Chapter 14: Integration of Nervous System Function

Terms and Definitions from Textbook

• Proprioception: Information about the position of the body and its various parts

• Sensation: the process initiated by stimuli acting on sensory receptors

• Perception: the conscious awareness of sensations

• Senses: means by which the brain receives information about the environment and the body

• Somatic senses: touch, pressure, temperature, pain, and proprioception

• Visceral senses: pain and pressure

• Special senses: localized to specific organs that have specialized receptors

Sensory Receptor Types based on Stimulus:

• Mechanoreceptors: a sensory receptor that responds to mechanical pressures, ex, pressure receptors in the carotid sinus or touch receptors in the skin

• Chemoreceptors: a sensory cell that is stimulated by a change in the concentration of chemicals to produce action potentials, ex, taste receptors, olfactory receptors, and carotid bodies

• Thermoreceptors: a sensory cell that responds to temperature changes at the site of the receptor

• Photoreceptors: a sensory receptor that is sensitive to light— for example, rods and cones and retina

• Nociceptors: a sensory receptor that detects painful or injurious stimuli; also called pain receptor

Sensory Receptors Types based on Position:

• Cutaneous receptors: associated with the skin

• Visceroreceptor: associated with the organs

• Proprioceptors: associated with joints and tendons

• Free nerve endings: unspecialized neuronal branches similar to dendrites

  • Merkel disks: involved with sensations of light touch and superficial pressure

  • Hair follicle receptors: Invagination of the epidermis into the dermis; contains the root of the hair and receives the ducts of sebaceous and apocrine glands.

  • Pacinian corpuscles:

  • Meissner corpuscles

  • Reffuni end organ: receptor located deep in the dermis and responding to continuous touch or pressure

  • Golgi tendon organ: proprioceptive nerve ending in a tendon

Sensation and Sensory System (Textbook)

Sensation is the process initiated by stimuli acting on sensory receptors. Perception is the conscious awareness of those sensations. The brain constantly receives sensations as action potentials from a wide variety of sensory receptors that receive stimuli from both inside and outside the body. Sensory receptors respond to stimuli by generating action potentials that are propagated along nerves to the spinal cord and brain. Perception results when the brain interprets the sensation-generated action potentials in the cerebral cortex. Some other parts of the brain are involved in modulating sensations before they are perceived. For example, the thalamus and amygdala receive and integrate pain signals.

The senses are the means by which the brain receives information about the environment and the body. Historically, five senses were recognized: smell, taste, vision, hearing, and touch. Today, the senses are divided into two basic groups: general and special.

The general senses have receptors distributed over a large part of the body. They are divided into two groups: somatic senses and visceral senses. The somatic senses provide sensory information about the body and the environment. Somatic senses include touch, pressure, temperature, pain, and proprioception. Proprioception (proh-pree-oh-SEP-shun) is the sense of your body's position and movement. An example of proprioception is the ability of a person to touch their nose when their eyes are closed. This is often used as a field sobriety test. The visceral senses provide information about various internal organs and consist primarily of pain and pressure.

The special senses are localized to specific organs that have specialized receptors. The special senses—smell, taste, vision, hearing, and balance—are considered in detail in chapter 15.

Not all of the sensory information detected by sensory receptors results in perception. Some action potentials reach areas of the brain where they are not consciously perceived. Two examples are proprioception and detection of change in homeostatic variables. For proprioception, although we can be consciously aware of body position and movements, much of this sensory information is propagated to the cerebellum, where it is processed at an unconscious level. For homeostasis, sensory information from receptors that monitor body variables, such as blood pressure, are processed unconsciously in the medulla oblongata. Because blood pressure must be regulated constantly to maintain homeostasis, if we had to regulate blood pressure consciously, we would not be able to think of much else. Homeostasis, therefore, is controlled largely without our conscious involvement.

Even the cerebral cortex screens much of what it receives and does not perceive many of the action potentials that reach it. In addition, humans exhibit selective awareness. We are more aware of sensations on which we focus our attention than on other sensations. If we were aware of all the sensory information that arrived at the cerebral cortex, we would probably not be able to function.

Sensory Receptors

Structurally, the simplest and most common sensory receptors are the free nerve endings.

There are five types of sensory receptors based on the type of stimulus they detect:

1. Mechanoreceptors respond to mechanical force such as compression, bending, or stretching of cells. The senses of touch, pressure, proprioception, hearing, and balance all depend on a variety of mechanoreceptors.

2. Chemoreceptors respond to chemicals. Smell and taste depend on chemoreceptors.

3. Thermoreceptors respond to changes in temperature at the site of the receptor and are necessary for the sense of temperature.

4. Photoreceptors respond to light striking the receptor cells and are necessary for vision.

5. Nociceptors, or pain receptors, respond to extreme mechanical, chemical, and thermal stimuli. Most sensory receptors typically respond to one type of stimulus, but some nociceptors respond to more than one.

There are three types of sensory receptors based on their location:

1. Cutaneous receptors are associated with the skin. Cutaneous receptors provide information about the external environment.

2. Visceroreceptors are associated with the viscera or organs. Visceroreceptors provide information about the internal environment.

3. Proprioceptors are associated with joints, tendons, and other connective tissue. Proprioceptors provide information about body position, movement, and the extent of stretch or the force of muscular contractions.

Free nerve endings are relatively unspecialized neuronal branches similar to dendrites. Free nerve endings are distributed throughout most parts of the body and are especially abundant in epithelial and connective tissues. These receptors are responsible for a number of sensations, including pain, temperature, itch, and movement.

The free nerve endings responsible for temperature detection respond to three types of sensations: (1) increase in skin temperature, (2) decrease in skin temperature, and (3) pain. Cold receptors increase their rate of action potential production as the skin is cooled. Cold receptors are also activated by menthol, which gives mint its cool taste. Warm receptors increase their rate of action potential production as skin temperature increases. Both cold and warm receptors respond most strongly to changes in temperature. Cold receptors are 10 to 15 times more numerous than warm receptors in any given area of skin. The third type of free nerve ending that responds to temperature is a pain receptor that is stimulated by extreme cold or heat. At very cold temperatures (0°-12°C), only pain receptors are stimulated. As the temperature increases above 15°C, the pain sensation ends. Between 12°C and 35°C, cold fibers are stimulated, and between 25°C and 47°C, nerve fibers from warm receptors are stimulated. Therefore, "comfortable" temperatures, between 25°C and 35°C, stimulate both warm and cold receptors. Temperatures above 47°C stimulate pain receptors but do not stimulate warm receptors. One pain receptor that is activated by high temperature is also activated by capsaicin, the chemical that gives chili peppers their hot taste.

A Merkel disk consists of axonal branches that end as flattened expansions, each associated with a specialized epithelial cell. These receptors are distributed throughout the basal layers of the epidermis just superficial to the basement membrane and are associated with dome-shaped mounds of thickened epidermis in hairy skin. Merkel disks are involved with the sensations of light touch and superficial pressure. These receptors can detect a skin displacement of less than 1 mm (1/25 inch).

Hair follicle receptors, or hair end organs, respond to very slight bending of the hair and are involved in light touch. These receptors are extremely sensitive and require very little stimulation to elicit a response. The sensation, however, is not very well localized. The dendritic tree at the distal end of a sensory axon has several hair follicle receptors. The field of hairs innervated by these receptors overlaps with the fields of hair follicle receptors of adjacent axons. The considerable overlap in the endings of sensory neurons helps explain why light touch is not highly localized; however, because of converging signals within the CNS, it is very sensitive.

Pacinian corpuscles, or lamellated corpuscles, are complex receptors that resemble an onion. Each Pacinian corpuscle consists of multiple layers of tissue surrounding a single dendrite of a sensory neuron. Compression of the Pacinian corpuscle alters the membrane potential of the dendrite, which can cause an action potential. The corpuscles associated with the skin are located within the subcutaneous tissue, where they are responsible for deep cutaneous pressure and vibration. Pacinian corpuscles associated with the joints help relay proprioceptive information about joint positions.

Meissner corpuscles, or tactile corpuscles, are distributed throughout the dermal papillae and are involved in point discrimination. Two-point discrimination (fine touch) is the ability to detect simultaneous stimulation of Meissner corpuscles in two distinct receptor fields by touching at two points on the skin. The distance between two points that a person can detect as separate points of stimulation differs for various regions of the body. This sensation is important in evaluating the texture of objects. Meissner corpuscles are numerous and close together in the tongue and fingertips but are less numerous and more widely separated in other areas, such as the back.

Ruffini end organs are located in the dermis of the skin, primarily in the fingers. They respond to pressure on the skin directly superficial to the receptor and to stretch of adjacent skin. These receptors are important in responding to continuous touch or pressure.

Muscle spindles consist of 3-10 specialized muscle fibers that are located in skeletal muscles; they provide information about the length of the muscle. Muscle spindles are important to the control and tone of postural muscles. Brain centers act through descending tracts to either increase or decrease action potentials in gamma motor neurons. Stimulation of the gamma motor system, caused by stretch of the muscle, activates the stretch reflex, which in turn increases the tone of the muscles involved.

Golgi tendon organs are proprioceptive receptors associated with the fibers of a tendon near the junction between the muscle and the tendon. They are activated by an increase in tendon tension, caused either by contraction of the muscle or by passive stretch of the tendon.

Responses of Sensory Receptors

Once a sensory receptor has been stimulated, the receptor produces a graded potential called a receptor potential. Recall that a graded potential is a local change in the membrane potential that can vary from small to large. If the receptor potential is large enough to reach threshold, an action potential is produced and is propagated toward the CNS. Exactly how that action potential reaches the CNS is yet another way receptors can be classified. Sensory receptor cells that conduct action potentials in response to the receptor potential are called primary receptors. Most general sensory receptors, including all those in table 14.2, belong to this category.

Pressure is applied to a mechanoreceptor (Pacinian corpuscle).

The applied pressure generates a graded potential, or receptor potential, at the dendrite of the associated sensory neuron.

When the graded potential reaches threshold, an action potential is generated in the axon and is propagated toward the CNS.

Secondary receptors have no axons or have short, axonlike projections and generally produce receptor potentials. The receptor cells of the special senses of taste, hearing, and balance belong to this category.

A chemoreceptor (taste receptor) is subjected to a chemical stimulus in the form of a salty substance.

The sensory receptor responds by generating a graded potential, also known as a receptor potential. In the case of taste receptors, an action potential can also be generated.

The receptor potentials cause the release of neurotransmitters from the receptor cell, which bind to receptor proteins on the membrane of a sensory neuron.

This causes a receptor potential in the neuron, and if threshold is reached, an action potential is propagated to the CNS.

Some sensations have the quality of adaptation, decreased sensitivity to a continued stimulus. After exposure to a certain stimulus strength for a time, the response of the receptors or the sensory pathways lessens from when the stimulus was first applied. The local graded depolarization that produces a receptor potential adapts, or returns, to its resting level, even though the stimulus is still applied. For example, when a person first gets dressed, tactile receptors and pathways relay information to the brain, creating an awareness that the clothes are touching the skin. After a time, the action potentials from the skin decrease, and the clothes are not perceived.

The rate of adaptation varies for different receptors, which can be classified as either tonic or phasic. (1) Tonic receptors generate action potentials as long as a stimulus is applied so that they adapt very slowly. Merkel disks and Ruffini end organs are examples of tonic receptors. (2) Phasic receptors adapt rapidly so they are most sensitive to changes in stimuli. Pacinian and Meissner corpuscles are examples of phasic receptors. Free nerve endings can be either tonic or phasic.

Proprioception is a good example of tonic and phasic receptor actions. Recall that proprioception provides information about the precise position and rate of movement of various body parts, the weight of an object being held in the hand, and the range of movement of a joint. This information is involved in activities such as walking, shooting a basketball, eating, and writing. Receptors for this system are located around joints and in muscles. Tonic receptors provide information that allows us to know where our little finger is at all times without having to look for it. Phasic receptors provide information that allows us to know where our little finger is as it moves; thus, we can control its movement through space and predict where it will be in the next moment. We are usually not conscious of tonic or phasic proprioceptive input because the higher brain centers ignore it most of the time. Through selective awareness, however, we can call up the information when we wish. For example, where is the thumb of your right hand at this moment? Were you aware of its position a few seconds ago?

Sensory Pathways

Sensory information from the periphery is transmitted via action potentials along sensory pathways, or tracts, to the brain. Sensory pathways involving the spinal cord are also called ascending spinal pathways. Each pathway is involved with specific modalities (the types of information transmitted). The neurons that make up each pathway are associated with specific types of sensory receptors. For example, thermoreceptors in the skin generate action potentials that are propagated along the sensory pathway for pain and temperature, whereas Golgi tendon organs located in tendons generate action potentials that are propagated along the sensory pathway involved with proprioception.

Conscious and unconscious sensory input are transmitted by different ascending pathways. The two major pathways involved in the conscious perception of external stimuli are (1) the spinothalamic tract of the anterolateral system and (2) the dorsal-column/medial-lemniscal system. The pathways carrying sensory input that we are not consciously aware of include some of the tracts of the anterolateral system (the spinomesencephalic and spinoreticular tracts) and the spinocerebellar tract.

Anterolateral System

The anterolateral system is one of the two major systems that convey cutaneous sensory information to the brain and includes three tracts: (1) spinothalamic, (2) spinoreticular, and (3) spinomesencephalic. The spinothalamic tract allows conscious perception of pain and temperature information, as well as light touch and pressure, tickle, and itch sensations. The spinoreticular tract and the spinomesencephalic tract carry pain and touch sensations to other parts of the brain, where the information is not consciously perceived. There is, however, considerable overlap among these three tracts within the anterolateral system.

The spinothalamic tract transmits sensory signals from peripheral receptors to the cerebral cortex through three neurons in sequence—the primary, secondary, and tertiary neurons.

• Primary neurons conduct action potentials that originate at the sensory receptor to the CNS. The axons of these neurons form the dorsal root of a spinal segment, and their cell bodies are located in the dorsal root ganglia. The axons then synapse with interneurons in the dorsal horn of the spinal cord, which are not specifically named in the three-neuron sequence. These interneurons synapse with secondary neurons.

• Secondary neurons in the spinal cord relay information to the brain. Within the spinal cord, axons from secondary neurons decussate (cross) to the contralateral (opposite) side of the spinal cord. Decussation occurs through the anterior portion of the gray and white commissures. The axons then enter the spinothalamic tract and ascend to the thalamus. In the thalamus, axons from the secondary neurons synapse with cell bodies of tertiary neurons.

• Tertiary neurons in the thalamus relay information to neurons in the somatosensory cortex of the cerebrum.

The spinoreticular and spinomesencephalic tracts ascend with the spinothalamic tract through the spinal cord but then divert to the midbrain and other brainstem nuclei. Some neurons in the spinoreticular tracts do not cross over but ascend on the ipsilateral (same) side of the spinal cord on which they enter. A portion of the spinomesencephalic tract, called the spinotectal tract, ends in the superior colliculi of the midbrain. The spinotectal tract transmits action potentials involved in reflexes that turn the head and eyes toward a point of cutaneous stimulation.

Spinocerebellar Tracts

The spinocerebellar tracts carry proprioceptive information to the cerebellum, allowing it to monitor actual movements and compare them with intended movements. Two main tracts extend through the spinal cord: the posterior and anterior spinocerebellar tracts.

The posterior spinocerebellar tract transmits information from the upper body, including the thoracic and upper lumbar regions, through uncrossed nerve fibers that enter the cerebellum via the inferior cerebellar peduncles. The anterior spinocerebellar tract carries information from the lower trunk and limbs. It consists of both crossed and uncrossed nerve fibers that enter the cerebellum via the superior cerebellar peduncle, with the crossed fibers recrossing within the cerebellum. Both tracts transmit proprioceptive information to the cerebellum from the same side of the body as the cerebellar hemisphere to which they project. The purpose of the anterior tract's double decussation remains unclear.

Proprioceptive information from the lower limbs, carried by the fasciculus gracilis of the dorsal-column/medial-lemniscal system, is often transferred through synapses in the inferior thorax to the spinocerebellar system and enters the cerebellum as unconscious proprioceptive information. The spinocerebellar tracts transmit minimal information from the upper limbs, which is instead projected to the thalamus and then enters the cerebellum via the inferior peduncle from the nucleus cuneatus. Consequently, the dorsal-column/medial-lemniscal system contributes to both conscious proprioception and unconscious neuromuscular functions.

Descending Pathways Modifying Sensation

The cerebral cortex and other brain regions can decrease conscious sensation perception, including pain, through descending pathways. These pathways originate in the brain and extend down to the spinal cord, where they influence sensory signals. Along the way, branches from these neurons interact with areas such as the thalamus, reticular formation, trigeminal nuclei, and the spinal cord. Neuromodulators, like endorphins and enkephalins, are released from these pathways and help reduce the frequency of action potentials in sensory tracts. This process, known as presynaptic inhibition, minimizes the sensation of pain.

Pain Pathways

Pain Characteristics

Definition: Pain is an unpleasant sensory and emotional experience that elicits physical (somatic), emotional (psychological), and involuntary (autonomic) responses.

Types of Pain Sensations:

• Sharp, Localized Pain:

  • Conducted by large-diameter, myelinated axons.

  • Results in a fast, distinct, pricking or cutting pain.

• Diffuse, Aching Pain:

  • Conducted by smaller, less-myelinated axons.

  • Slower transmission leads to a dull, burning, or aching sensation.

Factors Influencing Pain Perception:

• Receptor Stimulation: The way pain receptors (nociceptors) are activated affects the sensation.

• Action Potential Integration: Differences in how the nervous system integrates pain signals.

• Brain Processing: Complex interactions in key brain areas:

  • Cerebral Cortex: Processes and evaluates pain stimuli based on previous experience and context.

  • Thalamus: Relays pain signals to appropriate areas for processing.

  • Cingulate Gyrus: Associated with the emotional component of pain, such as discomfort or distress.

The sensation of pain involves sensory pathways that are activated by both pain receptors and nearby mechanoreceptors. Tactile mechanoreceptors in the dorsal-column/medial-lemniscal system are often activated by the same stimuli that affect pain receptors. Action potentials from tactile receptors provide information that allows the pain sensation to be localized. Superficial pain is highly localized, in part, because of the simultaneous stimulation of pain receptors and mechanoreceptors in the skin. Deep, or visceral, pain is not highly localized because of fewer mechanoreceptors in the deeper structures, and it is normally perceived as a diffuse pain.

Dorsal-column/medial-lemniscal system neurons are involved in what is called the gate-control theory of pain control. Primary neurons of this system send out collateral branches that synapse with interneurons in the dorsal horn of the spinal cord. These interneurons have an inhibitory effect on secondary neurons of the spinothalamic tract. Thus, pain action potentials traveling through the spinothalamic tract can be suppressed by action potentials that originate in neurons of the dorsal-column/medial-lemniscal system. The arrangement may act as a "gate" for pain action potentials transmitted in the spinothalamic tract. Increased activity in the dorsal-column/medial-lemniscal system tends to close the gate, thereby reducing pain action potentials transmitted in the spinothalamic tract. Descending pathways from the cerebral cortex or other brain regions can also regulate the gate. The gate-control theory may explain the physiological basis for reducing the intensity of chronic pain by rubbing the area around an injury, transcutaneous electrical stimulation, acupuncture, massage, and exercise. Exercise decreases the sensation of pain and is important in managing chronic pain not associated with illness. Acupuncture may lessen pain through the action of a gating mechanism that inhibits pain transmission upward in the spinal cord.

Analgesics are pain-relieving medications that act in much the same way as gate control. Some analgesics act in the periphery to reduce inflammation and the activation of peripheral nerves; others block the transmission of pain sensations in the spinal cord from primary neurons to neurons of the ascending pathways. Other analgesics function at the level of the cerebral cortex to modulate pain.

Referred pain is a painful sensation in a region of the body that is not the source of the pain stimulus. Most commonly, patients sense referred pain in the skin or other superficial structures when internal organs are damaged or inflamed. This sensation usually occurs because both the area of skin to which the pain is referred and the visceral area that is damaged are innervated by neurons that project to the same area of the cerebral cortex. The brain cannot distinguish between the two sources of painful stimuli, and the painful sensation is referred to the most superficial structures innervated by the converging neurons. This referral may occur because the number of receptors is much greater in superficial structures than in deep structures and the brain is more "accustomed" to dealing with superficial stimuli. Referred pain is clinically useful in diagnosing the actual cause of a painful stimulus. Heart attack victims often feel cutaneous pain radiating from the left shoulder down the arm. Other examples of referred pain are shown.

Sensation and Sensory System Notes

Sensation and Sensory System

general- widely distributed, no complex sense organ; ex, pressure, pain, vibration, proprioception, touch

special- localized with dedicated organ, ex, olfaction, gustation, vision, audition, equilibrium

sensation- perception= conscious awareness

• Receptor Types:

  • Mechano: ex, hair cell bending- hearing, equilibrium

  • Chemo: dissolved chemicals; taste and smell

  • Photo: light. retina

  • Thermo: cold, warm; Temperature extremes lead to pain

  • Noci: pain, free-nerve endings; somatic vs. visceral

• Positional Types:

  • Externo: cutaneous

  • Viscero: visceral

  • Proprio: joint position

Afferent Nerve Endings:

• Merkel’s discs- within stratum basale; light touch/pressure

• hair follicle receptors- hair bending

• Pacinian corpuscles- dermis/hypodermis; deep pressure//vibration

• Meissner’s corpuscles- within dermal papillae; 2 points. discriminations

  • increased count in tongue, lips, fingertips, and non-hairy surfaces

• Ruffini’s end organs- within dermis, pressure

• Golgi tendon organ- within joints, responds to increased tendon tension

• Muscle spindles- responds to increased muscle stretch; contraction minimizes damage

Sensory and Motor Nerve Tracts

Spinal pathways- involve primary, secondary, and tertiary neurons

• Ascending- sensory

  • Spinothalamic- lateral- carries pain/temperature information

  • Spinocerebellar- proprioception function

• Descending- motor

  • Pyramidal- muscle tone, skilled movement (conscious)

    • corticospinal- controls hand movements

    • corticobulbar- controls face/head movements

    • 75% of motor neurons crossover at medullary pyramids; others crossover within the cord

  • Extrapyramidal- controls unconscious movements

    • reburospinal- movement coordination

    • vestibulospinal- posture, balance

    • reticulospinal- posture adjustment during movement

    • tectospinal- head/neck movement associated with visual stimulus

• Proprioception- provides information about the precise position, rate of movement of body parts, and range of motion of joints

  • receptors located around joints and muscles

Pain:

• somatic- superficial

• referred- visceral pain “referred” to superficial locations

• phantom- seen with amputation