Sept 30 - Spinal Cord, Spinal Nerves and Plexuses Flashcards
Name the three meninges of the spinal cord
The dura mater
The arachnoid mater
The pia mater
Describe the dura mater of the spinal cord
It’s the outermost layer that is tough and fibrous and continous with dura of brain (through the foramen magnum) and ends at 2nd sacral vertebre. Unlike the brain, there is an epidural space between the dura and the spinal canal. It contains C.T., fat and veins
Describe the arachnoid mater of the spinal cord
Adherent to the dura. It is continuous with the arachnoid of the brain and ends at S2. The space between it and the pia is filled with CSF
What is the space between L1-2 and S2 called?
It’s called the lumbar cistern. It’s often used to access CSF without damaging the spinal cord (diagnostics, administering anesthetic)
Describe the pia mater of the spinal cord
It is the innermost layer that is adherent to the cord. It is continuous with the pia of the brain and ends with the dura at S2. It is thickened laterally as denticulate ligament (suspends cord with dural sac)
What is different about the meninges in the spinal cord, compared to the brain? (Name three differences)
The spinal cord is contained within a sac of dura but the dura is NOT attached to the bone of vertebral canal.
There is a “real” epidural space between the bone and the dura filled with fat, connective tissue and blood vessels.
Also, small “denticulate ligaments” that attach the spinal cord to the dural sac
Describe the location/enviromnent of the spinal cord
It is contained within a dura/arachnoid sac that extends from the foramen magnum down to the 2nd sacral vertebrae (S2). It is surrounded by CSF. The spinal cord ends as the conus medullaris between L1 and L2 vertebrae
What is the cauda equina?
Lumbar, sacral and coccygeal nerve roots that extend to exit the dural sac at a lower level
What is the filum terminale?
An extension of the pia that tethers the spinal cord to the end of the dural sac (anchor)
There are 2 enlargements of the spinal cord. Where are they? What are they called?
The cervical enlargement and the lumbosacral enlargement. The cervical enlargement of the cord is found between C4 and T1 and it is the site of innervation of the arm. The lumbosacral enlargement is found between L2 and S3 and it is the site of innervation of the leg
Describe the roots of the cervical enlargement of the spinal cord
They are short and horizontal
Describe the roots of the lumbosacral enlargement of the spinal cord
They are long and vertical forming the cauda equina
Describe the internal structure of the spinal cord
It consists of white matter (ascending and descending axons) surrounding the butterfly-shaped gray matter. The gray matter consists of a “dorsal horn” (sensory) and a “ventral horn” (motor)
Describe the role of the dorsal horn of the gray matter of the spinal cord
It is sensory; it processes and relays sensory information to the brain. Sensory information (touch, temperature, pain, proprioception, etc.) enters the cord via the dorsal root, which contains afferent fibres
Where are the cell bodies of sensory neurons found?
They are located in the dorsal root ganglion (DRG)
Describe the role of the ventral horn of the gray matter of the spinal cord
It contains large motor neurons that project to muscle (via the ventral root); referred to as motor efferents. The motor neurons are innervated by descending fibres (from the brain) carried in white matter
Describe the white matter of the spinal cord
It consists of nerve fibres, neuroglia and blood vessels (not neuronal cell bodies). It’s white because of the high fat content (myelin)
Fibres arise from cell bodies located in: ? (name three places
Brain: descending axons from cerebrum and brainstem
Spinal cord: Ascending axons from gray matter of spinal cord (dorsal horn)
Periphery: Ascending axons from the dorsal root ganglion
Comment on the arrangement of the axons in the spinal cord
The axons arranged in tracts/fasciculi that have a similar origin, termination and function (sensory/motor). Sensory information travels up the cord to the brain; sensory fibres are added to sensory pathways as the ascend, e.g., at the thoracic level, sensory fibres from the leg are ascending the cord, but sensory fibres from the arm have not yet entered the cord. Motor activity travels down the cord to the neurons in the gray matter of the cord
Where do you have the maximum amount of white matter
At the very top of the spinal cord
Describe the gray matter of the spinal cord
Butterfly- or H-shaped. It consists mainly of neuronal cell bodies and neuroglia. The dorsal horn receives sensory input via the dorsal root; it is the site of interneurons and projection neurons (project axons to the brain). The ventral horn has a motor function; it sends projections to skeletal muscles via the ventral root
Describe the nerves and roots of the spinal cord
There are 31 pairs of spinal nerves that leave the cord to pass through the intervertebral foramen and exit as peripheral nerves. Each nerve is attached to the spinal cord by a ventral root and a dorsal root
How are the spinal nerves named?
They are named according to their association with the vertebral column; there are 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal nerves
What is particular about the naming of the cervical nerves?
The cervical nerves exit ABOVE the vertebrae, except number 8, which exits below the 7th cervical vertebrae. The remaining nerves exit below the vertebrae.
Where are the cell bodies of the motor neurons?
They are found in the ventral horn of the spinal cord
The brain receives sensory information from a number of sensory systems. Explain
Receptors convert stimuli into action potentials. These action potentials enter the CNS via the dorsal roots of the spinal nerves. These afferent signals may be used in reflexes or the signals may be relayed via the neurons in the spinal cord or brainstem to higher centres
Name three different stimuli that the sensory system is capable of detecting
Mechanical stimuli (by mechanoreceptors), pain (by nociceptors) and temperature (by thermoreceptors)
Name different types of mechanoreceptors
Neuromuscular spindles and golgi tendon organs
What are the roles of neuromuscular spindles?
They are receptors for stretch, they respond to change in length (basis for stretch reflex, i.e., knee jerk)
How are Golgi tendon organs stimulated? What is their role?
They are stimulated by tension in tendons (protection against damage from excessive stretch)
Where are the cell bodies of sensory neurons in the PNS?
In the DRG. One process of the sensory cell projects from the spinal nerve into the periphery where it ends in a sensory receptor while another process projects to the CNS (spinal cord). Signal travels from the receptor to the CNS (i.e., afferent)
Explain the production of efferent signals
Descending fibres from the cerebral cortex and brainstem active the motor neurons of the ventral horn of the spinal cord that innervate skeletal muscle to produce a voluntary (and reflexive) movement. Efferent signals can also exit the brain through cranial nerves
Explain the travel of the dorsal and ventral roots from the spinal cord
Both roots pass through the intervertebral foramina to then join and form a spinal nerve (becomes a mixed nerve). They exit the foramen as a spinal nerve and divide into a dorsal ramus (smaller) and a ventral ramus (larger). They supply the myotomes and dermatomes that encircle the body
What is a myotome?
The group of muscles that a single spinal nerve root innervates with motor fibres
What is a dermatome?
The area of skin that is innervated by sensory fibres of a single spinal nerve root
Where do the dorsal rami go?
They travel posteriorly around the vertebral column to innervate muscles and skin of the back
Where do the ventral rami go?
They travel anteriorly to innervate muscles and skin of the anterolateral body wall and limbs. The ventral rami join together in a complex plexus of fibres in the cervical (brachial) and lumbar (sacral) regions
What is a nerve plexus?
It is formed by ventral rami of spinal nerves. Nerve fibres pass from one to another peripheral nerve and it results in a mixture of nerve fibres (from more than one spinal nerve) all bundled together in one peripheral nerve
Name four plexuses
Cervial, brachial, lumbar and sacral
What is the cervical plexus made up of? What does it innervate?
C1-C4. Motor innervation of muscles of neck and diaphragm. Sensory innervation of neck, diaphragm and ajacent peritoenum
What is the brachial plexus made up of? What does it innervate?
C5-T1. Motor and sensory innervation for shoulder and upper limb. It is the largest plexus
What is the lumbar plexus made up of? What does it innervate?
T12-L4. Motor for hip muscles, quadriceps and sartorious muscles. Sensory innervation of the lower anterior abdomen, ventral thigh and knee
What is the sacral plexus made up of? What does it innervate?
L4/L5-S3. Motor for pelvis, perineum and most muscles of the lower limb (not quadriceps or sartorious). Sensory for perineum and most of the lower limb
What is the potential consequence of peripheral nerve damage?
Motor and/or sensory changes. Flaccid paralysis followed by muscle degeneration (motor). Loss of cuteaneous sensibility (pinprick test; sensory)
