Nervous system

Dr Ganga singal

Spinal Cord

• Runs through the vertebral canal
• Extends from foramen magnum to second lumbar vertebra
• Regions
• Cervical
• Thoracic
• Lumbar
• Sacral
• Coccygeal
• Gives rise to 31 pairs of spinal nerves
• All are mixed nerves
• Not uniform in diameter
• Cervical enlargement: supplies upper limbs
• Lumbar enlargement: supplies lower limbs
• Conus medullaris- tapered inferior end
• Ends between L1 and L2
• Cauda equina - origin of spinal nerves extending inferiorly from conus medullaris.

Gross anatomical structures of the spinal cord

A. Two enlargements of the spinal cord occur in

regions where the amount of gray matter is greater than others; regions of sensory/motor control of the limbs.

1. Cervical enlargement - nerves supply the shoulder and upper limbs;

2. Lumbar enlargement - nerves supply the pelvis and the lower limb.

B. Conus medullaris: region at the end of the spinal cord where the tissue tapers into a cone shape; approximately in region of L2.

C. Filum terminale: connective tissue, continuous with the pia mater of the spinal cord.

D. Cauda equina: extension of nerves exiting from the base of the spinal cord; appears like a horse's tail.

The Spinal Cord

The Spinal Cord

Nerve Pathways into the Spinal Cord

Relationship b/t spinal &vertebral segment

• Add 1 in cervical region to no.of spine of vertebra.
• Upper thoracic +2.
• Lower thoracic+3.
• 11^th spine +4.
• 12^th spine+6.
• 1^st lumbar spine add requisites no.

Hence: 31 pairs of spinal nerves:

8 cervical

12 thoracic

5 lumbar

5 sacral

1 coccygeal

The Circulation of Cerebrospinal Fluid

Cross Section of Spinal Cord

On the surface :

• Deep anterior median fissure
• Shallower posterior median sulcus

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Spinal cord segment :

• Section of the cord from which a pair of spinal nerves are given off

Cross Section of Spinal Cord

• White matter:
• Myelinated axons forming nerve tracts
• Fissure and sulcus
• Three columns:
• Ventral
• Dorsal
• Lateral

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• Gray matter:
• Neuron cell ,neuroglia ,blood vessels.
• ‘Horns’:
• Posterior (dorsal)
• Anterior (ventral)
• Lateral

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• Commissures:
• Gray: Central canal
• White

(see later for white matter

pathways)

Neurons in grey matter

Structural classafication

• Golgi type 1.
• Golgi type 2.

Functional classification

• Motor neurons.
• Sensory neurons.
• Interneurons.

Lamina of Rexed

Lamina of Rexed

Lamina I Posteromarginal Nucleus

Lamina II Substantia Gelatinosa of Rolando

Lamina III

Lamina IV, V, VI ----- Nucleus Proprius

Lamina VII

- Intermediate Gray

- Intermediolateral cell column (ILM)

- Clarke’s column (Nucleus dorsalis)

- Intermediomedial cell column (IMM)

Lamina VIII

Lamina IX ---------- Anterior Horn (Motor) Cell

Lamina X ----------- Gray Commissure

Neuron-To-Neuron Transmission

White Matter: Pathway Generalizations

Descending (Motor) Pathways

• Descending tracts deliver motor instructions from the brain to the spinal cord
• Divided into two groups
• Pyramidal, or corticospinal, tracts
• Indirect pathways, essentially all others
• Motor pathways involve two neurons
• Upper motor neuron (UMN)
• Lower motor neuron (LMN)
• aka ‘anterior horn motor neuron” (also, final common pathway)

Descending Tracts

• Named for part of brain for point of origin and end in spinal
• Corticospinal tracts-carry conscious motor signals from cortex for precise, finely skilled coordinated limb movement
• Reticulospinal tracts-control muscles of upper and lower limbs and help maintain posture
• Rubrospinal tracts- unconscious coordination of movements.control muscle tone & synergy.
• Tectospinal tracts-movement of head-neck & eyes in response to visual stimuli

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Pyramidal (Corticospinal) Tracts

• Originate in the precentral gyrus of brain (aka, primary motor area)
• I.e., cell body of the UMN located in precentral gyrus
• Pyramidal neuron is the UMN
• Its axon forms the corticospinal tract
• UMN synapses in the anterior horn with LMN
• Some UMN decussate in pyramids = Lateral corticospinal tracts
• Others decussate at other levels of s.c. = Anterior corticospinal tracts
• LMN (anterior horn motor neurons)
• Exits spinal cord via anterior root
• Activates skeletal muscles
• Regulates fast and fine (skilled) movements

Corticospinal tracts

• Location of UMN cell body in cerebral cortex
• Decussation of UMN axon in pyramids or at level of exit of LMN
• Synapse of UMN and LMN occurs in anterior horn of s.c.
• LMN axon exits via anterior root

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Extrapyramidal Motor Tracts

• Includes all motor pathways not part of the pyramidal system
• Upper motor neuron (UMN) originates in nuclei deep in cerebrum (not in cerebral cortex)
• UMN does not pass through the pyramids!
• LMN is an anterior horn motor neuron
• This system includes
• Rubrospinal
• Vestibulospinal
• Reticulospinal
• Tectospinal tracts
• Regulate:
• Axial muscles that maintain balance and posture
• Muscles controlling coarse movements of the proximal portions of limbs
• Head, neck, and eye movement

� Extrapyramidal Tract ���

Note:�1. UMN cell body location�2. UMN axon decussates in pons�3. Synapse between UMN and LMN

occurs in anterior horn of sc

3. LMN exits via ventral root

4. LMN axon stimulates skeletal

muscle

Extrapyramidal (Multineuronal) Pathways

• Reticulospinal tracts – originates at reticular formation of brain; maintain balance
• Rubrospinal tracts – originate in ‘red nucleus’ of midbrain; control flexor muscles
• Tectospinal tracts - originate in superior colliculi and mediate head and eye movements towards visual targets (flash of light)

Main Ascending Pathways

• The central processes of first-order neurons branch diffusely as they enter the spinal cord and medulla
• Some branches take part in spinal cord reflexes
• Others synapse with second-order neurons in the cord and medullary nuclei

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Three Ascending Pathways

• The nonspecific and specific ascending pathways send impulses to the sensory cortex
• These pathways are responsible for discriminative touch (2 pt. discrimination) and conscious proprioception (body position sense).
• The spinocerebellar tracts send impulses to the cerebellum and do not contribute to sensory perception

Ascending Tracts

• Fasciculus gracilis- carries signals for deep touch, vibration, and position and movement of lower limbs and trunk
• Fasciculus cuneatus- same sensory signals as above, but begins at T6
• Spinothalamic tract- signals for pain, temp, pressure, light touch, tickle, and itch
• Spinocerebellar tract- signals for proprioception (coordination of muscle movement) from limbs and trunk

Specific and Posterior Spinocerebellar Tracts

• Dorsal Column Tract

1. AKA Medial lemniscal pathway

2. Fibers run only in dorsal column

3. Transmit impulses from receptors in

skin and joints

4. Detect discriminative touch and

body position sense =proprioception

• 1st O.N.- a sensory neuron
• synapses with 2nd O.N. in nucleus gracilis and nucleus cuneatus of medulla
• 2nd O.N.- an interneuron
• decussate and ascend to thalamus where it synapses with 3rd O.N.
• 3rd-order (thalamic neurons)
• transmits impulse to somato-

sensory cortex (postcentral gyrus)

Spinocerebellar Tract

• Transmit info. about trunk and lower limb muscles and tendons to cerebellum
• No conscious sensation

Nonspecific Ascending Pathway

• Include the lateral and anterior spinothalamic tracts
• Lateral: transmits impulses concerned with pain and temp. to opposite side of brain
• Anterior: transmits impulses concerned with crude touch and pressure to opposite side of brain
• 1st order neuron: sensory neuron
• 2nd order neuron: interneurons of dorsal horn; synapse with 3rd order neuron in thalamus
• 3rd order neuron: carry impulse from thalamus to postcentral gyrus

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Three Ascending Pathways

• The nonspecific and specific ascending pathways send impulses to the sensory cortex
• These pathways are responsible for discriminative touch (2 pt. discrimination) and conscious proprioception (body position sense).
• The spinocerebellar tracts send impulses to the cerebellum and do not contribute to sensory perception

Spinal Reflex Arc

Clinical anatomy

• syringomyelia.- dissociated sensory loss due to destruction of the spinal cord involving central canal & its surrounding area due to formation of elongated cavities.
• Results-bilateral loss of pain & temp sensation below the lesion (decussating spinothalamic fibres in anterior white commissure).other sensation preserved.

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Tabes dorsalis

• Cause-syphilitic degenerative lesion of the posterior white columns & posterior nerve roots.
• Results-impairment of proprioceptive sensibility .loss of position sense,vibratory sense,stereognosis,discriminative touch on the same side &below the level of cord lesion.
• Romberg’s sign positive.

Brown– Sequard Syndrome:

• Hemi-section of the cord
• Either due to penetrating injuries:

i) stab wounds

ii) gunshot wounds

• Fractures of lateral mass of vertebrae

Clinically:

• Paralysis on affected side (corticospinal)
• Loss of proprioception and fine discrimination (dorsal columns)
• Pain and temperature loss on the opposite side below the lesion (spinothalamic)

Blood supply of the spinal cord

Anterior spinal arteries

Posterior spinal arteries

Radicular arteries

Blood supply of the spinal cord

Radicular arteries

Anterior and posterior radicular arteries are form

arterial vasocorona

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Are branches from segmental arteries:

-ascending cervical a.

-deep cervical a.

-Intercostal a.

-lumbar a.

-sacral a.

Arterial vasocorona

Spinal vein

Anterior longitudinal venous trunk

Posterior longitudinal venous trunk

-posteromedian

-posterlateral

Internal vertebral venous plexus (epidural venous plexus)

-anteromedian

-anterolateral

ALVT

PLVT

Vasocorona vein

Anterior radicular vein

Posterior radicular vein

Epidural venous plexus