Neurophysiology 4 Flashcards
Describe the spinal cord
A major organ of the CNS which is tubular in structure and specialized into certain regions.
The spinal cord carries out sensory, integrative, and motor functions
Is an important structure between the body and the brain.
Continuous with the medulla up to the level of the 1st or 2nd lumbar vertebrae.
Forms a vital link between the brain and the body, and from the body to the brain
It has four regions: cervical (C), thoracic (T), lumbar (L) and sacral (S), each of which is comprised of several segments.
What are the functions of the spinal cord?
The spinal cord carries out sensory, integrative, and motor functions, which can be categorized as:
reflex, reciprocal activity (as one activity starts, another stops)
monitoring and modulation of sensory and motor mechanisms
transmission of impulses to the brain
How many spinal nerves are there?
There are 31 spinal nerve pairs:
8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal
All exit above their respective vertebrae from C1 down through C7.
C8 exits below C7 and above T1.
Thereafter, all spinal nerves exit below their respective vertebrae from T1 down through S5.
Describe, with reference to functionality, the cross sectional view of the spinal cord
The anterior midline is marked by the anterior median fissure, and the posterior midline is marked by the posterior median sulcus.
The terms dorsal (dorsal = “back”) and ventral (ventral = “belly”) are often used interchangeably with posterior and anterior, particularly in reference to nerves and the structures of the spinal cord.
The posterior regions are responsible for sensory functions and the anterior regions are associated with motor functions.
- This comes from the initial development of the spinal cord, which is divided into the basal plate and the alar plate.
- The basal plate is closest to the ventral midline of the neural tube, which will become the anterior face of the spinal cord and gives
rise to motor neurons.
- The alar plate is on the dorsal side of the neural tube and gives rise to neurons that will receive sensory input from the periphery.
Two consecutive rows of nerve roots emerge, from between the vertebra, on each sides of the cord.
• The ventral root is at the front of the spinal cord (also called motor or anterior root) and the dorsal root is at the back of the spinal cod (also called sensory or posterior root).
• The motor roots carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles.
• The sensory roots carry information to the brain from other parts of the body.
Describe the spinal nerves
• Corresponding dorsal and ventral roots join distally to form a spinal nerve.
• Each spinal nerve divides into a dorsal and a ventral ramus, and these are distributed to various parts of the body.
• Thus, a spinal nerve contains motor and sensory nerve fibers, to and from all parts of the body.
• Conus medullaris refers to the terminal part of the spinal cord.
• Below this is a collection of nerves known as the cauda equina, which is Latin for “horse’s tail”.
• The spinal cord terminates around L1-L2 of the vertebral column but the dura continues until S1-S2.
• Growth of vertebral column vs growth of spinal cord.
Describe the internal structure of the spinal cord
• A transverse section of the adult spinal cord shows white matter in the periphery, grey matter inside, and a tiny central canal filled with CSF at its center.
• A layer single layer of cells (ependymal) surrounds the canal.
• Surrounding the ependymal layer is the grey matter shaped like the letter “H” or a “butterfly”.
- This region contains cell bodies.
• The two “wings” of the butterfly are connected across the midline by the dorsal grey commissure and the anterior white commissure.
Describe the functional aspects of the grey matter within the spinal cord
• The posterior (dorsal) horn is responsible for sensory processing.
• The anterior (ventral) horn sends out motor signals to the skeletal muscles.
• The lateral horn, found only in the thoracic, upper lumbar, and sacral regions, contains cell bodies of motor neurons of the autonomic nervous system.
What are spinal cord laminae?
• The distribution of cells and fibers within the grey matter of the spinal cord exhibits a pattern of lamination.
• The cellular pattern of each lamina is composed of various sizes or shapes of neurons (cytoarchitecture) which led Rexed to propose a new classification based on 10 layers (laminae).
• This classification is useful since it is related more accurately to function than the previous classification scheme which was based on major nuclear groups.
List, with reference to function, the spinal cord laminae
• Laminae I to IV, in general, are concerned with exteroceptive sensation and comprise the dorsal horn. (Exteroceptive relates to stimuli that are external to an organism)
• Laminae V and VI are concerned primarily with proprioceptive sensations.
• Lamina VII is equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum.
• Laminae VIII-IX comprise the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle.
• Lamina X surrounds the central canal and contains neuroglia.
Describe the white matter aspect of the internal structure of the spinal cord
• Contains myelinated and unmyelinated nerve fibers.
• Just as the grey matter is separated into horns, the white matter of the spinal cord is separated into columns.
• These columns comprise nerve fibers that conduct information up (ascending) or down (descending) the spinal cord.
• Ascending tracts in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain.
• The columns of white matter include:
1. Dorsal (or posterior) column (or funiculus)
2. Lateral column
3. Ventral (or anterior) column
• The anterior white commissure resides in the centre, and it contains crossing nerve fibers that belong to the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts.
Describe the general nerve fiber types that can be distinguished in white matter part of the spinal cord
Three general nerve fiber types can be distinguished in the white matter:
1. Long ascending nerve fibers originally from the grey matter, which make synaptic connections to neurons in various brainstem nuclei, cerebellum and dorsal thalamus.
2. Long descending nerve fibers originating from the cerebral cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord gray matter.
3. Shorter nerve fibers interconnecting various spinal cord levels.
Explain the ratio of grey to white matter in the spinal cord
• In contrast to the cerebral hemispheres, grey matter is found in the interior, surrounded by white matter.
• The shape and size of the grey matter varies according to spinal cord level.
• At the lower levels, the ratio between grey matter and white matter is greater than in higher levels, mainly because lower levels contain less ascending and descending nerve fibers.
Therefore cervical has more white matter and less grey matter than sacral spinal cord
What layers surround the spinal cord?
• The spinal cord is bathed in cerebrospinal fluid (CSF) and surrounded by three protective layers called the meninges (dura, arachnoid, and pia mater) just as the brain is.
Where is a lumbar puncture done?
When conducting a lumbar puncture (spinal tap) the needle is inserted at L3/L4 or L4/L5 - low enough to avoid the spinal cord and high enough to be within the dural sac.
Epidural vs lumbar puncture
Describe the tracts within the spinal column
• The white matter of the spinal cord contains a mixture of ascending (sensory or afferent) and descending (motor or efferent) tracts.
• These tracts are bilaterally paired structures.
• Some cross the midline (decussate) at different levels to relay information to the side of the body, or from the side of the brain, on the side opposite to the point of origin.
