The Nervous System Spinal Cord, Spinal Nerves & Tracts Anatomy – Chapters 14 & 15

The Spinal Cord Begins at foramen magnum, runs through vertebral foramen (spinal canal), & ends at L2 vertebral level by forming conus medularis The spinal cord (as well as the brain) is well protected by bones, CT membranes (meninges), and fluid (cerebrospinal fluid (CSF))

Meninges Meninges – membranes that surround and protect the CNS Three layers: Dura mater Arachnoid mater Pia mater

Dura Mater – tough, fibrous CT outer membrane; one layer thick around spinal cord with epidural space external Arachnoid mater – “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 cerebrospinal fluid (CSF) Lumbar cistern – area of subarachnoid space below the conus medularis; site for lumbar puncture (“spinal tap”) Lumbar cystern

Spinal Cord Anatomy Begins at foramen magnum & ends at L2 vertebral level by forming conus medularis Has 2 thickened areas- cervical enlargement - supplies nerves to upper extremity lumbar enlargement - supplies nerves to lower extremity Made up of 31 spinal cord segments

Each spinal cord segment has a pair of Dorsal root ganglion (DRG) Dorsal root Ventral root Each spinal cord segment has a pair of dorsal roots with their associated dorsal root ganglia (DRG) ventral roots

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/sacral foramina. These spinal nerves make up the cauda equina Cauda equina

Sectional Anatomy of the Spinal cord Anterior median fissure Posterior median sulcus Anterior column Lateral column Posterior column Posterior gray horn - sensory Central canal Lateral gray horn (T1-L2, S2-S4) - autonomic Gray commissure Anterior gray horn - motor

The spinal cord has a narrow central canal surrounded by “horns” of gray matter connected by commissures. Gray matter horns contain sensory & motor nuclei (groups of cell bodies) & glial cells. Gray matter is surrounded by white matter “columns” (aka funiculi) which are made up of groups of myelinated axons creating organized ascending & descending tracts (aka fasciculi).

Tracts (Motor & Sensory Pathways) (Chap. 15) 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 (eventually terminating in cerebrum or cerebellum) Descending tracts – carry motor information from the brain down to specific levels of the spinal cord (eventually terminating on skeletal muscles)

Ascending Tracts 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 tracts Anterior 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 Pathways Carry motor signals from conscious & unconscious areas of the brain, down the spinal cord to control contraction of skeletal muscles Corticospinal pathways (aka Pyramidal tracts)- include anterior & lateral corticospinal tracts, and corticobulbar tracts) – conscious motor control Subconscious Motor Pathways – include medial and lateral pathways

Corticospinal (Pyramidal) Pathways Corticobulbar tracts – voluntary control of skeletal muscles of head & neck through cranial nerves 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 Pathways Originate from subconscious areas of the brain and are integrated with corticospinal pathways to allow for coordination of motor activity, maintenance of posture and muscle tone Medial pathways – unconscious control over head, eyes, neck, trunk & proximal limb muscles for gross muscle movements; include vestibulospinal, tectospinal, & reticulospinal tracts Lateral pathways – unconscious control over distal limb muscles for precise muscle movements; include rubrospinal tracts

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

Once formed, spinal nerves will branch into Rami Dorsal ramus – transmits sensations from skin of back & neck; provides motor control of deep muscles of back; found at all spinal nerves

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

Rami communicantes (white ramus & gray ramus) – carry autonomic motor fibers (ANS) to smooth muscles & glands in ventral body cavity; transmit visceral sensations; only found at T1-L2 spinal nerves

Nerve Plexuses Adjacent ventral rami will form complex interwoven networks of nerve fibers (axons) known as a nerve plexus Four plexuses – cervical, brachial, lumbar, & sacral 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) Motor control for muscles of neck region, levator scapulae, scalenes, SCM & trapezius (along with CN XI), and diaphragm Sensory from upper chest, shoulder, neck & ear regions Phrenic nerve (C3-C5)

Brachial plexus (C5-T1) Axillary nerve (C5-C6) Motor to & sensory from pectoral girdle region & upper extremities Axillary nerve (C5-C6) Musculocutaneous nerve (C5-7) Radial nerve (C5-T1) Median nerve (C6-T1) Ulnar nerve (C8-T1)

Lumbar plexus (T12-L4) Iliohypogastric nerve (T12-L1) Motor to muscles of abdominal, pelvic and lower extremity (anterior & medial) regions Sensory from skin of abdomen, pelvis & lower extremity Iliohypogastric nerve (T12-L1) Lateral femoral cutaneous nerve (L2-L3) Femoral nerve (L2-L4) Obturator nerve (L2-L4)

Sacral plexus (L4-S4) Sciatic nerve (L4-S3) Tibial nerve Motor to muscles of pelvis and lower extremity (gluteal, posterior femoral, lower leg & foot) Sensory from posterior pelvis, posterior thigh, anterior, posterior & lateral leg Sciatic nerve (L4-S3) Tibial nerve Common peroneal (fibular) nerve Sural nerve

Ventral rami from T2-T11 do not participate in a plexus Ventral rami from T2-T11 do not participate in a plexus. Instead they form individual intercostal nerves (aka thoracic nerves) Motor supply to intercostal & abdominal muscles; sensory from anteriolateral thorax/abdomen