is covered with pia, arachnoid, and dura. The epidural space contains blood vessels and fatty tissues. Denticulate ligaments, continuous with pia, connect the spinal cord with the meninges.
Spinal Nerves
There are 31 pairs of spinal nerves leaving the spinal cord.
Spinal Cord Tracts
Gray Matter (spinal cord nuclei)
Spinal cord nuclei are found in the gray matter, and are sensory and motor, and autonomic.
Sensory Nuclei
- receives information through Lissauer's tract
- extends logitudinally throughout spinal cord, capping dorsal horn
- relatively small neurons
- stains more lightly becuase it deals with finely myelinated/unmyelinated fibres related to pain
- local processing nucleus; synapses with other nuclei, which send signals further
- continuous in medulla and pons with spinal nucleus of the trigeminal
- at least some send denrtitic trees into substantia gelatinosa
- information passes from SG and ascend
- involved in transmitting info over longer distances
- largely in thoracic segments
- proprioceptive, especially of the lower limb
- posterior cerebellar tract ascends ipsilaterally to cerebellum
- located in intermediate gray of T1-L3
- sympathetic preganglionic neurons
- replaced by parasympathetic neurons in S2-S4
Motor Nuclei
- lower alpha motor neurons
- large cell bodies - 40-60 um across
- found at almost every level of the sc
- distributed to axial skeleton
- found primarily cervically and lumbosacrally; due to their number they form enlargements
- innervate limbs
- flexors tend to be dorsal, while extensors are ventral (as in CNS)
Descending Systems
Efferent fibres leave the spinal cord through ventral roots. They include:
- motor neurons, whose bodies are in the ventral gray horn
- most important of these is the corticospinal tract
- preganglionic autonomic neurons, whose bodies are in lateral gray horns
Ascending Systems
There is a tendency to think about sensory functions as being segragate, ie spinothalamic vs dorsal column). This is asctually not the case, and most kinds of sensory information reach the thalamus and cerebellum by more than one route. As a result, loss of single tract can often be compensated for , often to a very large degree.
Asecending systems convey sensory information. Afferent neurons, entering the dorsal root, have their cell bodies in dorsal root ganglia.
In general, ascending fibres synapse prior to decussating (crossing to the contralateral side)
- spinothalamic tract - pain, temperature, crude touch
- dorsal column and medial lemniscal system - proprioreception, fine touch, vibration
- spinocerebellar tract - proprioreception
Problems with the Spinal Cord
The different points of decussation aids in determining whether a problem is in the brain stem or spinal cord. A brain stem lesion will lead to contralateral deficits in touch, pain, and proprioreception, while a spinal cord lesion will lead to contralateral loss of pain but ipsilateral loss of proprioreception and touch.
Spinal shock is the period of flaccid paralysis, lasting a few days following spinal cord transection. Deep tendon reflexes then return and become hyperreflexive. This is not well understood.
Brown-Sequard Syndrome is a rare but informative condition where half the spinal cord is transected. Tactile, vibratory, and proprioreceptive information is lost ipsilaterally from the dorsal column, while pain and temperature are lost contralaterally from the spinothalamic tract.
Syringomyelia is a disease of the central part of the spinal cord in which a tube progressively enlarges. Bilateral pain and temperature sensation is usually the first to go due to the crossed fibres, followed by weakness and muscle atrophy due to progressive anterior horn damage.