Lecture 4: Nervous system III Flashcards
How are spinal nerves named?
According to their association with vertebral column
8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral
See figure
How is each nerve attached to the spinal cord?
By a ventral root and a dorsal root
See figure
Where does brain receive sensory info from?
Somatosensory systems
What happens after brain receives sensory info?
Receptors convert stimuli into action potentials that travel down the axons that enter the CNS
What root of the spinal nerves does sensory info enter the CNS?
Dorsal roots
What are afferent signals in the CNS used for?
In reflexes
Or signals may be relayed via neurons in spinal cord or brainstem to higher centres
What roots of spinal nerves carry motor impulses?
Ventral
What kind of stimuli is sensory system capable of detecting?
Mechanical stimuli (mechanoreceptors)
Pain (nociceptors) and
Temperature (thermoreceptors)
Mechanoreceptors (many types)
Types of mechanoreceptors
Neuromuscular spindles - receptors for stretch, respond to change in length (basis for stretch reflex i.e. Knee jerk)
Golgi Tendon organs - stimulated by tension in tendons (protection against damage from excessive stretch)
Where are the cell bodies of the sensory neurons of the PNS?
Dorsal root ganglia
Where do the processes of the sensory cells project?
One process of the sensory cell projects (in the spinal nerve) to the periphery where it ends in a sensory receptor
one process projects to CNS (spinal cord).
Afferent signal transduction
Signal (converted to action potential by receptor) travels from receptor to CNS
See figure
What root contains efferent fibres?
Anterior Root = Ventral Root = Motor Root
action potentials are leaving the cord
What do motor neurons innervate?
cell bodies are in the ventral horn of the cord
innervate skeletal muscle (via motor fibres)
cause contractions of the muscle (LMNs)
What do descending fibres from the cerebral cortex and brainstem activate?
Motor neurons
produce voluntary (and reflexive) movement
Where do the ventral and dorsal roots pass through?
Intervertebral foramina
They then join together and form a short spinal nerve (becomes a mixed nerve)
Supply myotomes and dermatomes
What is a myotome?
refers to muscles innervated by the motor fibres of a spinal nerve
What is a dermatome?
area of skin innervated by the sensory fibres of a spinal nerve
What would peripheral nerve injury cause (motor changes)?
flaccid paralysis followed by muscle
degeneration
What would peripheral nerve injury cause (sensory changes)?
loss of cutaneous sensibility (pinprick test)
However: the injury of one spinal cord segment
or spinal nerve rarely causes the loss of function of a whole muscle or loss of sensitivity in a dermatome
What does spinal nerve or root compression cause?
Prolapse of the nucleus pulposus
Causes herniation
Postero-lateral disc herniation
Most common herniation
Causes lower back pain, pain in buttock, thigh, leg
Prolapse of the L4/L5 disc causes compression of the L5 root
see figure
Postero-medial disc herniation
see figure
Where does the spinal cord extend from?
From foramen magnum to L1-L2
What structure marks the end of the spinal cord?
Conus medullaris (between L1 and L2)
See figure
What tethers the cord to the end of the dural sac
The filum terminale
Extension of the pia
What is below the conus medullaris?
Cauda equine
What are the two enlargements of the spinal cord
Cervical and lumbosacral
Where is cervial enlargement? What does it innervate?
C4-T1 of sc
Arm
Where is lumbosacral enlargement? What does it innervate?
L2-S3 of sc
Innervation of leg
Root length in cervical and lumbosacral region
Cervical - short and horizontal
Lumbosacral - cauda equina (horses tail)
What is caudal equina contained within?
Subarachnoid space