Deep to aracnhoid meninges
Contains nerve roots that compose cauda equina
Where are gray and white matter in the spinal cord?
Gray matter is the "H" in the center
White matter surrounds cord on the outside
(Note: this is opposite from the brain where gray matter is outside and white matter is inside)
Regions of the spinal cord
Gray matter: dorsal/ventral/lateral horns
White matter: columns, fasciculi
Note: dorsal = posterior; ventral = anterior
Sensory nerves with cell bodies in DRG go where?
Some go to dorsal horn, others go to dorsal column
(Also remember that sensory cells of DRG are pseudounipolar)
Motor neurons with cell bodies in ventral horn go where?
To skeletal muscle
Symp pre fibers from lateral horn (only T1-L2/3) go where?
To autonomic ganglia then to viscera
Spinal cord segments
Gray matter of the spinal cord--dorsal horn
"H" in middle of spinal cord
Gets input from DRG cells and neurons that participate in further processing of sensory information
Subdivision of dorsal horn into nuclei or laminae (Rexed's cyto-architectonic description)
Includes substantia gelatinosa (which has lots of cell bodies and is important for processing sensory information)
Gray matter of the spinal cord--ventral horn
Motor neurons that control skeletal muscles
Alpha motor neurons have large axons and innervate striated muscles ("final common path" of motor system)
Gamma motor neurons are smaller and innervate muscle spindle (sensory structure within skeletal muscles that contributes to muscle tone but does not directly cause contraction of skeletal muscle)
Motor neurons organized into columns or motor pools associated with certain muscle groups: proximal muscles medially in ventral horn, distal muscles laterally in ventral horn, flexor muscles dorsally in ventral horn, extensor muscles ventrally in ventral horn
Gray matter of the spinal cord--intermediate gray matter
Between dorsal and ventral horns
Contains interneurons that link sensory function to motor function
Many "polysynaptic" spinal reflexes involve interneurons in this region
Many descending pathways form contacts with neurons in this region
Note: interneurons form synaptic connections within same segment as well as in more distal segments of spinal cord
Gray matter of the spinal cord--lateral horn
Within intermediate zone of gray matter
Only in thoracic and upper lumbar segments
Mediates visceral motor function
From T1-L2 or L3, sympathetic preganglionic neurons form a column of cells (intermediolateral cell column) that occupies lateral horn --> axons exit thru ventral roots
Neurons in corresponding region of S2-4 form a sacral parasympathetic nucleus but do not form a distinct lateral horn
White matter of spinal cord
Contains columns (funiculi) in which axons ascend or descend
Dorsal columns: major sensory pathway divided into fasciculus gracilis (found at all levels and more medial) and fasciculus cuneatus (only in cervical and upper thoracic (to T6) levels; wedge-shaped)
Lateral and ventral columns: contain several motor and sensory tracts (location cannot be determined in conventional sections but has been shown experimentally)
Propriospinal system/fasciculus proprius: forms thin shell around gray matter; fibers interconnect different spinal cord levels
Why is amount of white matter greatest in cervical cord?
Everything has to synapse in the brain so there is the highest traffic of myelinated axons in the cervical region
Why are some ventral horns larger at some levels than others?
Some spinal segments innervate more muscle than others
Ex: cervical has big ventral horn because lots of muscles to supply but thoracic has small ventral horn because muscles don't need fine detail control
Anterior median septum
Break in spinal cord between two ventral horns
Where arterial supply to spinal cord is? Anterior spinal artery comes from 2 branches off vertebral artery that come together to form anterior spinal artery?
Classification of peripheral nerve fibers
Ia: sensory; muscle spindle primary endings
Ib: sensory; golgi tendon organs
alpha: motor; efferents to extrafusal muscle fibers
gamma: motor; efferents to intrafusal muscle fibers
Do reflexes require higher brain centers to operate?
"Spinal cord" reflexes!
Reflexes can be modulated by descending influences from more rostral brain regions, but do not require them
How many neurons do reflexes utilize?
One class of spinal cord reflexes uses only one synapse between sensory and motor elements (monosynaptic reflex)
However, most reflexes involve one or more interneurons within the pathway (polysynaptic reflexes)
Ways to classify axons within a peripheral nerve
Letters (for sensory and motor fibers): Group A-B are larger fibers that are myelinated and have highest conduction velocities; Group C are smallest fibers that are unmyelinated
Roman numerals (sensory fibers only): Groups I-IV
3 basic spinal cord reflexes
1) Stretch reflex
2) Golgi tendon organ reflex
3) Flexor withdrawal reflex
Major components of stretch reflex (deep tendon reflex)
1) Muscle spindle: connective tissue capsule that encloses intrafusal muscle fibers; receives sensory innervation from primary and secondary nerve (afferent?) fibers; receives motor innervation from small gamma motor neurons; is located within regular skeletal muscle and attached to muscle fibers by connective tissue; spindles in parallel with regular muscle fibers
2) Ia primary afferent fibers: have primary ending wrapped around intrafusal fiber; form excitatory connections with alpha motor neurons; send projections to dorsal nucleus of Clarke; send excitatory connections to motor neurons of synergistic muscles; send inhibitory (disynaptic) signals to motor neurons of antagonistic muscles
3) Gamma motor neurons: innervate polar ends of intrafusal muscle fibers so can influence sensitivity of muscle spindle; do not innervate regular muscle fibers directly; not contacted by primary sensory endings; innervated by other afferents from dorsal roots and by descending motor pathways that influence both alpha and gamma motor neurons
4 things the Ia fiber sends signals to
First, the Ia fiber senses stretch of the primary endings of the muscle spindle (?), then it sends signals:
1) Alpha motor neuron to same muscle
2) Dorsal nucleus of Clarke in dorsal horn (--> alpha motor neuron goes up dorsal spinocerebellar tract); also up dorsal column for proprioception
3) Alpha motor neuron to synergistic muscle
4) Inhibitory Ia nerve --> alpha motor neuron to antagonist muscles
Gamma motor neuron
Influence sensitivity of muscle spindle
Sense stretch of intrafusal fibers with sensory endings that are wrapped around intrafusal fibers
Produce contractions at polar ends of muscle spindle (this makes it easier to activate the stretch reflex, doesn't contract the muscle itself!!)
Not contacted by primary afferent Ia fibers
Activated by descending motor pathways and other afferents from dorsal roots (cutaneous afferents)
Ensures that we can still signal length of muscle even when muscle contracted (without gamma motor neuron, would have no afferent activity coming in!)
Could contribute to hyperreflexia if over-active!
Effects of stretch and contraction on discharge rate of Ia afferent fibers from muscle spindle
Stretch of muscle = stretch of spindle = increased discharge rate of Ia fiber
Contraction of muscle = decreased stretch of spindle = decreased discharge rate of afferent Ia fiber
Makes sense because muscle spindle reflex CONTRACTS the muscle
What does the gamma motor system do during muscle contraction in the "silent" period of Ia afferents
Gamma motor system fills in continued discharge because it is activated by polar ends of spindle
Muscle spindle can respond to chanes in load by providing information to us about muscle length
Roles of the muscle spindle in motor control
Negative feedback system that monitors muscle length
1) Participate in automatic adjustments of body to maintain posture (need something operating on a lower level to help us maintain balance)
2) Compensate for changes in load during motor activity
3) Contribute to normal muscle tone
4) Contribute to sense of limb position and movement (proprioception and kinesthesia)
What do primary afferent fibers of the muscle spindle respond to?
Stretch of the regular muscle fibers
Contraction of the polar ends of the spindle (intrafusal muscle fiber)
Do gamma motor neurons contribute directly to muscle tension?
They exert their effects through the stretch reflex pathway
What initiates the stretch reflex?
Tendon tap stretches the muscle
You're tapping the tendon but it's really the muscle that is the site of action
Extrafusal vs. intrafusal muscle fibers
Extrafusal = regular muscle fibers
Intrafusal = muscle spindle = in parallel with extrafusal muscle fibers