Ch 14 - Somatosensory Regulation, Pain, and Headache

Neuron

electrically excitable, amniotic, structure and functional unit of the NS

Dendrites

Receive signals from other neurons and conduct them towards the body

• brings toward the cell body

Cell bodies

contains nucleus and cellular organelles

• also called soma

Axon or nerve fibers

conducts electrical impulses (action potentials) away from the cel body toward other neurons, muscles, or glands

Other parts of the neuron

axon terminals: contain synaptic end bulbs that contain neurotransmitters

axon hillock: trigger zone

myelin sheath: formed by shwaann cells in PNS and oligodendrocytes in CHS (speeds up chemical messaging)

nodes of Ranvier: parts of the axon with no myelin sheath

Sensory (afferent) neurons vs Motor (efferent) neurons

In the CNS

• sensory transmit impulses from sensory receptors to CNS

• motor neurons carry impulses from the CNS to effectors

Interneurons (association neurons)

entirely within the CNS, lie between motor and sensory neurons and are integrators of signals

Nerves

Bundles of neuron processes in the PNS

• connect effectors and organs to CNS

• is myelinated

Tracts

Bundles of neuron processes in the CNS

• conducts impulses in the CNS

• is myelinated

white matter contains myelinated neurons that form tracts

CNS

brain and spinal cord

main function: receives sensory signals, process information, and send out motor signals

PNS

nerves outside the brain and spinal cords

Main functions: connect the CNS to the rest of the body and to the external environment

Somatic NS

Subdivision of the PNS

• controls voluntary body movement and transmits sensory information to and from the CNS

Autonomic NS

Subdivision of the PNS

• Controls involuntary visceral functions (heart rate, digestion, respiration)

• responsible for the withdrawal reflex

Divided into three branches

Sympathetic → fight or flight, responds during stressful situations

Parasympathetic → rest and digest, responds during calm states

Enteric → regulates voluntary activities in the GI tract

Cranial Nerves

One part of the PNS

• Originate from the cerebrum and brainstem: connect the CNS to the head, face, and neck

identified by roman numerals

Sensory cranial nerves

I, II, VIII

Motor sensory nerves

III, IV, VI, XI, XII

Mixed cranial nerves

V, VII, IX, X

Spinal nerves

Division of the PNS

• Originate from the spinal cord connects CNS to the trunk and limbs

• Named by the level of the spinal cord at which they emerge

Thoracic nerves (T1-T12) → need to KNOW

All spinal nerves are mixed → need to KNOW

Dorsal root ganglion and Ventral Root

dorsal root ganglion → contains the cell bodies of sensory neurons

ventral root → contains cell bodies of motor neurons

Neuroglia (glial) cells

supporting cells in the CNS and PNS

Supporting cells in the CNS

Oligodendrocytes → produce the myelin sheath that promotes the efficient transmission of nerve impulses around axons (note: cannot support regeneration or healing)

astrocytes → maintain exchanges between capillaries and neurons

microglial cells → remove cellular debris from sites of injury or normal cell turnover

ependymal cells → form cerebrospinal fluid (CSF) in cavities of the brain

Schwann cells

supporting cells of the PNS

• the produce myelin, vital in the regeneration of damaged PN fibers

Somatosensory System

organized around three major levels of neural integration

• sensory units

• ascending pathways (tracts)

• central processing centers in the thalamus and cerebral cortex

First-order neurons

Within the somatosensory system

• they receive sensory stimuli and carry signals to the CNS

end of this is in gray matter, where it synapses with 2nd order neuron

Second-order neurons

They relay signals within the spinal cord or brainstem

• form ascending pathways

• within the somatosensory system

Usually within gray matter in the dorsal horn of the spinal column, it extends from the gray matter of the spinal cord up to synapse with a 3rd-order neuron

• receives input from the primary order neuron

• are excitatory and inhibitory interneurons

• cross over the cord and ascend or descend

Third-order neurons

They transmit info to the cerebral cortex for perception

• within the somatosensory cortex

They are interneurons in the thalamus that carry signals from the thalamus to the primary somatosensory cortex in the parietal lobe

Sensory Unit

basic functional structure in the NS that detects and transmits sensory information

• contains two main parts:

  • sensory receptor → specialized structure that detects a specific type of stimulus (e.g., pressure, heat, light)

  • sensory neuron (afferent neuron) → carries the signal from the receptor to the CNS; this is a first-order neuron

components of somatic sensory receptors in the skin

Meissner corpuscle → light touch

Merkel receptor → pressure and texture (deeper in dermis)

Ruffini corpuscle → stretch

Free Nerve Endings → pain, touch, temperature

Pacinian Corpuscle → vibration

Sensory Nerve

Ascending Pathways

Sensory information travels from the spinal cord to the thalamus through second-order neurons

• goes through the discriminative and spinothalamic pathways

axons in both these pathways transmit sensory information from one side of the body to the opposite side of the brain

Discriminative Pathway

transmits information about body position and discriminative touch (ability to sense and localize touch)

• axons of the 2nd order neurons (also known as the dorsal column neurons) cross over (decussate) in the medulla where they form a band of myelinated white matter (medial lemniscus)

they then form a synapse with the 3rd order neurons in thalamus

• is an ascending pathway

Spinothalamic pathway

transmits information about pain, temperature, pressure, and non-discriminative touch (the ability to sense but not localize touch)

• has two subdivisions:

  • neospinothalamic tract → sequence of few neurons with long axons (directly conducts impulses to thalamus) → goes medial

  • paleospinothalamic tract → sequence of many neurons that sends branches to the reticular formation in the brainstem → goes lateral

the axons of the 2nd order neurons cross over in the same segment of the spinal cord as the cell body, and then they ascend in the opposite side of the spinal cord to the thalamus

Reticular formation

network of neurons that receives information and sends stimulatory signals to higher areas in brain

Central Processing

They are in the cerebral cortex and are responsible for recognizing a stimulus, differentiating its characteristics, and interpreting its meaning

• info is relayed to the thalamus to the cerebral cortex via the third-order neurons

Primary Somatosensory Cortex

The main cortical area that receives and processes sensory information from the body

• 3rd order neurons

• it identifies the location of the pain, intensity, and physical characteristics of the painful stimulus

Somatosensory association cortex

Integrates sensory input from the primary somatosensory cortex

• Behind the primary association area

• required to transform the sensation into meaningful learned perception, you can feel it but you cannot recognize it

Dermaome

Area of skin integrated by sensory neurons from a single spinal nerve root

• There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)

• C1 is an exception with no associated dermatome

damage to a specific nerve will result in a reduced, but not total loss of sensory perception

Factors that influence the reaction to pain

anxiety, culture, and past experiences

• pain is a protective complex that is a phenomenon made up of dynamic interactions among physical, cognitive, spiritual, emotional, and environmental factors

Specificity theory

Proposes that pain is a distinct sensation with its own specialized receptors and neural pathways

• does not explain chronic pain

the amount of pain is related to the amount of tissue injury

accounts for many types of injuries but does not explain psychologic contributions to pain or chronic pain

Pattern theory

Suggests that pain does not depend on specialized receptors

• describes the role of impulse intensity

• strong or repeated stimulation produces a pattern of neural activity that the brain interprets as pain

Nociceptors

Sensory receptors that are activated by noxious stimuli

• Primary-order neurons begin with nociceptors

• stimulated by severe mechanical deformation or any potentially harmful stimuli and initiate the sensation of pain

Unimodal nocicpetors and polymodal nocicpetors

unimodal → respond to one type of noxious stimulus, such as mechanical or thermal stimuli

polymodal → they respond to multiple types of noxious stimuli, including mechanical, thermal, and chemical stimuli

Myelinated A-delta

Detect sharp localized pain

• transmission is fast and causes reflex withdrawal of the affected body part from the stimulus before the pain sensation is perceived

• axon in the first order neuron that relays pain from the nociceptors to the spinal cord

Unmyelinated C

stimulated by mechanical, thermal, and chemical nociceptors (polymodal)

• transmission is slower and conveys dull, aching, or burning sensations

• usually conveys chronic pain

Sequence a sequence

1. Skin receptor

2. Sensory neuron (1st order)

3. Interneuron (2nd order)

4. Spinal Cord

5. Spinothalamic tract to medulla oblongata

6. Thalamus

7. Interneurons (3rd order)

8. Somatosensory cerebral cortex

Pain pathways

steps that carry pain signals from the site of injury to the brain and then regulate those signals

Transduction

Begins when tissue is damaged by exposure to chemical, mechanical, or thermal noxious stimuli and is converted to electrophysiologic activity

• nociceptors convert the harmful stimulus into electrical impulses

first step of the pain pathway

Transmission

Conduction of pain impulses along the A (fast) and C (slow) fibers into the dorsal horn of the spinal cord and eventually to the reticular formation, thalamus, hypothalamus, limbic system, and cerebral cortex

second step of the pain pathway

Perception

Conscious awareness of pain

The third step of the pain pathway

• The brain (somatosensory cortex) interprets the pain signal

• pain perception is influenced by emotions, attention, past experiences, and cultural factors

pain modulation

refers to the body’s ability to increase or decrease pain signals

• brain can suppress pain signals through pathways descending to the spinal cord

Periaqueductal Gray (PAG)

The region of gray matter that surrounds the cerebral aqueduct in the midbrain plays a major role in pain modulation'

• It activates the descending inhibitory pathways to the dorsal horn of the spinal cord. Suppresses transmission of pain signals from peripheral nerves to the brain

MAKE SURE TO GO OVER SLIDE 37 ON THIS LECTURE

PAG-mediated pain controls system uses endogenous opioids

Endogenous Opiodis

Naturally produced by the body that help regulate pain, stress response, mood, and reward

• enkephalins, endorphins, and dynorphins

they inhibit pain impulses in the spinal cord, brain, and periphery

their binding to the opioid receptors in the spinal cord prevents the ascending transmission of pain impulses through the spinothalamic pathway

• opioid analgesics act by binding to the same receptors

Pain classification

duration

• Acute - <3 months

• Chronic - >3 months

location

• cutaneous → arises from superficial structures

• deep somatic → arises from joints, muscles, bone, and skin

• deep visceral → arises from the internal organs and lining of body cavities

site of referral

• referred pain → is in an area distant from its point of origin (very common); area is usually supplied by the same spinal segment as the actual site

Referred pain

area is distant from the actual site

• usually is supplied by the same spinal segment as the actual site

• visceral and somatic afferent neurons converge on the same dorsal horn neurons, making it difficult for the brain to correctly identify the source of pain

ex) myocardial infraction pain is referred to the left shoulder, arm, neck, and jaw (lower jaw or a part of facial skin)

Alterations in pain sensitivity

Changes in how the NS perceives pain

• results in pain being felt more easily, more intensely, or in response to normally non-painful stimuli

Hyperalgesia

increased response to a painful stimuli (more extreme pain than there should be)

• caused by an increased response to a painful stimulus

Allodynia

when a non-painful stimulus is perceived as painful

• caused by abnormal processing in the CNS

• non-nociceptive inputs begin activating pain pathways

Hypoalgesia

decreased sensitivity to painful stimuli

• result from nerve damage to activation of descending inhibitory pathways that suppress pain transmission

Analgesia

absence of pain perception despite a painful stimulus

• caused by severe nerve injury, pharmacologic effects (such as opioids)

Paresthesia

Abnormal sensation felt in the skin without an obvious external stimulus

• described as tingling, pins and needles, numbness, burning, or a “crawling” sensation

• caused by irritation, compression, or damage to sensory nerves

Neuropathic pain

Caused by damaged or dysfunction of nerves

• described as burning, shooting, tingling, or electric shock-like pain

• neuralgia is severe, brief, and often repetitive attacks of throbbing pain that occur along the distribution of a spinal or cranial nerve (e.g., trigeminal neuralgia)

Phantom pain

pain perceived in a body part that has been amputated or is no longer present

• multiple theories - spontaneous stimulation of spinal cord neurons, nerve damage

Headaches and types of headaches (tension, migraine, and cluster)

headache is a pain or discomfort in the head, scalp, face, or upper neck

• Tension Headache → most common type; it feels like a tight band or pressure around the head (most people get this from stress)

• Migraine → usually genetic; it is moderate to severe throbbing pain, often on one side of the head, accompanied by nausea, vomiting, or sensitivity to light and sound (lying down helps to relieve it)

• Cluster Headache → severe pain usually around one eye, occurring in repeated attacks (comes and then goes away for weeks or months)

also, secondary headaches occur as a symptom of another underlying condition (some are indications of serious disorders like meningitis or brain tumor)

Migraine

Affects 20% of the general US population

• Prevalence is higher in women compared to men, and it may occur in children

• special characteristics: aura occurs before most migraine episodes

Aura

set of sensory, visual, or mental symptoms that occur before a neurological event (migraine, seizure)

• visual hallucinations (shimmering spots, zigzag lines, flashing lights); numbness or tingling sensations; weakness (usually on one side of the body); speech disturbances

Pathogenesis of migraine

1. Trigger factors activate the trigeminovascular system (network formed by pain-sensitive intracranial BVs, meninges, and sensory fibers of the trigeminal nerve)

2. Release of CGRP (calcitonin gene-related peptide) and other neuropeptides occurs; CGRP triggers vasodilation and an inflammatory response

3. meningeal BVs dilate and become inflamed

4. Pain signals travel through the trigeminal pathway to the brainstem and thalamus

5. Pain is perceived as a throbbing headache

Pain threshold

The lowest intensity of pain that a person can recognize

• intense pain at one location may increase the threshold in another location

• indv. with many painful sites may report only the most painful

• after the dominant pain is diminished, the individual may then identify other painful areas

Pain tolerance

The greatest intensity of pain that an individual can endure

• very individualized; varies among people and in the same person over time

• may increase in certain situations, such as during intense focus, stress response, or through descending inhibitory pathways

Pain assessment

the evaluation of a patient’s pain to determine its intensity, location, quality, duration, and impact on function

• numeric pain intensity scale asks to select which number best represents the intensity of their pain

0 represents no pain

10 represents the most intense pain

5 is moderate pain