Nerve Pathways:Nerve Pathways:Functions, Lesions and AdhesionsFunctions, Lesions and Adhesions

D.Robbins

Spinal cordSpinal 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 rootsNerve 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 NervesAfferent Nerves

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.

Primary Afferent Neuron = the first neuron in a spinal reflex or ascending spinal pathway.

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 SectionSpinal Cord Cross Section

Image taken from: http://cas.bellarmine.edu/tietjen/HumanBioogy/SpinalCord01.gif

Ascending Pathways: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 pathwaysNerve pathwaysAscending Tracts

Tract Signal function

Dorsal columns Vibration, tactile sensation, conscious proprioception

Spinocerebeller Proprioception

Spinothalamic (lateral and anterior)

Pain, temperature, itch (lateral), crude touch (anterior)

Spinoreticular Pain

Spinomesencephalic Pain

Spino-cervico-thalamic Pain (touch?)

Spinohypothalamic Pain

Dorsal Column and Spinocerebellar Dorsal Column and Spinocerebellar PathwaysPathways

• 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

Clarkes Column (L1-T1)

C

G

Z

Thalamus

N.B . cerebellar feedback actually occurs posteriorly not laterally, however in a 2D diagram its easier to represent it this way.

Spinoreticular and Spinothalimic Spinoreticular and Spinothalimic pathwayspathways

• 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 Thalamus

P

M

N.B. cerebellar feedback actually occurs posteriorly not laterally, however in a 2D diagram its easier to represent it this way.

Descending Spinal Pathways: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).

Descending Tracts

Tract Signal function

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) PontineRegulates 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) MedialIt 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.

Nerve pathwaysNerve pathways

Corticospinal tractCorticospinal 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 tractRubrospinal 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 TractReticulospinal 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.

P

M

Vestibulospinal TractVestibulospinal 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 PathwaysDescending Pathways

Pathway Upper limb Lower limb

Cortico/-pyramidalThis Tract functions to modulate the activity of Alpha or Gamma Motor Neurons as directed by the Motor Cortex.

Rubro-spinal Stimulates flexors

Reticulo-spinalMedullary inhibits extensors and excites flexors

Pontine excites extensors and inhibits flexors

(Generally upper limb)

Vestibulo-spinalDoesn’t affect upper limbs but helps position head and neck in response to body tilting (medial)

Stimulates extensors (lateral)

Tecto-spinal Control of head, neck and eye movements.

Spinal Cord Cross SectionSpinal Cord Cross Section

Image taken from; http://img.medscape.com/pi/emed/ckb/clinical_procedures/1134815-1148570-1177.jpg

Gray matter organisationGray matter organisation

• Two schemes have evolved for organizing neuron cell bodies within gray matter. Either may be used according to which works best for a particular circumstance.

• 1) Spinal Laminae—spinal gray matter is divided into ten laminae (originally based on observations of thick sections in a neonatal cat). The advantage is that all neurons are included. The disadvantage is that laminae are difficult to distinguish.

I-VI: Posterior/Dorsal horn Lamina I: Posterormarginal nucleus Laminae II/III: Substansia gelatinosa Laminae III/IV/V: Nucleus propius Lamina VI: Nucleus dorsalisVII-IX: Anterior/Ventral horn Lamina VII: Intermediolateral nucleus Lamina VIII: Motor interneurons Lamina IX: Motor neurons which also contain the Onuf’s nucleus in the sacral region Lamina X: Neurons bordering central canal

Spinal NucleiSpinal 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 NeuronsMotor 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.lower motor neurons.

Motor neuronsMotor 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 neuronsAlpha motor neurons (α-MNs) innervate extrafusal muscle fibers, the most numerous type of muscle fibre and the one involved in muscle contraction.

– Gamma motor neuronsGamma 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 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.Descending Lesions cont.

• Damage to lower motor neurons, lower motor neurone lesionslower 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 LesionsAscending 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 signsLesion 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 Difference between positive and negative signs of lesionnegative 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.

AdhesionsAdhesions

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.

The following information and images were all taken from: Biomechanics of the Nervous System: Breig Revisited (http://www.neurodynamicsolutions.com/breig-revisited.php)

Fissure FormationFissure Formation

Sites of tearing in the cervical cord resulting from compression by a body impinging on it from (A) anterior and (B) posterior directions.

A.) A transverse tear in the posterior side results from an anterior compression combined with cervical extension.

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 tissueEffects 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 pulpFormation 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 adhesionsInfluence 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!!

Questions?Questions?

Thanks for listeningThanks for listening