Transcript Lecture 2: The Spinal Cord

Spinal Cord
Maryann Martone
1/20/2011
Functions of Spinal Cord
•Final common pathway for
the somatomotor system
•Conveys somatosensory
information from the body
•Autonomic neurons
•Reflexes
•Central pattern generators
Spinal cord segments, nerves and vertebrae
Cervical (C) - Neck
(C1-8)
Thoracic (T) - Chest
(T1-12)
Lumbar (L) - Back
(L1-5)
Sacral (S) - Pelvis
(S1-5)
Coxygeal
Vertebra
Spinal segment
Spinal root
Caudal End
• conus medullaris
– (Spinal cord terminates
around vertebra L1)
• filum terminale (composed
of pia mater): anchors
spinal cord
• cauda equina
Meninges
• Dura transitions to epineurium at spinal nerve
Sensory and Motor Roots
Dorsal: sensory
Ventral: somatic motor + visceral motor
Alar vs Basal Plates
Internal structure
Posterior/
Dorsal
Horn
Posterior
Funiculus (white
matter)
Posterior/Dorsal
Section of
human spinal
cord (C8)
myelin stain
Lateral
Column/
Funiculus
Anterior/
Ventral
Horn
Anterior
Column
Anterior/Ventral)
White matter external; gray matter
internal
Anterior median fissure
Posterior median sulcus
Central canal
Anterior white commissure
Levels of the Spinal Cord
Cervical
Lumbar
Upper Thoracic
Sacral
The ratio of white matter to gray matter increases from
caudal to rostral
Principles of Cord
Organization
1) Longitudinal Arrangement
Fibers (White Matter) ------------- White Column / Fasciculus
Cell Groups (Gray Matter) ------- Gray Column / Nucleus
2) Transverse Arrangement
Afferent & Efferent Fibers
Crossing (Commissural and Decussating) Fibers
3) Somatotopical Arrangement
Rexed’s Laminae
Lissauer tract
Lamina I: posteromarginal nucleus
Lamina
Lamina
Lamina
Lamina
II: substantia gelatinosa
III, IV: nucleus proprius
V, VI
VII: intermediate gray
Clarke’s column (Nucleus dorsalis; C8-L2,
sensory relay)
intermediolateral cell column -sympathetic
T1-L2, parasympathetic in sacral cord)
Lamina VIII
Lamina IX: anterior horn motor neurons
Lamina X: gray commissure
Somatic Motor Neurons
•
•
•
Multipolar
Transmitter: acetylcholine
Output: One motor neuron to several muscle cells = one motor unit
-motor neurons: muscle fiber innervation
-motor neurons:muscle spindle innervation
•
Input from –
•
•
Integrate
Somatotopic Map
dorsal root ganglion cells
spinal interneurons
long tracts from brain
proximal to distal muscle: medial to lateral in spinal cord
Flexor-extensor muscle: dorsal to ventral in spinal cord
Autonomic neurons
• Sympathetic and
parasympathetic
preganglionic
neurons are
segregated in the
spinal cord
– Parasympathetic=
cervical and sacral
– Sympathetic =
thoracic and lumbar
• Intermediate horn
Dorsal root ganglion neurons
• Pseudomonopolar
• Cells have specialized
endings for each
modality
• Encapsulated
– (touch,
proprioception)
• Unencapsulated (free
nerve endings)
– Pain and temperature
• Differ in size of axon,
cell body, myelination,
transmitter
Dermatomes
The region of skin innervated by a
single dorsal root ganglion
• Human Nervous System:
– 31 pairs of spinal nerves
• Divided into cervical,
thoracic, lumbar and sacral
levels
– Spinal nerves impose
segmentation; actual
cord consists of columns
of cells
– Innervation of body is
segmented (dermatomes)
Mono- vs. polysynaptic reflexes
• Monosynaptic reflex
– Sensory neuron synapses directly on a motor neuron
• Polysynaptic reflex
– At least one interneuron between sensory afferent and motor
efferent
– Longer delay between
stimulus and response
– Length of delay is
proportional to the number
of synapses involved.
– more complicated
responses than monosynaptic
reflexes as the interneurons
can control several muscle
groups.
Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Spinal Cord Pathways
•
•
•
•
Descending motor
Ascending sensory
Intraspinal
Sympathetic and parasympathetic
• Nomenclature: usually origin to target e.g.
corticospinal: from cortex to spinal cord
Descending pathways
1) Medial Pathways (vestibulo-, reticulo-, tectospinal)
–
–
Termination in ventromedial gray matter
Axial and proximal innervation
2) Lateral pathways (cortico-, rubrospinal)
–
–
Termination in dorsolateral gray matter
Distal muscle innervation
3) Aminergic pathways (coerulo-, raphaespinal)
–
–
Diffuse termination, modulatory systems
serotonin, noradrenalin
Medial pathways
• Vestibulospinal
– Balance and posture / vestibular labyrinth
• Reticulospinal
– posture
• Ipsilateral projection, mostly on propriospinal and
interneurons
• Tectospinal
– Contralateral, only cervical, head and eye movement
coordination
Aminergic pathways
• Coerulospinal tract-Locus coeruleus
• Raphaespinal system- Modulation of pain
Lateral pathways
• Corticospinal tract (CST)
– From motor cortex
• Rubrospinal tract
– From red nucleus in midbrain
Corticospinal tract
•Also called pyramidal tract
•Arises primarily from primary motor cortex,
premotor and supplementary motor cortex
•Somatosensory cortex also contributes
•70-90% of fibers cross in the lower medulla
(decussation of pyramids)
•Crossed = lateral corticospinal tract
•Uncrossed = anterior/ventral corticospinal tract
•Synapses with:
•Alpha and gamma motor neurons
• Propriospinal neurons
• Interneurons
Internal capsule
Midbrain
pons
medulla
decussation
Crus cerebri
spinal cord
Longitudinal pontine fibers
Pyramids
Species differences in
mammalian
corticospinal tract
anatomy
• change in location of the
corticospinal tract from rat to ape.
• Function: CST is far more
important to humans and
monkeys than it is to rats, while
the rubrospinal tract becomes less
significant in humans.
• Increase in bilateral projections
Kuypers, 1977
Three major sensory systems
• Dorsal Column system
– Fine, discriminitive touch
– Conscious proprioception
• Spinocerebellar system
– Unconscious proprioception
• Spinothalamic system=anterolateral system
– Pain, temperature, pressure and vibration
Major Ascending Tracts
• Dorsal columns
– Fine, discriminitive touch, conscious proprioception, pressure
and vibration sense
– Facisculus gracilis vs fasciculus cuneatus
– First synapse: relay nuclei in the medulla (nucleus gracilis and
nucleus cuneatus), ascend ipsilaterally in the spinal cord
• Anterolateral system=spinothalamic system
– Pain and temperature
– Form Lissauer’s tract=posterolateral tract
– First synapse: dorsal horn
– 2nd order neuron crosses in anterior white commisure: ascend as
spinothalamic tract (also spinoreticular and spinomesencephalic tracts)
• Spinocerebellar
– Dorsal and ventral: mostly ipsilateral
– Unconscious proprioception
– (First synapse: Clarke’s nucleus neurons or acessory cuneate nucl.)
Dorsal
Columns
•Somatotopically organized:
Fasciculus gracilis &
cuneatus above T7
•Ascends ipsilaterally
• First synapse: medulla: nucl. gracilis and
nucl. cuneatus
• Output from relay nuclei to thalamus
crosses
• Conscious touch
on contralateral side
of the body
Dorsal column/medial lemniscus
1. Dorsal Root Ganglion
dorsal root - dorsal column
2. Dorsal Column Nuclei (N.
gracilis or N.cuneatus)
internal arcuate fiber - lemniscal
decussation- medial lemniscus
3. Thalamus (VPL)
internal capsule -corona radiata
4. Primary sensory cortex (S I)
Anterolateral system
(spinothalamic)
• DRG axons form Lissauer tract
• Pain and temperature on contralateral side
of body
• Many peptidergic neurons, transmitter:
substance P
• Crosses in spinal cord
• Thalamus via spinal lemniscus
(spinoreticular, spinomesencephalic tracts)
Summary
Ascending
• Voluntary motor control
• Reflexes and spinal circuitry
Descending
Facsciulus Proprius
• Propriospinal neurons in dorsal horn (lamina III+IV)
• Intersegmental coordination of various muscles
• ~30% of white matter derived from propriospinal
axons
• Axons mostly surrounding
gray matter
Corticospinal tract=pyramidal tract
• Projections from large, layer V pyramidal
neurons (including Betz cells)
Macaca mulatta (brainmaps.org)