The Nervous SystemThe Nervous SystemSpinal Cord & Spinal Spinal Cord & Spinal

NervesNerves

The Nervous SystemThe Nervous SystemSpinal Cord & Spinal Spinal Cord & Spinal

NervesNervesAnatomy Anatomy

Dr. Edita RevayDr. Edita Revay

The Central Nervous System

Meninges – Connective tissues that surround and protect the brain and spinal cord

The CNS is well protected by bone, CT, and fluid

Dura Mater – tough, fibrous outer layer; 2 layers thick around brain with creation of dural sinuses between layers; 1 layer around spinal cord with epidural space external

Arachnoid – “spidery” web-like middle layer

Pia Mater – delicate, thin inner layer; filum terminale - extension of pia mater extends from tip of cord to coccyx to anchor cord in place; denticulate ligaments anchor cord laterally

Subarachnoid space – between arachnoid & pia mater; contains cerebral spinal fluid (CSF)

Lumbar cistern – area of subarachnoid space below the conus medularis; site for lumbar puncture (“spinal tap”)

Lumbar cystern

The Spinal Cord Begins at foramen magnum & ends at L2 vertebral level by forming conus medularis

Made up of 31 spinal cord segments

Has 2 thickened areas-

cervical enlargement - supplies nerves to upper extremity

lumbar enlargement - supplies nerves to lower extremity

•Each spinal cord segment has a pair of

• dorsal roots with their associated dorsal root ganglia (DRG)

• ventral roots

Dorsal root ganglion (DRG)

Dorsal root

Ventral root

• Each dorsal root contains the axons of sensory neurons

• Each dorsal root ganglion contains the cell bodies of these sensory neurons

• Each ventral root contains the axons of motor neurons

The dorsal & ventral roots of each segment come together at the intervertebral foramen (IVF) to form a mixed spinal nerve

Spinal Nerves• Part of the PNS

• Contain both motor & sensory fibers

• 31 pair of nerves – each nerve forms from union of dorsal/ventral root of spinal cord segment & exits between vertebra at IVF

•8 pair cervical spinal nerves – 1st cervical nerve exits between occipital bone & C1, 8th cervical nerve exits the IVF between C7-T1

• 12 pair thoracic spinal nerves

• 5 pair lumbar nerves

• 5 pair sacral nerves

• 1 pair coccygeal nerves

Below the conus medularis, spinal nerves must angle downward (in the subarachnoid space) before exiting their IVF. These spinal nerves make up the cauda equina

Cauda equina

Sectional Anatomy of the Spinal cord

Anterior median fissure

Posterior median sulcus

Lateral gray horn (T1-L2, S2-S4) - autonomic

Anterior gray horn - motor

Central canal

Gray commissure

Anterior column

Lateral column

Posterior column

Posterior gray horn - sensory

Key Note

The spinal cord has a narrow central canal surrounded by “horns” of gray matter connected by a commissure. Gray matter horns contain sensory & motor nuclei (groups of cell bodies).

Gray matter is surrounded by white matter “columns” which are made up of groups of myelinated axons creating organized ascending & descending tracts.

Tracts (Motor & Sensory Pathways)

• Groups of axons found in the white matter columns of the spinal cord that carry specific information

• Ascending tracts - carry sensory information up the spinal cord to areas of the brain (cerebrum or cerebellum)

• Descending tracts – carry motor information from the brain down to specific levels of the spinal cord

Ascending Tracts (Pathways)Three major groups of pathways transmit somatic sensory information originating from receptors, up the spinal cord to the brain –

Spinothalamic tracts

Posterior column pathways

Spinocerebellar tracts

Spinothalamic tractsAnterior spinothalamic tract (ASTT) – crude touch & pressure

Lateral spinothalamic tract (LSTT) – pain & temperature

THALAMUS

Posterior Column Pathways

Fasciculus cuneatus & fasciculus gracilis –

conscious proprioception (joint position)

discriminitive (fine) touch (2-point discrimination, stereognosis, graphism)

vibration

pressure

Spinocerebellar Tracts

Anterior spinocerebellar tract (ASCT) & Posterior spinocerebellar tract (PSCT) –

unconscious proprioception (from golgi tendon organs, muscle spindles & joint capsules)

muscle tone

balance

Descending PathwaysThree major pathways carry motor signals from conscious & unconscious areas of the brain, down the spinal cord to control contraction of skeletal muscles

Corticospinal tracts

Medial pathways

Lateral pathways

Corticospinal (Pyramidal) Tracts Corticobulbar tracts – voluntary control of skeletal muscles of head & neck

Lateral corticospinal tracts (LCST) – voluntary control of skeletal muscles in neck & body; fibers cross in pyramidal decussation of M.O.

Anterior corticospinal tracts (ACST) - voluntary control of skeletal muscles in neck & body; fibers cross at spinal cord level in anterior commissure

Medial & Lateral PathwaysIntegrated with corticospinal pathways to allow for coordination of motor activity, maintenance of posture and muscle tone

Medial pathways – unconscious control over trunk & proximal limb muscles for gross muscle movements

Lateral pathways – unconscious control over distal limb muscles for precise muscle movements

Tracts include: vestibulospinal, tectospinal, reticulospinal & rubrospinal

In order for sensory information to enter the spinal cord and ascend in a sensory tract, and for motor information to get from a descending tract to reach a skeletal muscle, impulses must travel through peripheral nerves (spinal nerves & cranial nerves)

Spinal Nerves 31 pair

Part of PNS

Formed by union of ventral (motor) root and dorsal (sensory) root

Dorsal ramus – transmits sensations from skin of back & neck; provides motor control of deep muscles of back

Once formed, spinal nerves will branch into Rami

Ventral ramus – provides motor control to muscles of extremities, anterior & lateral trunk; transmits sensations from all but skin of back

Rami communicantes – carry autonomic motor fibers (ANS) to smooth muscles & glands in ventral body cavity; transmit visceral sensations

Nerve Plexuses

Adjacent ventral rami will form complex interwoven networks of nerve fibers known as a nerve plexus

Emerging from each plexus will be specifically named peripheral nerves, which will contain fibers from multiple spinal cord levels

Cervical plexus (C1-C5) Phrenic nerve (C3-C5)

Brachial plexus (C5-T1)

Axillary nerve (C5-C6)

Musculocutaneous nerve (C5-7)

Radial nerve (C5-T1)

Median nerve (C6-T1)

Ulnar nerve (C8-T1)

Lumbar plexus (T12-L4)

Femoral nerve (L2-L4)

Iliohypogastric nerve (T12-L1)

Obturator nerve (L2-L4)

Sacral plexus (L4-S4)

Sciatic nerve (L4-S3)

Tibial nerve

Common peroneal (fibular) nerve

Ventral rami from T2-T11 do not participate in a plexus. Instead they form individual intercostal nerves

Dermatomes

Dorsal & ventral rami of spinal nerves provide control of skeletal muscle contraction & carry sensory input from the skin

Dermatomes – segmental sensory “map” of the body related to spinal nerve levels