Biology 220
Anatomy & Physiology I
Unit VII
SPINAL CORD AND
TRACTS
Chapter 12, pp. 461-467
Chapter 15, pp. 538-546
E. Gorski/ E. Lathrop-Davis/ S. Kabrhel
Spinal Cord Functions
•
communications link between brain and body
° carries ascending, sensory impulses toward brain
° carries descending, motor impulses to synapse with
motor neurons that will serve effectors
• reflex centers -- allow faster response without direct
brain input
° somatic reflexes (skeletal muscles and skin)
°
stretch reflex
deep tendon reflex
flexor reflex
crossed extensor reflex
visceral reflexes (glands & smooth muscle)
Spinal Cord Structure
Spinal cord enlargements:
• cervical -- origin of
nerves to upper limb
• lumbar -- origin of
nerves to lower limb
Fig. 12.24, p. 462
Spinal Coverings: Meninges
•
Dura mater -- tough, outer covering
° epidural space - between dura and vertebra;
contains fat and blood vessels
- epidural anesthetic
•
Arachnoid mater -- middle layer
° subarachnoid space -- between arachnoid mater and
pia mater
- contains cerebrospinal fluid
- lumbar puncture (inferior to L1, safe at L3) - in lower
lumbar region, CSF may be removed for testing with less
risk to nerves/spinal cord
•
Pia mater - delicate inner layer
White Matter
•
myelinated (and unmyelinated) fibers carrying
information (impulses) between brain and spinal cord
and from one level to another within spinal cord
• organized as white columns = funiculi
° dorsal (posterior) funiculus
° ventral (anterior) funiculus
° lateral funiculus
• columns (funiculi) may contain both ascending and
descending tracts
Nerve Roots: Dorsal Root
Dorsal root contains afferent fibers for both somatic and
visceral sensory neurons
• dorsal root ganglion (DRG) contains cell bodies of
afferent neurons (these are unipolar neurons)
Dorsal Root
DRG
Fig. 12.29, p. 466
Nerve Roots: Ventral Root
Ventral root contains efferent (motor) fibers to
effectors
• largest in the in cervical and lumbar regions where
somatic motor neurons arise for nerve plexuses to
limbs
• autonomic motor neurons also leave via ventral
root
Ventral Root
Fig. 12.29, p. 466
Spinal Cord Pathways and Tracts
most pathways cross over (decussate) – so they are
contralateral
• most pathways consist of a chain of 2-3 neurons
• pathways exist as right/left pairs
•
Fig. 12.29, p. 466
Ascending (Sensory) Tracts
•
conduct sensory impulses from general sensory
receptors toward brain
° from skin
- touch [Meissner’s corpuscles], pressure [Pacinian
corpuscles], temperature, pain
°
from proprioceptors in joints, tendons, muscles
- stretch of muscles, tendons, joints
•
sensory pathways contain 2 or 3 neurons
° first order: from receptor to spinal cord
° second order: spinal cord to thalamus or cerebellum
° third order: thalamus to cerebral cortex
Ascending (Sensory) Tracts
3 main ascending pathways on each side:
• dorsal column pathways/lemniscal (specific)
• spinothalamic pathways/anterolateral (nonspecific)
• spinocerebellar pathways (to cerebellum)
Specific Pathway:
Dorsal Column Tracts
•
axons involved with highly localized (fine) touch,
pressure, vibration, conscious proprioception to
opposite side of brain (i.e., these are contralateral)
• allows fine localization of sensation by cerebral cortex
° fasciculus cuneatus -- consists of neurons with
impulses arising from upper half of trunk, upper
limbs and neck
° fasciculus gracialis -- consists of neurons with
impulses arising from lower trunk and lower limbs
*first-order sensory neurons are part of PNS
*second- and third-order neurons are part of CNS
Specific Pathway:
Dorsal Column Tracts
•
•
•
first-order neuron enters via dorsal
root and ascends dorsal column
without synapsing
° first-order neuron synapses with
second-order neuron in medulla
oblongata (in either nucleus
cuneatus or nucleus gracilis)
second-order neuron crosses to other
side in medulla (medial lemniscal
tract)
° second-order neuron synapses with
third-order neuron in thalamus
third-order neuron carries impulse
from thalamus to primary
somatosensory area of cerebral cortex
Fig. 15.2, p. 538
Nonspecific Pathway:
Spinothalamic Tracts
•
•
also called anterolateral tracts
carry sensory information regarding crude touch
sensation, pressure, temperature and pain
• impulses may arise from several different receptors or
even different types of receptors
• contralateral
*first-order sensory neurons are part of PNS
*second- and third-order neurons are part of CNS
Nonspecific Pathway:
Spinothalamic Tracts
•
first-order neuron enters dorsal root
° first order neuron synapses with
second-order neuron in posterior
gray horn
• second-order neuron crosses over to
other side in spinal cord before
ascending to thalamus
° second-order neuron synapses
with third-order neuron in
thalamus
• third-order neuron carries impulse
from thalamus to primary
somatosensory area of postcentral
gyrus
Fig. 15.2, p. 538
Spinocerebellar Pathway Tracts
•
•
•
carry subconscious proprioception information to
cerebellum (aid balance)
posterior spinocerebellar tract and anterior spinocerebellar
tract carry impulses from lower limbs and trunk (tracts
from upper trunk and limb are more complicated)
carry impulses related to position, stretch, and tension in
skeletal muscle, joints, tendons
*first-order sensory neurons are part of PNS
*second-order neurons are part of CNS
Spinocerebellar Pathway Tracts
•
first-order neuron enters
spinal cord via dorsal root
° first-order neuron
synapses with secondorder neuron in dorsal
gray horn
• second-order neuron enters
cerebellum via cerebellar
peduncles
° second-order neuron ends
up on same side of body
(ipsilateral)
Fig. 15.2, p. 538
Descending (Motor) Tracts
•
Two groups of tracts:
° direct (pyramidal) tracts
° indirect (extrapyramidal) tracts
• Two neurons in pathway
° upper motor neurons
° lower motor neurons
Descending (Motor) Tracts
•
upper motor neurons originate in gray matter of
cerebral cortex or other gray matter
° upper motor neurons run through spinal cord to
level at which the spinal nerve to their effector
originates
° upper motor neurons synapse with the lower motor
neuron in the anterior gray horn of the spinal
segment
• lower motor neurons exit via ventral root and carry
impulses to effectors
*Upper motor neuron is within CNS
*Lower motor neuron is part of PNS
Direct (Pyramidal) Tracts
•
•
•
precise activation of contralateral skeletal muscles
carry impulses for voluntary movement
upper motor neuron
° extends from pyramidal cells of primary motor
cortex through brain stem
° synapses with lower motor neuron in anterior gray
horn of spinal cord at appropriate level
• lower motor neuron extends from spinal cord to
effector
• lateral and anterior tracts differ in where they
decussate
*Upper motor neuron is within CNS
*Lower motor neuron is part of PNS
Indirect (Extrapyramidal) Tracts
•
•
•
Formed from neurons originating in areas other than
primary motor cortex
Subconscious (involuntary) movements
Involved in regulating:
° axial muscles that maintain balance and posture
° muscles controlling gross movements of proximal parts
of limbs
° head, neck and eye movements to follow objects
Indirect (Extrapyramidal) Tracts
•
rubrospinal tract - questionable function in humans
(affects distal limb flexors in other animals)
• vestibulospinal tract - posture and balance during
standing and movement (extensor muscles)
• reticulospinal tract - involved with visceral motor
functions; skeletal muscle tone; breathing; unskilled
movements
• tectospinal tract - control reflexive movements of eye,
head, neck, arm in response to visual stimuli
Descending Tracts
Fig. 15.4, p. 545
Spinal Cord Trauma
•
Paralysis - loss of motor function due to damage of
motor neurons or anterior horn
° flaccid paralysis
- caused by damage to ventral (anterior) root or
anterior horn (LMN are damaged)
- no stimulation to muscle, which atrophies
from disuse
° spastic paralysis
- caused by damage to UMN (brain damage or
spinal cord damage)
- muscles stimulated in response to somatic
reflexes (loss of muscle tone without atrophy)
Spinal Cord Trauma
Paresthesia - abnormal sensory input (e.g., “pins
and needles”) as from damage to sensory neuron
• Transection of cord - spinal cord itself is damaged
(“cut”) results in paralysis and sensory loss below
(inferior to) level of injury
° quadriplegia -- results when damage is done in
cervical region; all limbs are affected
° paraplegia -- results when damage is done in
thoracic region (between T1 and L1); only lower
limbs are affected
•
Spinal Cord Trauma
•
Spinal shock - transient period of functional loss
° depression of somatic (e.g., stretch, withdrawal)
and visceral reflexes (e.g., bowel, bladder, BP)
° lasts few to 48 hours (if longer, get permanent
paralysis)
• Hemiplagia - paralysis on one side; reflects brain
damage (e.g., stroke)