Download Gross Anatomy Lecture 1: Spinal Cord and Nerves I. Basic

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Gross Anatomy Lecture 1: Spinal Cord and Nerves
Basic Terminology
A. Neuron
1. Cell body – processes called neuritis
a. Dendrites – receive information
b. Axon – carries away information from cell body
2. Nucleus – collection of nerve cell bodies that perform a common function within
brain or spinal cord
3. Ganglion – a collection of nerve cell bodies outside brain and spinal cord
4. Afferents – nerve fibers that carry information or signals to a ganglion or nucleus
5. Efferents – nerve fibers that carry info away from cell body
6. Types of Neurons:
a. Sensory neuron (Dorsal root ganglia)
b. Motor neuron (spinal cord)
B. Central Nervous System (CNS)
1. Neurons whose cell bodies and neuritis are contained within brain and SC
C. Peripheral Nervous System (PNS) – carry motor, sensory and/or autonomic signals
between CNS and body (composed of cranial, spinal, splanchnic nerves)
1. Somatic Nervous System – innervates skeletal muscles of the body wall or limbs and
maintain voluntary control over body movement
-Somatic nerves that innervate skeletal muscles at SC levels are SPINAL
nerves; at brain or brainstem level are CRANIAL nerves
a. Motor – Somatomotor: voluntary and reflex contraction of skeletal muscle
b. Sensory – somatosensory: sensory innervation of skin, muscles, and joints
2. Visceral Nervous System –innervates internal organs, smooth/cardiac muscle, and
sweat glands. Controls most of unconscious homeostatic mechanisms of the body
and innervate smooth muscle of BV, cardiac muscle of heart, glands in skin and
various organs or viscera in body cavity
-Splanchnic nerves – innervate visceral nervous system
a. Motor - Visceromotor (Autonomic Nervous System): involuntary contraction
of smooth muscle, cardiac muscle and glands
i. Divided into sympathetic and parasympathetic divisions
b. Sensory – Viscerosensory: modulates blood pressure and chemistry,
respiration, etc.
Spinal Cord
A. General morphology
1. Extends from foramen magnum and ends in cone shaped structured (conus
medularis between L1 and L2 of vertebral column
a. Filum terminale – ligament attaches SC to vertebral column caudally
2. Cauda equine – collection of dorsal and ventral roots exiting and entering SC below
L1 or L2 (continue below level of conus medularis)
3. Lumbar cistern: space between conus medularis and end of vertebral canal
a. Lumbar punctures are performed in patients to assess the content of the CSF
and are done at the level of the cauda equina in order to prevent damage to
the spinal cord
b. Needle inserted in midline between spinous processes of L3 and L4 (or L4
and L5) vertebrate
4. 2 enlargements towards cranial and caudal ends
a. Cervical enlargement – area of cord where additional motor neurons are
located that innervate upper limb
b. Lumbar enlargement – contains neurons that innervate lower limb
III.
5. Spinal nerves exit vertebral canal between individual vertebrae through
intervertebral foramen to innervate muscles from neck to feet
a. 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1
coccygeal)
6. Relationship between spinal cord level and vertebral level in adult
a. Vertebral level – position of an individual vertebrate along vertebral column
b. Spinal cord level – location of an individual spinal cord segment along the SC
i. Spinal cord segment – region of spinal cord that contains all the
neurons and nerve roots that make up individual spinal nerve
c. In adults SC and vertebral elves not equivalent because vertebral column
elongates at faster rate than the SC
i. SC shorter than vertebral column – ends at L1 (cauda equina extend to
end of vertebral column)
ii. 31 pairs of spinal nerves and only 30 vertebrate
d. Position of spinal nerve as it exist vertebral column changes in relation to the
vertebrate it exists adjacent to
i. Between C1-C7, spinal nerve exists above the vertebrate with same
anatomical name
ii. SN C8 exits between C7 and T1 vertebrate
iii. Caudal to T1 all spinal nerves exit column below vertebrae with the
same name
e. In cervical region, SC segment and vertebral level closely linked
f. In lumbar region, they are far apart
B. Gray and white matter
1. Gray matter: Central parts of cord contain the cell bodies
a. Shape of gray matter has H or butterfly appearance – 3 parts
i. Dorsal (posterior) horn: specialized for receipt of sensory information
from peripheral nerve
ii. Ventral (anterior) horn: contains motor neurons whose axons exit the
spinal cord and innervate skeletal muscle
iii. Intermediate (lateral) horn at levels T1-L2: contains neurons that are
part of the ANS and exit spinal cord to modulate autonomic activity in
the periphery
2. White matter – area surrounding gray matter that contains myelinated axons that
travel up and down the cord
3. Grey/White Matter Characteristics of spinal cord – grey/white matter ratio differs in
different parts of SC
a. Cervical SC
i. Large ventral horns for innervation of upper limb
ii. Large amount of dorsal white matter for transmission of sensory
information from all parts of body
b. Thoracic spinal cord
i. Special area of gray matter (lateral horn) that houses cell bodies for
sympathetic (preganglionic) neurons)
Spinal Nerves
A. Nerve roots
1. Dorsal root – carries sensory information from periphery into dorsal horn of spinal
cord
a. Composed of sensory axons whose cell bodies are located in dorsal root
ganglion (somatosensory and viscerosensory axons)
IV.
b. Dorsal root ganglion – contains somatosensory and viscerosensory cell
bodies
2. Ventral root – carries motor information form spinal cord to periphery
a. Somatomotor axons in ventral root derived from motor neurons in ventral
horn
b. Ventral roots of thoracic and lumbar spinal nerves carry visceromotor
(autonomic –sympathetic preganglionic) axons from cell bodies in
intermediate horn of spinal cord
3. Spinal nerve – anatomical definition – small part of nerve where roots and rami
come together and sensory and motor fibers cross
a. At intervertebral foramen, two nerve roots join to form spinal nerve which
divides into two primary rami
B. Rami
1. Dorsal primary ramus- first branch of spinal nerve that courses dorsally
(somatosensory, Somatomotor, post-ganglionic sympathetics)
a. Supplies fibers (sensory and motor) to synovial joints of vertebral column,
deep muscles of the back and the skin that overlays the deep back
musculature
b. Posterior cutaneous nerve: begin at surface of erector spinae muscle and
continue to the skin, passing though superficial back muscles without
innervating them
2. Ventral primary ramus – carries both sensory and motor fibers to and from rest of
the body including anterior and lateral muscles of trunk and limbs
a. Lateral or anterior cutaneous nerve: part of ventral ramus that innervates
skin on the lateral and anterior surface of the trunk
3. Cutaneous nerve: nerve that innervates the skin and contains somatosensory and
visceromotor (ANS) axons
C. Components of spinal nerve – have two targets (skeletal muscles and skin)
1. Innervates muscles – all three components
a. Somatomotor, somatosensory, visceromotor
2. Innervates skin
a. Contains somatosensory and visceromotor ANS axons
D. Dermatome - area of skin innervated by sensory fibers of single spinal nerve or spinal cord
segment
1. Sensory or cutaneous fibers of primary rami that innervate skin remained
segregated and have segmented distribution on skin surface
2. Sensory fibers from dorsal and ventral rami of each SC segment come together to
form a continuous band of sensory input from a localized area of body wall
Processing of sensory information
A. Sensory motor circuit
1. Sensory info conveyed to CNS by sensory fibers located in spinal and cranial nerves
2. Sensory limb (afferent) gives info to brain/SC which gives info to motor limb
(efferent)
3. Sensory motor circuit integrates information between peripheral and central NS
B. Two pathways to respond of sensory stimulus
1. Voluntary – sensory info can be evaluated at higher centers such as cortex to
formulate conscious (voluntary) motor plan to guide desired response
a. Ex picking up glass of water
b. Sensory limb  spinal cord cortex spinal cord  motor limb
2. Reflex - rapid involuntary movement in response to sensory stimulus that does not
require conscious thought
a. Somatic reflex – contraction of skeletal muscle and occur naturally to prevent
injury from harmful stimulus (stepping on tack or touching hot pot)
i. Integrity of somatic reflex tested clinically using stretch reflex
 Knee jerk reflex – tests L2, l3, L4
 Ankle jerk reflex – tests S1, S2
ii. Decreased reflex response indicates peripheral nerve injury
iii. Exaggerated reflex indicates injury to the CNS
b. Cranial nerve reflex -involve components of either or both visceral and
somatic nervous system as well s our special senses
i. Pupillary light reflex – regulates size of pupil based on intensity of
light
ii. Accommodation reflex – adjusts thickness of lens to maintain focus of
objects both near and far
iii. Corneal reflex – eyes blink to lubricate eye to remove foreign object
iv. Gag reflex – prevent something from entering throat except as part of
normal swallowing, also evoked by strong negative taste