Chapter 13 - Spinal Cord & Spinal Nerves Flashcards
Spinal Cord
- 2 Main Functions: Conduction pathway & reflex center
- Located within vertebral canal
- 3 Main Protections: Vertebral column, 3 meninges, cerebrospinal fluid
3 Meninges
- Dura Mater
- Arachnoid Mater
- Pia Mater
Denticulate Ligaments
- Supports/protects/centers spinal cord within subarachnoid space
- Located between anterior & posterior roots of spinal nerves
Cerebrospinal Fluid
- Made by choroid plexuses & secreted by ependymal cells of brain ventricles
- Circulates in brain ventricles, connecting channels, subarachnoid space & central canal of spinal cord
3 Spaces Between Meninges
- Epidural: Between Dura & bone
- Subdural: Under Dura
- Subarachnoid: Between Pia & Arachnoid
Lumbar Puncture
- Insertion of a needle between L3-L4 or L4-L5
- Used for extraction of CSF/injection of anesthesia
- Supracristal line identifies L4
- AKA “Spinal Tap”
External Anatomy of Spinal Cord
- Extends from foramen magnum to the space between vertebrae L1 & L2
- Conus Medullaris = Tapered end
- Filum Terminale = Anchoring cord
- Cauda Equina = Spinal nerve roots
31 Spinal Nerve Pairs (Typing)
- 8 Cervical
- 12 Thoracic
- 5 Lumbar
- 5 Sacral
- 1 Coccygeal
2 Enlarged Areas of the Spinal Cord
- Cervical (C4 - T1) = Upper Limbs
2. Lumbar (T9 - T12 L1 & Cauda Equina) = Lower Limbs
Posterior Median Sulcus
Divides the posterior side of the spinal cord
Anterior Median Fissure
Divides the anterior side of the spinal cord
Gray Matter of Spinal Cord
- Contains sensory axon terminals, interneurons, and motor neurons (all unmyelinated)
- Also contains neuroglia
- Integrating center for spinal reflexes
- Is “H-shaped”
Gray Commissure
Cross-bar of the gray matter
Central Canal
- Extends the length of spinal cord
- Continuous w/ 4th ventricle
- Contains CSF
Sensory Nuclei
Cell bodies of interneurons within gray matter
Motor Nuclei
Cell bodies of motor neurons within gray matter
3 Types of Gray Horns
- Anterior Gray Horn: Contains somatic motor neurons (connects to skeletal muscle)
- Lateral Gray Horn: Contains visceral motor neurons (connects to visceral smooth muscle)
- Posterior Gray Horn: Contains interneurons (connects to incoming sensory axons)
2 Spinal Nerve Roots on Each Side (Gray Matter)
- Anterior Root & Rootlets
2. Posterior Root & Rootlets
Anterior Root & Rootlets
- Contains axons of lateral & anterior horn motor neurons
- Motor fibers exit cord via anterior roots
Posterior Root & Rootlets
- Contains axons of sensory neurons & posterior root ganglion
- Sensory fibers enter cord via posterior roots
Spinal Nerve
Combination of both anterior root & posterior root
Spinal Nerve Root Damage
- Most common cause: Spinal disc herniation
- Effects: Loss of sensation, muscle coordination + pain
White Matter of Spinal Cord
- Contains tracts (Bundles of myelinated axons of motor & sensory neurons)
- Anterior white commissure = found right in front of the gray commissure
- 3 White columns: Posterior, Lateral & Anterior (Each representing a sensory or motor fiber tract)
5 Generalizations About Spinal Tracts
- Multi-neuron pathways between brain & periphery
- Pyramidal Tract = CNS->UMN->LMN->Skeletal Muscle
- Spinothalamic Tract = Periphery->Neurons->CNS - Axons w/ similar destinations & functions
- Purely motor tracts OR purely sensory tracts
- Come in pairs
- Most decussate/cross over
2 Main Spinal Cord Functions
- Information transmission (Via sensory & motor white matter tracts)
- Information integration (Reflexes & reflex arcs)
3 Main Sensory Tracts
- Spinothalamic Tract: Located in the anterior & lateral white columns
- Posterior White Column Sensory Tracts (Gracile & Cuneate): Located in the posterior white column
- Anterior & Posterior Spinocerebellar Tracts: Located in the lateral white column
2 Types of Motor Tracts
- Direct Motor Pathways
2. Indirect Motor Pathways
Direct Motor Pathways
- 2-neuron Pathways: Upper motor neuron (UMN) & Lower motor neuron (LMN); the starting points of the pathways
- Anterior & Lateral Corticospinal Tracts: Located in the Anterior & Lateral White Columns
Indirect Motor Pathways
- UMN arises from motor nuclei in brainstem
- Rubrospinal & Vestibulospinal Tracts: Located in the Lateral White Column
- Tectospinal Tract: Located in the Anterior White Column
- Lateral & Medial Reticulospinal Tracts: Located in the Lateral & Medial White Columns
Neural Reflex
- Automatic motor response by nervous system to changes in external/internal environment in order to preserve homeostasis
- Examples: Somatic reflexes (withdrawal reflex) & Autonomic reflexes (heartbeat regulation)
5 Components of a Neural Reflex Arc
- Stimulus activates receptor (specialized cell/tissue that monitors internal/external environment)
- Receptor activates sensory neuron
- Integrating center (CNS info processing site; can be monosynaptic or polysynaptic)
- Integrating center activates motor neurons
- One ore more effectors respond
6 Classifications of Reflexes & Reflex Arcs
- Spinal Reflexes
- Cranial Reflexes
- Somatic Reflexes
- Visceral Reflexes
- Ipsilateral Reflex Arc
- Contralateral Reflex Arc
Spinal Reflex vs. Cranial Reflex
- Spinal = Process of the spinal cord; mostly innate
- Cranial = Processed in the brain; mostly learned reflexes
Somatic Reflex vs. Visceral Reflex
- Somatic = Controls skeletal muscles
- Visceral = Controls visceral organs
Ipsilateral vs. Contralateral
- Ipsilateral: Sensory stimulus & motor response on the same side of the body
- Contralateral: Motor response on side opposite to the stimulus
Monosynaptic vs. Polysynaptic
- Monosynaptic = Sensory neuron synapses directly on motor neuron (E.g, Stretch Reflex)
- Polysynaptic = Reflex involving at least one interneuron between sensory & motor neurons (3 Main Examples)
5 Features of a Polysynaptic Reflex
- Interneuron involvement
- May be intersegmental
- May involve reciprocal innervation (if there are opposing muscles)
- May use reverberating circuits (prolonged exposure)
- Several reflexes may cooperate
Stretch Reflex
Regulates skeletal muscle length & muscle tone; sensory receptors = muscle spindles
- Sensory Nerve endings wrapped around intrafusal fibers
- Example: Patellar reflex (AKA “Knee Jerk”); CNS sets muscle tone
Steps of a Stretch Reflex
- Generator potential
- AP in sensory neuron
- EPSPs in motor neuron
- AP in motor neuron
- ACh release
- EPSPs in extrafusal fibers
- AP in extrafusal fibers
- Muscle contraction
- Relieved strain of stretch
Postural Reflexes
- Stretch reflexes used to maintain upright posture
- An example of reciprocal innervation: normally, when a stretched muscle (agonist) contracts, its antagonists relax
3 Examples of Polysynaptic Reflexes
- Tendon Reflex
- Flexor (Withdrawal) Reflex
- Crossed Extensor Reflex
Tendon Reflex
- Golgi tendon organ prevents stretching damage to tendons when muscles contract vigorously
- Relaxation of stimulated muscle (via inhibitory interneuron) & contraction of antagonist (via excitatory neuron)
- Tendon reflex overrides stretch reflex
Flexor (Withdrawal) Reflex
- Move affected body part away from stimulus
- Usually initiated by pain stimuli
- Ipsilateral
- Polysynaptic
- Intersegmental
- Reciprocal innervation
Crossed Extensor Reflex
- Complements withdrawal reflex
- Polysynaptic
- Intersegmental
- Contralateral
- Example: Step on a nail, flexor reflex pulls away right foot, crossed extensor reflex stiffens left leg
3 Reflexes for Diagnosing Disease
- Babinski Sign
- Patellar Reflex
- Achilles Reflex
Babinski Sign
- Positive: Dorsiflexion of great toe when lateral sole is stroked (+ possible fanning of other toes); normal spinal reflex for infants under 18 months; may be sign of CNS injury of oldar than 18 months
- Negative: Plantar flexion reflex; represents modification of the spinal reflex due to influences of myelinated descending corticospinal tract fibers; normal in children older than 18 months & adults
Patellar & Achilles Reflexes
If either reflex is diminished or exaggerated, CNS damage may have occured
Spinal Nerve Anatomy
- Spinal nerves = union of posterior & anterior spinal roots
- Mixed sensory/motor nerves
- 3 connective tissue sheaths
3 Spinal Nerve Sheaths
- Endoneurium: Wraps fibers
- Perineurium: Wraps fascicles
- Epineurium: Wraps whole nerve; continuous w/ dura mater
4 Branches (Rami) of Spinal Nerves
- Posterior Rami: Connects to muscles & skin of back
- Anterior Rami: Connects to limbs & muscles of lateral/ventral trunk
- Thoracic Anterior Rami = Intercostal Nerves
- Other Anterior Rami = Nerve Plexuses - Meningeal Rami
- Rami Communicantes
4 Spinal Nerve Plexuses
- Cervical Plexus
- Brachial Plexus
- Lumbar Plexus
- Sacral & Coccygeal Plexuses
Cervical Plexus
- Anterior rami of C1-C5
- Innervate skin & muscles of neck, head & shoulders
- Phrenic nerve emerges from cervical plexus (specifically C3, C4, C5
Brachial Plexus
- Anterior rami of C4-T12
- Subdivided into roots, trunks, divisions & cords
- Nerves include: radial, median & ulnar nerves
- Innervate skin & muscles of shoulder, arm & hand
Lumbar Plexus
- Anterior rami of T12-L4
- Subdivided into roots & divisions
- Nerves include: femoral & obturator nerves
- Innervates muscles of anterolateral abdominal wall, external genitals & parts of thigh, leg & foot
Sacral Plexus
- Anterior rami of L4-S4
- Subdivided into roots & divisions
- Nerves include: sciatic nerve (common peroneal + tibial)
- Innervate skin & muscle of buttocks, pelvis. thigh, leg & foot
Coccygeal Plexus
- Anterior rami of S4 & S5
- Nerves include: anococcygeal nerve
- Innervate skin & muscle of buttocks, pelvis. thigh, leg & foot
Dermatomes
- Skin areas supplied by a spinal nerve
- Branches of cranial nerve V (trigeminal) has dermatomes w/ little overlap
- Help to identify damaged spinal nerves or spinal cord segments
- The C1 spinal nerve has no dermatomes
