Spinal cord has ____ segments and ______ enlargements
31 segments
2 enlargements

Spinal enlargements
<answer>
<strong>Receive sensory (afferent) information from limbs</strong>
<strong>Send motor (efferent) axons to the limbs</strong>

Cervical = upper limb
Lumbosacral = lower limb
</answer>

Termination of adult spinal cord
L1 / L2

Termination of dural (thecal) sac
<answer>
S2 (PSIS)

End of arachnoid
</answer>

Distal attachment of spinal cord
Attached to coccyx via filum terminale

Layers of filum terminale
Internal filum = pia mater
External flium = dura mater

Origins of spinal ganglion
<answer>
Neural crest

(PNS!)
</answer>

Ventral Root
<strong>Motor / Efferent</strong>
Anterior

Dorsal Root
<strong>Sensory / Afferent</strong>
Posterior

Spinal ganglia contain ______
<answer>
Sensory neurons

(DRG's)
</answer>

Dermatome
<strong>Area of skin innervated by a single spinal segment</strong>

Myotomes
<answer>
<strong>Mass of muscle innervated by a particular spinal cord segment</strong>

Myotomes include portions of several muscles
Many muscles contain portions fo more than one myotome
</answer>

Radiculopathy
<answer>
Injury to spinal nerve<strong>
- Affects dermatome innervated by segment from which nerve arises</strong>
<strong>- Causes muscle weakness (not paralysis)</strong>

Myotome contributes to more than one muscle!
</answer>

Neuropathy
Injury to peripheral nerves
<strong>- Affects multiple adjacent dermatomes
- Causes muscle paralysis
</strong>

Lower Motor Neurons
- Anterior horn of spinal cord
- Axons innervate skeletal muscle (form motor part of peripheral nerves)
- "final common pathway" = receive information from brain (UMN)

Upper Motor Neurons
- Higher centers of brain (e.g. motor cortex)
- Cross in pyramidal decussation (i.e. medulla)
- Axons excite / inhibit lower motor neurons

Anterior Horn Somatotopy
<answer>
<span style="text-decoration:underline;">Gray Matter</span>
<strong>Medial LMNs = proximal muscles</strong>
<strong>Lateral LMN's = distal muscles</strong>

<strong>Dorsal LMN's = flexors
Ventral LMN's = extensors </strong>
</answer>

Spinal cord directional landmark
Anterior fissure = ventral

<question>
Somatic Reflexes: 
Afferent Component
</question>
<strong><span style="text-decoration:underline;">Afferent component
</span></strong>- 1<sup>o</sup> sensory / afferent neurons (Ia) in DRG
- Peripheral axons carry info from receptors in skin / muscles / tendons / joints
- Central axons carry info into spinal cord via dorsal roots

<question>
Somatic Reflexes:
Efferent Component
</question>
<answer>
<span style="text-decoration:underline;"><strong>Efferent component</strong></span>
- Spinal LMN's (efferent) in anterior horn
- Cholinergic

<strong>&alpha; motor neurons -&gt; extrafusal fibers</strong> 
     - generate muscle force
<strong>&gamma; motor neurons -&gt; intrafusal fibers</strong> 
     - regulate tension on muscle spindle
</answer>

Motor Unit
<answer>
<strong>One &alpha; motor neurons + extrafusal fibers</strong> = motor unit

<span style="text-decoration:underline;">Large motor unit</span> -&gt; large muscles w/ coarse movements
<span style="text-decoration:underline;">Small motor unit</span> -&gt; small muscles w/ fine movements
</answer>

Muscle Spindle
<span style="text-decoration:underline;">Proprioceptor (length)</span>
&gamma; motor neuron
1<sup>o</sup> (Ia) afferent neuron
2<sup>o</sup> (II) afferent neuron

Golgi Tendon Organ
<span style="text-decoration:underline;">Proprioceptor (force)</span>
GTO (Ib) afferent neuron

Lateral Corticospinal Tract
<answer>
- Arises from UMN in motor cortex
- 85-90% axons descend / cross in pyramidal decussation (caudal medulla)
- Innervates LMNs (which innervate limbs)
- Trunk muscles tend not to cross 

Clinically most important descending tract
</answer>

Somatosensory Systems
<answer>
Primary sensory cortex = postcentral gyrus (BA 3,1,2)

<strong>Two major sensory (ascending) pathways
</strong><span style="text-decoration:underline;">Dorsal columns (lemniscal system)</span>
     - discriminative touch / proprioception / vibration
     - crosses in medulla

<span style="text-decoration:underline;">Anterolateral (spinothalamic) system</span> 
     - crude touch / pain / temperature
     - crosses in spinal cord
</answer>

Meninges
- Periosteum
- Epidural space
- <strong>Dura mater
</strong>- Subdural space (potential space)
- <strong>Arachnoid mater
</strong>- CSF
- <strong>Pia mater </strong>(tightly adherent to cord)

Denticulate ligaments
- Formed by pia mater
- Suspends spinal cord

Pachymeninx
<strong>Dura Mater</strong>
- Derived from mesenchyme
- Dense CT 
     - longitudinal collagen fibers w/ circular elastic fibers

Leptomeninx
<strong>Arachnoid / Pia Mater
</strong>- Derived from neural crest
- Contains blood vessels