Nerve Pathways:
Functions, Lesions and Adhesions
D.Robbins
Spinal cord
• The spinal cord is a cylinder of CNS. The spinal cord exhibits subtle cervical
and lumbar (lumbosacral) enlargements produced by extra neurons in
segments that innervate limbs. The region of spinal cord caudal to the
lumbar enlargement is conus medullaris. Caudal to this, a terminal filament
of glial tissue extends into the tail.
• A spinal cord segment = a portion of spinal cord that gives rise to a pair
(right & left) of spinal nerves. Each spinal nerve is attached to the spinal
cord by means of dorsal and ventral roots composed of rootlets. Spinal
segments, spinal roots, and spinal nerves are all identified numerically by
region, e.g., 6th cervical (C6) spinal segment.
Nerve roots
• Both the spinal cord (CNS) and spinal roots
(PNS) are enveloped by meninges within
the vertebral canal. Spinal nerves (which are
formed in intervertebral foramina) are
covered by connective tissue (epineurium,
perineurium, & endoneurium) rather than
meninges.
• Sacral and caudal spinal roots (surrounding
the conus medullaris and terminal filament
and streaming caudally to reach
corresponding intervertebral foramina)
collectively constitute the cauda equina.
MORE SUITABLE IMAGE NEEDED
Image taken from
wikipedia
Afferent Nerves
Primary Afferent Neuron = the first neuron in a spinal reflex or ascending spinal pathway.
Primary afferent neurons have their unipolar
cell bodies in spinal ganglia.
Their axons traverse dorsal roots, penetrate
the spinal cord (at the dorsolateral sulcus)
and bifurcate into cranial and caudal
branches which extend over several
segments within white matter of the dorsal
funiculus.
Collateral branches from the cranial and
caudal branches enter the gray matter to
synapse on interneurons and projection
neurons (or directly on efferent neurons
for the myotatic reflex).
In some cases (discriminative touch), the
cranial branches of incoming axons
ascend directly to the brainstem where
they synapse on projection neurons of
the pathway.
Spinal Cord Cross Section
Image taken from: http://cas.bellarmine.edu/tietjen/HumanBioogy/SpinalCord01.gif
Ascending Pathways:
In general, pathways may be categorised into three broad
functional types:
1) Conscious discrimination/localisation (e.g., pricking pain, warmth, cold,
discriminative touch, kinesthesia) requires a specific ascending spinal pathway to
the contralateral thalamus which, in turn, sends an axonal projection to the cerebral
cortex. Generally there are three neurons in the conscious pathway and the axon of
the projection neuron decussates and joins a contralateral tract.
2) Affective related (emotional & alerting behavior) information involves ascending
spinal pathways to the brainstem. Projection neurons are non-specific. They receive
synaptic input of different modalities and signal an ongoing magnitude of sensory
activity, but they cannot signal where or what activity.
3) Subconscious sensory feedback for posture/movement control involves ascending
spinal pathways principally to the cerebellum or brainstem nuclei that project to the
cerebellum. Generally there are only two neurons in a subconscious pathway and
the axon of the projection neuron joins an ipsilateral tract.
Nerve pathways
Ascending Tracts
Tract
Signal function
Dorsal columns
Vibration, tactile sensation, conscious
proprioception
Spinocerebeller
Spinothalamic (lateral and
Proprioception
anterior)
Spinoreticular
Spinomesencephalic
Spino-cervico-thalamic
Spinohypothalamic
Pain, temperature, itch (lateral), crude
touch (anterior)
Pain
Pain
Pain (touch?)
Pain
Dorsal Column and Spinocerebellar
Pathways
•
Dorsal column pathway carries
info on tactile sensation, pressure
and proprioception.
•
In the dorsal tract, the sensory
neurons synapse in an area
known as Clarke's nucleus or
"Clarke's column".
•
This is a column of relay neuron
cell bodies within the medial gray
matter within the spinal cord in
layer VII (just beneath the dorsal
horn), specifically between T1-L1.
These neurons then send axons
up the spinal cord and form
synapses in the accessory (lateral)
cuneate nucleus, lateral to the
cuneate nucleus in the medulla.
•
Spinocerebellar pathway carries
info on proprioception
Thalamus
Z
G
C
Clarkes
Column (L1-T1)
N.B . cerebellar feedback actually occurs posteriorly not laterally,
however in a 2D diagram its easier to represent it this way.
Spinoreticular and Spinothalimic
pathways
Thalamus
•
The Spinothalamic Tract, like the
Dorsal Column-Medial Lemniscus
Tract, use three neurons to convey
sensory information from the
periphery to conscious level at the
cerebral cortex.
•
The Spinothalamic tract carries
information on pain, temperature and
crude touch.
•
The Spinoreticular pathway carries
info on pain, temperature and crude
touch.
Thalamus
M
P
N.B. cerebellar feedback actually occurs posteriorly not laterally,
however in a 2D diagram its easier to represent it this way.
Descending Spinal Pathways:
Axons of brain projection neurons travel in descending tracts in spinal white
matter. They arise from various locations in the brain and synapse primarily
on interneurons within the spinal cord.
By synapsing on interneurons, descending tracts regulate:
1) spinal reflexes;
2) excitability of efferent neurons (for posture and movement); and
3) excitability of spinal projection neurons, i.e., the brain is able to regulate
sensory input to itself. In some cases, descending tracts affect axon
terminals of primary afferent neurons, blocking release of neurotransmitter
(presynaptic inhibition).
Nerve pathways
Descending Tracts
Signal function
Tract
Corticospinal (pyramidal)
Fine voluntary motor control of the limbs. The
pathway also controls voluntary body posture
adjustments.
Rubrospinal
Involved in involuntary adjustment of arm position in
response to balance information; support of the body.
Reticulospinal (1) Pontine
Regulates various involuntary motor activities and
assists in balance (leg extensors). Some pattern
movements e.g. stepping
(2) Medullary
Inhibits firing of spinal and cranial motor neurons,
control of antigravity muscles.
Vestibulospinal (1) Medial
It is responsible for adjusting posture to maintain
balance (neck muscles).
(2) Lateral
It is responsible for adjusting posture to maintain
balance (body/lower limb).
Tectospinal
Controls head and eye movements, Involved in
involuntary adjustment of head position in response to
visual information.
Corticospinal tract
Travels from the cerebral cortex down to
the spinal cord.
CST actually consists of two separate
tracts in the spinal cord: the lateral
corticospinal tract and the anterior
corticospinal tract. Contains mostly
motor axons.
Referred to as a pyramidal tract as
when the tract passes the medulla, it
forms a dense bundle of nerve fibres
that is shaped somewhat like a pyramid
Lateral
CST
Anterior
CST
Rubrospinal tract
•
Travels from the cerebral cortex down to
the spinal cord via the red nucleus. An
extra-pyramidal motor tract.
•
Its main role is the mediation of voluntary
movement. It is responsible for large
muscle movement such as the arms and
the legs as well as for fine motor control. It
facilitates the flexion and inhibits the
extension in the upper extremities
Reticulospinal Tract
• An extra-pyramidal motor tract
which travels from the reticular
formation.
•
The tract is divided into two parts, the
medial (or pontine) and lateral (or
medullary) reticulospinal tracts (MRST
and LRST).
•
1. Integrates information from the
motor systems to coordinate automatic
movements of locomotion and posture.
•
2. Facilitates and inhibits voluntary
movement, influences muscle tone.
M
P
Vestibulospinal Tract
•
Inputs originate from the labyrinthine
system via the vestibular nerve and
from the cerebellum.
•
The medial part of the
vestibulospinal tract project
bilaterally down the spinal cord and
triggers the cervical spinal circuits,
controlling a correct position of the
head and neck.
•
The lateral part of the
vestibulospinal tract projects
ipsilateral down to the lumbar
region. There it helps to maintain an
upright and balanced posture by
stimulating extensor motor neurons
in the legs.
V
Descending Pathways
Pathway
Cortico/-pyramidal
Rubro-spinal
Reticulo-spinal
Vestibulo-spinal
Tecto-spinal
Upper limb
Lower limb
This Tract functions to modulate the activity of Alpha
or Gamma Motor Neurons as directed by the Motor
Cortex.
Stimulates flexors
Medullary inhibits extensors and excites flexors
Pontine excites extensors and inhibits flexors
(Generally upper limb)
Doesn’t affect upper limbs
but helps position head and
neck in response to body
tilting (medial)
Stimulates extensors
(lateral)
Control of head, neck and eye movements.
Spinal Cord Cross Section
Image taken from; http://img.medscape.com/pi/emed/ckb/clinical_procedures/1134815-1148570-1177.jpg
Gray matter organisation
• Posterior/Dorsal
Two schemes have
I-VI:
hornevolved for organizing neuron cell
bodies
within gray matter.
Lamina
I: Posterormarginal
nucleus Either may be used
Laminae
II/III: Substansia
gelatinosa
according
to which
works best for a particular
Laminae
III/IV/V: Nucleus propius
circumstance.
Lamina VI: Nucleus dorsalis
VII-IX: Anterior/Ventral horn
Lamina
Intermediolateral
nucleus gray matter is divided
• 1) VII:
Spinal
Laminae—spinal
Lamina
VIII:
interneurons
into
tenMotor
laminae
(originally based on observations of
Lamina
IX: Motor
neurons
also cat).
contain
theadvantage
Onuf’s nucleus
in
thick
sections
in a which
neonatal
The
is that
the sacral region
all neurons are included. The disadvantage is that
Lamina X: Neurons bordering central canal
laminae are difficult to distinguish.
Spinal Nuclei
2) Spinal Nuclei—recognizable clusters of cells are identified as nuclei [a
nucleus is a profile of a cell column]. The advantage is that distinct nuclei are
generally detectable; the disadvantage is that the numerous neurons outside of
distinct nuclei are not included
Image taken from: http://images3.wikia.nocookie.net/psychology/images/thumb/c/c0/Medulla_spinalis_-_Substantia_grisea__English.svg/400px-Medulla_spinalis_-_Substantia_grisea_-_English.svg.png
Motor Neurons
• Motor neurons are split into two groups: Upper and Lower
motor neurons.
• Upper motor neurons originate in the motor region of the cerebral
cortex of the brain stem and carry motor information down to the
final common pathway, that is, any motor neurons that are not
directly responsible for stimulating the target muscle.
• The cell bodies of these neurons are some of the largest in the
brain, approaching nearly 100μm in diameter.
• These neurons connect the brain to the appropriate level in the
spinal cord, from which point nerve signals continue to the muscles
by means of the lower motor neurons.
Motor neurons
• Lower motor neurons (LMNs) are the motor neurons connecting
the brainstem and spinal cord to muscle fibers, transmitting nerve
impulses from the upper motor neurons to the muscles. A lower
motor neuron's axon terminates on an effector (muscle).
• Lower motor neurons are classified based on the type of muscle
fibre they innervate:
– Alpha motor neurons (α-MNs) innervate extrafusal muscle fibers, the most
numerous type of muscle fibre and the one involved in muscle contraction.
– Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers, which
together with sensory afferents compose muscle spindles. These are part of
the system for sensing body position (proprioception).
Descending Pathway Lesions
• An upper motor neuron lesion is a lesion of the neural pathway
above the anterior horn cell or motor nuclei of the cranial nerves.
• This is in contrast to a lower motor neuron lesion, which affects
nerve fibers travelling from the anterior horn of the spinal cord to the
relevant muscle(s).
• Upper motor neuron lesions are indicated by:
–
–
–
–
–
Spasticity, increase in tone in the extensor muscles (lower limbs) or flexor muscles (upper
limbs)
Clasp-knife response where initial resistance to movement is followed by relaxation
Weakness in the flexors (lower limbs) or extensors (upper limbs), but no muscle wasting
Increase Deep tendon reflex (DTR)
Presence of Babinski sign
Descending Lesions cont.
• Damage to lower motor neurons, lower motor neurone lesions (LMNL)
causes:
– Decreased tone
– Decreased strength
And:
– Decreased reflexes in affected areas.
• These findings are in contrast to findings in upper motor neurone lesions.
– LMNL is indicated by:
– Abnormal EMG potentials, fasciculations, paralysis, weakening of muscles, and
neurogenic atrophy of skeletal muscle.
Ascending Pathway Lesions
• Loss of sensory input from relevant pathway
– E.g. Spinothalamic tract
• Unilateral lesion usually causes contralateral anaesthesia (loss of sensation (pain and
temperature)). Anaesthesia will normally begin 1-2 segments below the level of lesion,
affecting all caudal body areas. This is clinically tested by using pin pricks.
– If lesion is hemisection (halfway across the spinal cord) (causing hemiplegia)) it
is known as Brown-Séquard syndrome.
• Brown-Séquard syndrome may be caused by a spinal cord tumour, trauma (such as a
gunshot wound or puncture wound to the neck or back), ischemia (obstruction of a
blood vessel), or infectious or inflammatory diseases such as tuberculosis, or multiple
sclerosis.
– Any presentation of spinal injury which is an incomplete lesion can be called a
partial Brown-Séquard or incomplete Brown-Séquard syndrome, so long as it has
characterized by features of a motor loss on the same side of the spinal injury
and loss of sensation on the opposite side.
Lesion signs
• Lesions have positive or negative signs.
– Positive (also called release phenomena) = abnormal and stereotyped
responses that are explained are explained by the withdrawal of tonic
inhibition (e.g. decerebrate rigidity).
– Negative signs reflect the loss of particular capacities normally
controlled by the damaged systems.
Difference between positive and
negative signs of lesion
• 1.) Diseases affecting the descending pathways give rise to
spasticity whereas diseases of motor neurons do not.
• 2.) Diseases affecting motor neurons directly result in denervation
atrophy and reduced muscle volume, whereas this does not occur
with damage to the descending pathway.
• 3.) Damage to the descending systems tend to be distributed more
diffusely in limb or face muscles and often affects large groups of
muscles e.g. the flexors. In contrast, degeneration in the local
groups of motor neurons tends to affect muscles in a patchy way
and may even be limited to single muscles.
Adhesions
The following information and images were all taken from: Biomechanics of the Nervous System: Breig Revisited
(http://www.neurodynamicsolutions.com/breig-revisited.php)
A.) Anteriorly located
foramnum magnum
tumour.
B.) Spondylotic
protrusions into the
cervical canal.
C.) Intramedullary
glial tissue scar or
circumscribed
oedema, as in
multiple sclerosis
and spinal cord
injury.
D.) Fracture of the
odontoid process.
E.) Compression
fracture of thoracic
process, with
kyphtoic
angulation.
F & G.) Pedicles
deformed by
osteophytic spurs.
Fissure Formation
A.) A transverse tear in
the posterior side results
from an anterior
compression combined
with cervical extension.
Sites of tearing in the
cervical cord resulting
from compression by a
body impinging on it
from (A) anterior and
(B) posterior directions.
B.) A transverse tear in
the anterior side of the
cord occurs from a
posterior compression
irrespective of whether
the cervical canal is
flexed or extended.
Effects of scar tissue
Scar tissue occurs in normal
tissue after damage and
forms with higher
collagenous content than that
of the original tissues.
This results in a stiffer
structure that adapts
differently to pressure in
either tension or
compression that the original
tissues.
Formation of vortices in cord pulp
Extrusion of cord substance by
fractured or displaced bone usually
continues for some time after a
transverse fissure has appeared.
Viscous tissue elements are therefore
forced into the pial sheath and flow in
cranial and caudal directions.
The flow is augmented by the elastic
retraction of the membranes of the
severed nerve fibres. The resistance to
flow can set up vortices.
Influence of posture on adhesions
Impingement e.g.
margin of petrous
bone, calcified tissue,
tumour.
Clivus tumour, or
anterior located
foramen magnum
tumour.
Intramedullary firm
body setting up
bending tensile
stresses.
Herniated lumbar disc
creating stress in
nerve roots.
Flexion
exacerbates all
stresses in the
spinal cord – no
matter what the
level!!
Thanks for listening
Questions?