Unit V - The Nervous System II

Anatomy of a Nerve

• Epineurium covers the nerves

• Perineurium surrounds a fascicle

• Endoneurium separates individual nerve fibers

  • Blood vessels only penetrate the perineurium

Ganglia

Clusters of nerve cell bodies

Rami

Branches of spinal nerves that split after exiting the spine

Plexuses

Networks where rami intermix and reorganize before serving the limbs

Spinal Nerves and Plexuses

The Spinal Nerves

• 31 pairs of spinal nerves (1st cervical above C1)

  • Mixed nerves exiting at intervertebral foramen

• Proximal Branches

  • Dorsal root is sensory input to spinal cord

  • Ventral root is motor output of spinal cord

  • Cauda Equina is roots from L2 to C0 of the cord

• Distal branches

  • Dorsal ramus supplies dorsal body muscle and skin

  • Ventral ramus to ventral skin and muscles and limbs

  • Meningeal branch to meninges, vertebrae and ligaments

Branches of a Spinal Nerve

• Spinal nerves:

  • 8 Cervical

  • 12 Thoracic

  • 5 Lumbar

  • 5 Sacral

  • 1 Coccygeal

• Each has dorsal and ventral ramus

Shingles

• Skin eruptions along path of nerve

• Varicella-zoster virus (chicken pox) remains for life in dorsal root ganglia

• “Occurs after age 50 if immune system is compromised”

  • Not true! Can occur at any age with re-exposure to Varicella

• No special treatment

Nerve Plexuses

• Ventral rami branch and anastomose repeatedly to form 5 nerve plexuses

  • Cervical in the neck, C1 to C5 

    • supplies the neck and phrenic nerve to the diaphragm

  • Brachial in the armpit, C5 to T1

    • supplies the upper limb and some of the shoulder and neck

  • Lumbar in the low back, L1 to L4

    • supplies the abdominal wall, the anterior thigh, and the genitalia

  • Sacral in the pelvis, L4, L5 and S1 to S4

    • supplies the remainder of the lower trunk and lower limb

  • Coccygeal, S4, S5 and C0

Cutaneous Innervation and Dermatomes

• Each spinal nerve receives sensory input from a specific area of skin called a dermatome

• Overlap at edges by 50%

  • A total loss of sensation requires anesthesia of 3 successive spinal nerves

Sensory Input

Vital to the integrity of personality and intellectual function

Sensory Deprivation

Withholding sensory stimulation

Sensory Receptor

• A structure specialized to detect a stimulus

  • Bare nerve ending

  • Sense organs - nerve tissue surrounded by other tissues that enhance response to a certain type of stimulus

    • Added epithelium, muscle, or connective tissue

General Properties of Receptors

• Transduction – the conversion of one form of energy to another

  • Fundamental purpose of any sensory receptor

  • Conversion of stimulus energy (light, heat, touch, sound, etc.) into nerve signals

• Receptor Potential – small, local electrical change on a receptor cell induced by an initial stimulus

  • Results in release of neurotransmitter or a volley of action potentials that generates nerve signals to the CNS

• Sensation – a subjective awareness of the stimulus

  • Most sensory signals delivered to the CNS produce no conscious sensation

    • Filtered out in the brainstem

    • Visceral stimuli do not require conscious awareness (pH and body temperature)

Receptors Transmit Four Kinds of Information

• Modality

• Location

• Intensity

• Duration

Modality

• Type of stimulus or the sensation it produces

  • Vision, hearing, taste

• Labeled Line Code

  • All action potentials are identical

  • Each nerve pathway from sensory cells to the brain is labeled to identify its origin, and the brain uses these labels to interpret what modality the signal represents

Location

• Encoded by the nerve fibers that issue signals to the brain

• Receptive Field – area that detects stimuli for a sensory neuron

  • Receptive fields vary in size – fingertip versus skin on the back

  • Two-point touch discrimination

• Sensory Projection - the brain identifies the site of stimulation

• Projection Pathways – the pathways followed by sensory signals to their ultimate destination in the CNS

Intensity

• Encoded in 3 ways:

• The brain can distinguish intensity by:

  • Which fibers are sending signals 

  • How many fibers are doing so

  • How fast are these fibers firing

Duration

• How long does the stimulus last

  • Change in firing frequency over time

  • Sensory adaptation – prolonged stimulation → firing of the neuron gets slower over time → less aware of the stimulus

  • Phasic Receptor –burst of action potentials on stimulation, quick adaptation & sharply reduce or stop signaling even though the stimulus continues

    • Smell, hair movement, and cutaneous pressure

  • Tonic receptor - adapts slowly, generates nerve signals more  steadily 

    • Proprioceptors - body position, muscle tension, and joint motion

Adaptation to Stimulation

• Tonic: AP frequency determined by the amplitude of the stimulus

• Phasic: AP frequency determined by the rate of change of the amplitude of the stimulus

  • Slowly adapting approaches, Tonic

  • Rapidly adapting, similar to Phasic

Receptor Level Processing

Receptor Potentials

• Generator Potential: 

  • The receptor region is part of a sensory neuron 

  • Include free dendrites or encapsulated receptors of most general sense receptors

  • Graded potential that generates action potentials 

• Receptor Potential:

  • The receptor is a separate cell

  • Graded potential occurs in separate receptor cells and changes the amount of neurotransmitter released by the receptor cell 

  • Most special senses

Classification of Receptors

• By Modality:

  • Thermoreceptors, photoreceptors, nociceptors, chemoreceptors, and mechanoreceptors 

• By Origin of Stimuli

  • Exteroceptors - detect external stimuli

  • Interoceptors - detect internal stimuli

  • Proprioceptors - sense body position and movements

• By Distribution

  • General (somesthetic) senses - widely distributed

  • Special senses - limited to the head

  • Vision, hearing, equilibrium, taste, and smell

General Senses

• Structurally simple receptors

  • One or a few sensory fibers and a little connective tissue

    • Unencapsulated nerve endings

    • Encapsulated nerve endings

• Physiologically simple receptors

Unencapsulated Nerve Endings

• Dendrites are not wrapped in connective tissue

• Free Nerve Endings

  • For pain and temperature

  • Skin and mucous membrane

• Tactile (Merkel) Discs

  • For light touch and texture

  • Associated with Merkel cells at the base of epidermis

• Hair Receptors (Peritrichial Endings)

  • Wrap around the base of the hair follicle

  • Monitor the movement of hair

Encapsulated Nerve Endings

• Tactile (Meissner) Corpuscles

  • Light touch and texture

  • Dermal papillae of hairless skin

• Krause End Bulbs

  • Tactile

  • In mucous membranes

• Bulbous (Ruffini) Corpuscles

  • Heavy touch, pressure, joint movements, and skin stretching

• Lamellar (Pacinian) Corpuscles

  • Deep pressure, stretch, tickle, and vibration 

  • Periosteum of bone, and  deep dermis of skin

• Muscle Spindles

• Golgi Tendon Organs

Vibration Sense

• Texture sense and pitch sense vary according to the frequency of vibration

• Tuning Curves measure sensitivity to varying frequency of stimulation

• Meissner’s is more broadly tuned, less sensitive, and less rapidly adapting compared to Pacinian

Somesthetic Projection Pathways

• From receptor to final destination in the brain, most somesthetic signals travel by way of three neurons

• 1^st order neuron (afferent neuron)

  • From the body, enter the dorsal horn of the spinal cord via the spinal nerves

  • From the head, enter the pons and medulla via the cranial nerves

  • Touch, pressure, and proprioception on large, fast, myelinated axons

  • Heat and cold on small, unmyelinated, slow fibers

• 2^nd order neuron

  • Decussation to the opposite side in the spinal cord, medulla, or pons

  • Ends in the thalamus, except for proprioception, which ends in the cerebellum

• 3^rd order neuron

  • The thalamus to the primary somesthetic cortex of the cerebrum

Peripheral Nerves Undergo Regeneration

• Undergo regeneration through Schwann Cells

• Mature neurons do not divide, but they can regenerate in the PNS if the cell body remains intact

• CNS regrowth is inhibited by oligodendrocytes and astrocyte-derived scar tissue

Three Levels of Motor Control

• Cerebellum and basal nuclei (ganglia) plan and coordinate complex motor behaviors.

• Motor control at lower levels is mediated by reflex arcs or governed by complex motor patterns

• Three Levels:

  • Precommand Level (Highest)

  • Projection Level (Middle)

  • Segmental Level (Lowest)

Segmental Level

• Contains the reflexes and spinal cord circuits

• Proximate executor of movement: direct connection to myofibers

• Mediates automatic reflex movements

• Segmental circuits

  • Activate ventral horn motor neurons in a group of segments

  • Stimulate specific groups of muscles

• Central pattern generators

  • Circuits that control repeated motor activities

  • Inhibitory and excitatory neurons with rhythmic patterns of activity

Projection Level

• Contains descending projection fibers

• Convey information to lower motor neurons (LMNs)

• Provide feedback to precommand level

• Direct (pyramidal) pathways

• Indirect pathways

Precommand Level

• Contains the cerebellum and the basal nuclei

• Unconscious planning—acts in advance of willed movements to control outputs of motor cortex

• Precisely start/stop movements, coordinate them with posture, block unwanted movements

• Cerebellum: 

  • Integration of motor and sensory feedback: compares intentions to actions

  • “Fine tunes” motor activity by projecting to cortex (via thalamus) and brainstem

• Basal nuclei:

  • Receives inputs from all cortical areas

  • Output to premotor and prefrontal cortex via the thalamus

  • Inhibits motor centers at rest, activation leads to motor initiation

Nature of Somatic Reflexes

• Quick, involuntary, stereotyped reactions of glands or muscles to sensory stimulation 

  • Automatic responses to sensory input that occur without our intent or often even our awareness

• Functions by means of a somatic reflex arc

  • Stimulation of somatic receptors

  • Afferent fibers carry a signal to the dorsal horn of the spinal cord

  • One or more interneurons integrate the information

  • Efferent fibers carry impulses to skeletal muscles

  • Skeletal muscles respond

The Muscle Spindle

• Sense organ (proprioceptor) that monitors length of muscle and how fast muscles change in length

• Composed of intrafusal muscle fibers, afferent fibers and gamma motor neurons

The Stretch (Myostatic) Reflex

• When a muscle is stretched, it contracts and maintains increased tonus (stretch reflex)

  • Helps maintain equilibrium and posture

    • Head starts to tip forward as you fall asleep

    • Muscles contract to raise the head

  • Stabilize joints by balancing tension in extensors and flexors, smoothing muscle actions

• A very sudden muscle stretch causes a tendon reflex

  • The knee-jerk (patellar) reflex is a monosynaptic reflex

  • Testing somatic reflexes helps diagnose many diseases

• Reciprocal inhibition prevents muscles from working against each other

Simple Stretch Reflex

Functions of the Alpha - Gamma Coactivation Mechanism

• Detect changes in muscle length, even when muscle is partially contracted

• Monitor if the muscle (extrafusal fibers) shortened as commanded

  • A feedback mechanism to determine if recruitment is needed

The Patellar Tendon Reflex Arc

Flexor Withdrawal Reflexes

• Occurs during withdrawal of foot from pain

• Polysynaptic reflex arc

• Neural circuitry in spinal cord controls sequence and duration of muscle contractions

Crossed Extensor Reflexes

• Maintains balance by extending other leg

• Intersegmental reflex extends up and down the spinal cord

• Contralateral reflex arcs explained by pain at one foot causes muscle contraction in other leg

Golgi Tendon Reflex

• Proprioceptors in a tendon near its junction with a muscle -- 1mm long, encapsulated nerve bundle

• Excessive tension on tendon inhibits motor neuron

  • Muscle contraction decreased

• Also functions when muscle contracts unevenly

Spinal Cord Trauma / Injury

• 10-12,000 people/ year are paralyzed

• 55% occur in traffic accidents

• This damage poses risk of respiratory failure

• Early symptoms are called spinal shock

• Tissue damage at time of injury is followed by post-traumatic infarction

Spinal Cord Injuries

• Trauma

• Tumours

• Ischemia

• Developmental Disorders

• Neurodegenerative Diseases

• Demyelinated Diseases

• Transverse Myelitis

• Vascular Malformations

Spinal Cord Trauma

• Automobile Crashes

• Falls

• Gunshots

• Diving Accidents

• War Injuries

• Etc…

Spinal Cord Tumours

• Right Tumour

• Ependymomas

• Astrocytomas

• Metastatic Cancer

Spinal Cord Ischemias

• Occlusion of Spinal Blood Vessels

  • Including Dissecting Aortic Aneurysms

• Emboli

• Arteriosclerosis

Spinal Cord Developmental Disorders

Spina Bifida

Meningomyolcoele

Etc…

Spinal Cord Neurodegenerative Diseases

Friedreich's Ataxia

Spinocerebellar Ataxia

Etc…

Spinal Cord Demyelinative Diseases

• Multiple Sclerosis

• Etc…

Spinal Cord Transverse Myelitis

• From Spinal Cord Stroke

• Inflammation

• Etc…

Spinal Cord Vascular Malformations

• Arteriovenous Malformation (AVM)

• Dural Arteriovenous Fistula (AVF)

• Spinal Hemangioma

• Cavernous Angioma

• Aneurysm

Spinal Cord Injury (SCI) Immediate Response

• Initial mechanical trauma secondary to traction and compression forces

• Direct compression of neural elements by bone fragments, disc material, and ligaments damages CNS and PNS

• Blood vessel damage leads to ischemia

• Rupture of axons and neural cell membranes also occurs

• Microhemorrhages occur within minutes in the central gray matter and progress over the next few hours

• Massive cord swelling within minutes, leading to secondary ischemia

• Loss of autoregulation and spinal shock cause systemic hypotension, exacerbate ischemia

• Ischemia, toxic metabolic compounds, and electrolyte changes cause a secondary injury cascade

Spinal Cord Injury (SCI) Delayed Response

• Hypoperfusion of gray matter extends to the white matter, altering propagation of action potentials along the axons, contributing to spinal shock

• Massive release of glutamate leads to excitotoxicity - overstimulation of neighbor neurons, production of free radicals, death of healthy neurons

• Excitotoxic mechanisms, via glutamate receptors, kill neurons & oligodendrocytes, leading to demyelination

• Wave of apoptosis affects oligodendrocytes up to 4 segments from the trauma site days and weeks after the initial trauma

Spinal Cord Plegia

• Plegia: paralysis, stroke, or a significant loss of muscle function

• Extent of plegia dependent on position of injury

• Cervical Injury:

  • Quadriplegia, loss of autonomic control

• Thoracic Injury:

  • Paraplegia, respiration intact

• Lumbar & Sacral Injury

  • Decreased control of legs, hips, urinary system and anus

Pain

• Discomfort caused by tissue injury or noxious stimulation, and typically leading to evasive action

  • Important since helps protect us 

  • Lost in diabetes mellitus – diabetic neuropathy

Nociceptors

• Two types provide different pain sensations

  • Fast pain travels in myelinated fibers at 12 - 30 m / sec

    • Sharp, localized, stabbing pain is perceived with injury

  • Slow pain travels through unmyelinated fibers at  0.5 - 2 m / sec

    • Longer-lasting, dull, diffuse feeling

Somatic Pain

From skin, muscles and joints

Visceral Pain

• From the viscera

  • Stretch, chemical irritants or ischemia of viscera (poorly localized)

Chemicals Released from Injured Tissue

• Stimulates pain fibers

• Bradykinin  - most potent pain stimulus known

• Makes us aware of injury and activates cascade or reactions that promote healing

• Histamine, prostaglandin & serotonin also stimulate nociceptors

Projection Pathway for Pain

• Two main pain pathways to the brain, and multiple subroutes

  • Head

  • Neck

Pain Signals from the Head

• First-order neuron cell bodies in dorsal root ganglion of spinal nerves or cranial nerves V, VII, IX, and X

• Second-order neurons decussate and send fibers up spinothalamic tract or through medulla to thalamus

  • Gracile fasciculus carries visceral pain signals

• Third-order neurons from thalamus reach postcentral gyrus of cerebrum

Pain Signals from the Neck Down

• Travel by way of three ascending tracts:

  • Spinothalamic Tract – most significant pain pathway

    • Carries most somatic pain signals

  • Spinoreticular Tract – carries pain signals to reticular formation

    • Activate visceral, emotional and behavioral reactions to pain

  • Gracile Fasciculus – carries signals to the thalamus for visceral pain

Referred Pain

• Pain in viscera often mistakenly thought to come from the skin or other superficial site

  • Results from convergence of neural pathways in CNS

  • Brain “assumes” visceral pain is coming from skin

    • Brain cannot distinguish source

  • Heart pain felt in shoulder or arm because both send pain input to spinal cord segments T1 to T5

Steps of Pain Perception