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 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
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
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
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 (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 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)
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)
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)
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)
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.
Instead they form individual
intercostal nerves (aka
thoracic nerves)
Motor supply to intercostal &
abdominal muscles; sensory
from anteriolateral
thorax/abdomen