The Nervous System
Spinal Cord & Spinal Nerves
Anatomy – Chapter 14 & 16
(p. 432 - 439)
The Central Nervous System
The CNS is well protected by bone, CT, and fluid
Meninges – Connective tissues that surround and
protect the brain and spinal cord
Three layers: Dura mater,
Arachnoid, Pia mater
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 cerebrospinal 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
 Has 2 thickened areascervical enlargement - supplies
nerves to upper extremity
lumbar enlargement - supplies
nerves to lower extremity
 Made up of 31 spinal cord
segments
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 C7T1
• 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
Posterior median sulcus
Posterior column
Posterior gray horn sensory
Central canal
Lateral column
Gray commissure
Anterior column
Lateral gray horn (T1-L2, S2S4) - autonomic
Anterior gray horn motor
Anterior median fissure
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)
(Chap. 16, p. 432-439)
• 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
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 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 (2point 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 pathway (includes anterior & lateral
corticospinal tracts, and corticobulbar tracts)
 Medial pathways
 Lateral pathways
Corticospinal (Pyramidal) Pathway
 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
Integrated with corticospinal pathways to allow for
coordination of motor activity, maintenance of posture and
muscle tone
Medial pathways – unconscious control over neck, 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
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)
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 T2T11 do not participate
in a plexus. Instead
they form individual
intercostal nerves