The Spinal Cord and Spinal Nerves
1. Introduction:
a. The spinal cord and spinal nerves mediate reactions
to environmental changes.
b. The spinal cord has several functions:
--Process reflexes.
--Site for integration of EPSP & IPSP that arise
locally or are triggered by nerve impulses from
the periphery & brain.
-It is a conduction pathway for sensory and motor
nerve impulses.
2. Anatomy of spinal cord:
A. Spinal cord is covered by sheath:called
Meninges and also by
--Vertebra.
--Cerebrospinal fluid (CSF)
a. Meninges: they are
--Dura mater (outermost layer)
--Arachnoid mater (middle layer)
--Pia matter (innermost layer). It adheres to the
surface of spinal cord & brain.
Meningitis refers to inflammation of the meninges.
b. Vertebral column provides a bony covering of the
spinal cord.
c. Cerebrospinal fluid (CSF): fluid formed in the brain
& remains in the subarachnoid space (between
arachnoid &pia matter).It protects and give cushion
to the spinal cord.
B. External anatomy:
i.
Spinal cord begins as a continuation of medulla
oblongata & terminates at about second lumbar
vertebra in the adult.
ii. The tapered portion of the spinal cord is the
conus medullaris, from which arise the filum
terminate & cauda equina.
iii. Contains cervical and lumber enlargements that
serve as point of origin for nerves to the
extremities.
iv. Spinal nerves:
 31 pairs named and numbered according to their
region & & level of the spinal cord from which they
emerge
 There are 8 pairs of cervical nerves, 12 pairs of
thoracic nerves, 5 pairs of lumber nerves, 5 pairs of
sacral nerves & 1 pair of coccygeal nerve.
 Spinal nerves are the path of communication
between the spinal cord & most of the body.
 Spinal roots are the two points of attachment that
connect each spinal nerve to a segment of the spinal
cord.
a. Posterior dorsal root (sensory):
Contain sensory nerve fibers and conducts nerve
impulses from the periphery into the spinal cord.
The posterior root ganglia contains the cell bodies
of the sensory neuron
b. The anterior or ventral (motor) root: contains
motor neuron axons, and conduct impulses from
the spinal cord to the periphery. Cell bodies of
the motor neurons are located in the gray matter
of the spinal cord.
v.
Lumbar puncture or spinal tap: By this procedure
CSF is removed from the subarachnoid space. It
id used:
 to diagnose any pathology in the spinal cord.
 To introduce antibiotic, contrast media,
anesthetics and chemotherapeutic drugs in the
spinal cord.
C. Internal anatomy of the spinal cord:
i.
The anterior median fissure and posterior
median sulcus penetrate the white matter &
dendrites into right and left side
ii. The gray matter is shaped like the letter ‘H’ or a
butterfly and is surrounded by the white matter.
a. Gray matter consists primarily of cell bodies of
neurons and neuroglia, unmyelinated axons,
dendrites and motor neurons.
b. The white matter consists of bundles of myelinated
axons of motor & sensory neurons.
iii. Central canal: It is the center of ‘gray
commissure’ which forms the cross bar of the H
shaped gray matter.
iv. Anterior white commissure is situated anterior
to the gray commissure. It connects the white
matter of the both side of the spinal cord.
v. The gray matter is divided into ‘horns’, which
contain cell bodies of neurons.
vi. The white matter is divided into columns. Each
column contains distinct bundles of nerve axons
& carry similar information. These bundles are
called ‘tracts’.
--Sensory or ascending tracts conducts impulses
towards brain.
--Motor or dscending tracts conduct impulses
down the cord.
3. Spinal cord physiology:
The spinal cord has two principal functions:
I. Gray matter receives and integrates incoming &
outgoing information.
II. The white matter tracts are highways for nerve
impulse conduction to & from the brain.
A. Sensory & motor tracts:
 Sensory tracts are: spinothalamic tract.
Posterior (dorsal) column tract.
 Motor tracts are the pyramidal tract.
The extra-pyramidal tract.
The axons of various nerves & CNS tracts develop
myelin sheath at different times, which explains the
poor sensory & motor development of newborns.
B. Reflexes:
--Spinal cord serves as an integrating center for spinal
reflexes.
--A reflex is an involuntary motor response to a
stimulus.
--A reflex may be somatic, & autonomic (visceral).
C.Reflex arc:
i. The components of a reflex arc are:
--receptors.
--Sensory neuron or afferent pathway.
--Integrating center.
--Motor neuron or efferent pathway.
--Effector organ
ii. Reflexes help to maintain homeostasis by making
rapid adjustments.
D. Somatic spinal reflexes include:
--Stretch reflex, Tendon reflex.
--Flexor or Crossed extension reflex or
Withdrawal reflex.
All the above reflexes exhibit reciprocal innervation.
a. Stretch reflex:
--The stretch reflex is important in maintaining muscle
tone and muscle coordination during exercise.
 it is monosynaptic reflex arc meaning that it
has one synapse in between one sensory
neuron and a motor neuron.
 Example of stretch reflex: knee jerk, biceps
jerk etc.
 it operates as a feedback mechanism to control
muscle length by causing muscle contraction.
b. Tendon reflex:
--it operates as a feedback mechanism to control
muscle tension by causing muscle relaxation when
muscle force become too extreme. Example: claspknife reflex.Receptor: Golgi Tendon Organ
c. Flexor or crossed extension reflex:
--Flexor or withdrawal reflex.
--It is a protective reflex that moves a limb to avoid
pain
--It results in contraction of flexor muscles of the
same side & contraction of extensor muscles of the
opposite side producing crossed extension reflex to
maintain balance.
4. Spinal nerves:
A. Spinal nerves connect the CNS to sensory
receptors, muscle & gland.
 They are part of PNS
 31 pairs of spinal nerves: cervical
Thoracic.
Lumbar & sacral
 Spinal nerve connects to the cord via an
anterior and a posterior root Posterior root
contains sensory axon & the anterior root
contains motor axon—so a spinal nerve is a
mixed nerve.
B. Distribution of spinal nerves:
I. After passing through its intervertebral
foramen, a spinal nerve divides into several
branches—known as ‘rami’
II. Branches of spinal nerves include:
--Dorsal ramus.
--Ventral ramus.
--Meningeal branch.
--Rami communicantes.
iii. Plexuses::
The ventral rami of spinal cord
form a network of nerves called Plexuses
a. The cervical plexus: supplies the skin & muscles of
the head, neck & upper part of the shoulder connect
with some cranial nerves.
 supplies the diaphragm by phrenic nerve.
 Clinical application: Damage to the spinal cord
above the origin of the phrenic nerve cause
respiratory arrest.
b. The brachial plexus: constitutes the nerve supply
for the upper extremity and to a number of neck &
shoulder muscles.
Clinical application: Injury to brachial plexus leads to
Erb-Duchene Palsy or Waiter’s tip palsy, wrist
drop,(Radial nerve injury) , carpal tunnel syndrome,
claw hand etc
c. The lumbar plexus: supplies
--Anterolateral abdominal wall.
--External genital
--Part of the lower extremity
Clinical application: i. Femoral nerve injury
 Leads to inability to extend the leg.
 Leads to loss of sensation in the skin over
antero-lateral part of the thigh.
ii. Obturator nerve injury:
--a complication of child birth.
--Result in paralysis of the adductor muscle
of the leg and loss of sensation over medial
aspect of the thigh.
d. The sacral plexus: supplies the buttock, perineum
and part of lower extremity
Clinical application:
--Sciatic nerve injury: Results in Sciatica where pain
extends from the buttock down to back of the leg.
--Can occur due to herniated disc, dislocated hip, and
osteoarthritis of lumbosacral joint (spine)
5. Dermatomes:
i.
All spinal nerves except C1 innervate specific,
constant segment of the skin, the skin segments
are called Dermatomes.
ii.
Knowledge of Dermatomes helps a physician to
determine which segment of the spinal cord or
which spinal nerve is malfunctioning.
6. Disorders:
--Neuritis
--Shingles: an acute infection of peripheral nerve
by herpes Zoster virus—causing pains, skin
discoloration & blisters ( Chiken Pox).
 Poliomyelitis (infantile paralysis): a viral
infection leads to fever, headache, stiffness of
back & neck, deep pain, weakness & loss of
some reflexes.
 Virus destroys motor neuron cell bodies.
The Spinal cord and Spinal nerves
1. Introduction.
2.Anatomy of spinal cord:
External anatomy
Internal anatomy
3. Spinal cord physiology
Sensory and motor tracts
Reflexes
Reflex arc
Somatic reflex: stretch reflex
Tendon reflex
Flexor & crossed
extension reflex
4. Spinal nerves: distribution
Plexuses: cervical, brachial
Lumber, sacral
5. Dermatome
6. Disorder