Principles of Anatomy: Spinal Cord and Spinal Nerves

spinal cord segments

31 segments, each segment = one spinal nerve

cervical spinal nerves

8 segments = 8 pairs 

Thoracic spinal nerves 

12 segments, pairs 

lumbar spinal nerves 

5 segments, pairs 

conus medullaris

end of the spinal cord (L1-L2)

cauda equina

“horse tail” region distal to the medullary cone

pia mater

layer of meninges closest to the cord

subarachnoid space

space between arachnoid and pia where CSF flows

arachnoid mater

transparent and thin layer of meninges

subdural space 

space between dura and arachnoid

dura mater

thick outer layer of the meninges

epidural space

space between dura (periosteal) and bone filled with fat tissue where epidural is placed

lumbar cistern and lumbar puncture (spinal tap)

• largest collection of fluid in subarachnoid space at the bottom of spinal cord (L2-S2)

• ideal space for needle to draw CSF, easy to avoid hitting the spinal cord

• Needle is inserted between L3 and L4

Grey matter 

divided into dorsal and ventral horns

• cell body of motor neuron lives in ventral horn and axon leaves horn to form ventral root

white matter

Dorsal column, ventral column, lateral column

myelinated axons that run up and down cord carrying sensory (ascending) and motor (descending) info

Dorsal horn

found in the grey matter

• central process of sensory neuron innervates 

• receives incoming sensory info from peripheral and central processes 

ventral horn

found in grey matter

• where cell body of the motor neuron lives (efferent pathway)

column 

white matter tracts 

• lateral, dorsal, ventral

dorsal column 

ascending spinal tract (red)

• gracile fasciculus (medial)

• cuneate fasciculus (lateral)

gracile fasciculus

• dorsal column

• medial

• fine touch, proprioception for LOWER BODY

• decussates at medulla oblongata

Fasciculus cuneatus 

• dorsal column

• lateral

• fine touch and proprioception for UPPER BODY 

• decussates at medulla oblongata 

spinothalamic tract

ascending spinal tract

• lateral and ventral columns 

• pain and temperature signals to brain

• decussates in spinal cord 

corticospinal tract

• descending spinal tract

• anterior and lateral columns

• carry info from motor cortex of brain to cord 

lateral corticospinal 

descending

lateral dorsal column

voluntary limb movement 

90% cross at medulla (brainstem)

anterior corticospinal

descending

ventral dorsal column (medial)

voluntary truck control 

crosses at spinal cord

decussating

crossing over of motor tracts, left brain control right side of body

Upper motor neuron 

• myelinated by ogliodendrocytes in the CNS

• travel in spinal cord (corticospinal tracts)

• descends to level of cord where it can synapse with LMN

• TRIGGER MUSCLE CONTRACTION 

lower motor neuron 

• myelinated by schwann cells in the PNS 

• communicates with muscle cells that begin to release Ca to trigger muscle contraction 

multipolar neuron

neuron with many processes that leave cell body (motor neuron)

• cell body lives in the ventral horn and axon leaves through ventral root 

unipolar neuron (pseudo)

neuron with one process leaving the cell body that splits into a peripheral and central process

• cell body lives in dorsal root ganglion

• peripheral process carries sensory info to cell body from receptors

• central process sends info to the dorsal horn

dorsal root

connected by rootlets to dorsal root ganglion

ventral root

ventral, no root ganglion, rootlets connect to spinal cord

root

either motor OR sensory, not both

rootlets

connect root to the spinal cord

dorsal root ganglion

where cell body of sensory (afferent) neurons live

mixed spinal nerve

ventral root joins dorsal root (motor AND sensory)

Intervertebral foramen

• formed by pedicle and vertebral notches

• allow spinal nerves to leave canal

• where mixed spinal nerves (joined ventral and dorsal roots) exit and SPLIT after IVF into dorsal and ventral ramus

dorsal ramus

sends motor and sensory fibers to muscle and skin of back

ventral ramus 

contains nerves that go everywhere else in the body

cervical spinal nerves

8 total

thoracic spinal nerve

12 total

lumbar spinal nerve

5 total

sacral spinal nerve

4 total

lumbar & thoracic spinal nerves

spinal nerves leave below/inferior to corresponding vertebrae

cervical spinal nerves

• 8 cervical spinal nerves, but inly 7 cervical vertebrae

• exits above corresponding vertebrae

  • C3 SN would exit between C2 and C3

plexus

jumble of ventral rami

• 4 somatic plexus 

cervical plexus 

ventral rami C1 though C4

gives rise to the phrenic peripheral nerve

innervates the diaphragm 

brachial plexus

ventral rami C5 through T1

gives rise to median peripheral nerve 

provides motor and sensory to upper limb structures

lumbar plexus

ventral rami L1 through L5

gives rise to femoral peripheral nerve

innervates quads (motor) and skin of thigh (sensory)

sacral plexus

Ventral rami S1 through S4

gives rise to sciatic peripheral nerve 

innervates all other muscles of the lower limbs 

spinal nerve fascicle

• collection of nerve fibers

• composed of motor (efferent) and sensory (afferent) neurons 

• make up spinal nerves (mixed)

• neurons within are myelinated and unmyelinated

endoneurium

outside CT sheath around a neuron 

perineurium 

outside CT sheath around a fascicle

epineurium

outside CT sheath around an entire nerve

relfex arc

NO BRAIN INVOLVED

1. stimulation of receptor in muscle 

2. peripheral process of sensory neuron sends an afferent signal up a nerve fiber, to the cell body in the DRG, and into the central process that innervates the dorsal horn 

3. Synapse on cell body of multipolar motor neuron (LMN)

4. Efferent response is sent down a motor nerve fiber

5. LMN stimulates an effector