1. Descending pathways

2. 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. The nerve fibers that descend in white matter from supraspinal levels are segregate into nerve bundles –descending tracts.
The cell bodies of these neurons are some of the largest in the brain, approaching nearly 100μm in diameter.

3. Location of descending Pathways
Lateral corticospinal tract
Anterior corticospinal tract

4. ANATOMICAL ORGANIZATION
FIRST ORDER- CEREBRAL CORTEX
SECOND ORDER(INTERNUNCIAL NEURON- ANTERIOR GRAY COLUMN
THIRD ORDER(LOWERMOTOR NEURON)- ANTERIOR GRAY COLUMN

5. Nerve pathways 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) 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.

6. Spinal cord organization
dorsal
White matters (nerve fibres)
Sensory inflow
Dorsal
horn
lateral
medial
Ventral
horn
Motor commands
Grey matters (cell bodies)
ventral

7. corticospinal tracts
Axonsof pyramidal cells situated in fifth layer of cerbral cortex.
One third –primary motor cortex
One third-secondary motor cortex
Descending fibers converge in corona radiata
Midbrain-basispedunculi
Pons-transverse pontocerebellar fibers
Medulla-pyramids
Junctionof medulla and spinal cord-decussating of fibers
Lateral corticospinal tract

8. DESCENDING TRACTS OF MOTOR FIBERS

9. Motor neurons
The motor neurons situated in the anterior gray column send axon to innervate muscles through anterior roots of spinal nerve. 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.
Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers,

10. 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).

11. 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

12. Descending Lesions cont.
Damage to lower motor neurons, lower motor neurone lesions (LMNL) causes:
Decreased tone
Decreased strength
And:
Decreased reflexes in affected areas.

13. 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.

14. Questions?

15. Draw labelled diagram showing tracts in spinal cord.

17. Spinal cord organization5 6 7

18. Spinal Cord Cross Section4 3 18

19. Spinal Cord, Roots & Nerves1 2 19