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Chapter 16

Sensory, Motor, and Integrative Systems

In this chapter we explore the levels and components of pathways that convey sensory nerve impulses from the body to the brain, and the general sensations (somatic and visceral) that result. We will

also examine the activation of

motor pathways and movements

In chapter 17 we will look at

the special senses of sight,

hearing, taste, and smell

General Sensations

As sensory impulses reach the CNS, they become part of a large pool of sensory input

(though not every one

will elicit a response)

Each piece of

incoming information

is combined with other arriving

and previously stored information

in a process called integration

General Sensations

Integration occurs at many places along pathways in the spinal cord, brain stem, cerebellum, basal nuclei,

and cerebral cortex. Sensations result in

and evoke a conscious perception

or subconscious awareness that

changes have occurred in the

external or internal environment.

The motor responses are also

modified at several of these levels.

General Sensations

Examples of complex integrative functions of the brain include wakefulness and sleep, and learning and memory

Sensory Modalities

Each unique type of sensation is called a sensory modality, and a given sensory neuron carries information for only one modality, be it somatic, visceral, or “special”

Somatic senses include tactile sensations (touch, pressure, vibration, itch, and tickle), thermal sensations (warm and cold), pain sensations, and proprioception (awareness of limb and joint position in space)

Visceral senses provide information about conditions within internal organs

Sensory Modalities

The process of sensation begins in a sensory receptor, which can be either a specialized cell or the dendrites of a sensory neuron

A particular kind of stimulus (a change in the environment) activates certain sensory receptors, while other sensory receptors respond only weakly or not at all – a characteristic known as selectivity

Sensory Modalities

For a sensation to arise, four events typically occur:

Stimulation of the sensory receptor - an appropriate stimulus must occur within the receptor’s receptive field

Transduction of the stimulus - a sensory receptor converts energy in a stimulus into a graded potential. Recall that graded potentials (but not APs) vary in amplitude depending on the strength of the stimulus that causes them, and are not propagated

Sensory receptors produce two different kinds of graded potentials—generator potentials and receptor potentials—in response to a stimulus. When stimulated, the dendrites of free nerve endings, encapsulated nerve endings, and the receptive part of olfactory receptors produce a generator potential (Figure 16.1a, b). When a generator potential is large enough to reach threshold, it triggers one or more nerve impulses in the axon of a first-order sensory neuron. The resulting nerve impulse propagates along the axon into the CNS. Thus, generator potentials generate action potentials.

By contrast, sensory receptors that are separate cells produce graded potentials termed receptor potentials.

Sensory Modalities

The four events that bring about a sensation, cont’d…

Generation of nerve impulses – occurs when the sum of graded potentials reach threshold in first-order neurons (the first neuron in a specific tract – in this case from the PNS into the CNS)

Integration of sensory input – occurs when a particular region of the CNS integrates a number (and even a variety) of sensory nerve impulses and results in a conscious sensations or perceptions

Sensory Modalities

Sensory Receptors

Sensory receptors can be grouped into several classes

based on structural and functional characteristics:

Microscopic structure – free nerve endings vs encapsulated endings, for example

Location…of the receptors and the origin of the stimuli that activate them

The type of stimulus detected (nociceptors for pain, mechanoreceptors for pressure, etc.)

Interoceptors (visceroceptors) are located in blood vessels, visceral organs, muscles, and the nervous system and monitor conditions in the internal environment. The nerve impulses produced by interoceptors usually are not consciously perceived; occasionally, however, activation of interoceptors by strong stimuli may be felt as pain or pressure.

Sensory Receptors

Sensory receptors can be grouped into several classes

based on structural and functional characteristics:

Microscopic structure – free nerve endings vs encapsulated endings, for example

Location…of the receptors and the origin of the stimuli that activate them – exteroceptors near the external surface vs interoceptors (visceroceptors), for example

The type of stimulus detected (nociceptors for pain, mechanoreceptors for pressure, etc.)

Sensory Receptors

Receptors named according to their location include:

Exteroceptors, which are located at or near the external surface of the body and respond to external stimuli

Interoceptors (visceroceptors), which are located in blood vessels, organs, and muscles and produce impulses which usually are not consciously perceived

Proprioceptors, which are located in muscles, tendons, joints, and the inner ear. They provide information about body position and movement of joints

Exteroceptors provide information about the external environment.

Interoceptors monitor the internal environment. Occasionally activation of interoceptors by strong stimuli may be felt as pain or pressure.

Sensory Receptors

Receptors can also denote the type of stimulus that excites them

Sensory Receptors

Receptors named according to mode of activation are:

Mechanoreceptors, which are sensitive to deformation

Thermoreceptors, which detect changes in temperature

Nociceptors, which respond to painful stimuli

Photoreceptors, which are activated by photons of light

Chemoreceptors, which detect chemicals in the mouth (taste), nose (smell) and body fluids

Osmoreceptors, which detect the osmotic pressure of body fluids

Mechanoreceptors provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. They also monitor the stretching of blood vessels and internal organs.

Sensory Receptors

A characteristic feature of most sensory receptors is adaptation, in which the generator potential or receptor potential decreases in amplitude during a sustained or constant stimulus

Because there is an accommodation response at the receptor level, the frequency of nerve impulses traveling to the cerebral cortex decreases and the perception of the sensation fades even though the stimulus persists

receptors vary in how quickly they adapt (rapidly adapting and slowly adapting receptors)

Receptors associated with pressure, touch, and smell are rapidly adapting.

Mechanoreception

Many of the mechanoreceptors and nociceptors previously described are

located in

the skin

Mechanoreception

This graphic illustrates some representative examples of general somatic mechanoreceptors and the first-order neurons to which they belong. Receptors for special senses are not shown.

All of our sensory modalities are important, but pain serves a protective function and is indispensable for survival

Nociceptors are chemoreceptive free nerve

endings activated by tissue damage from intense thermal, mechanical, or chemical stimuli - they’re found in every tissue of the body except the brain

Nociception

There are two types of pain: fast and slow

The perception of fast pain (acute, well localized) occurs rapidly because the nerve impulses propagate along medium-diameter, myelinated A fibers

By contrast, slow pain begins after a stimulus is applied and gradually increases in intensity over a period of several seconds or minutes. Impulses for slow pain conduct along small-diameter, unmyelinated C fibers. This type of pain may be excruciating and often has a burning, aching, or throbbing quality

An example of slow pain is the pain associated with a toothache.

Nociception

Pain that arises from stimulation of receptors in the skin is called superficial somatic pain; stimulation of receptors in skeletal muscles, joints, tendons, and fascia causes deep somatic pain
Visceral pain results from stimulation of nociceptors in visceral organs

In many instances of visceral pain, the pain is felt in or just deep to the skin that overlies the stimulated organ, or in a surface area far from the stimulated organ. This phenomenon is called referred pain

Nociception

Common patterns of referred visceral pain are shown in this graphic

Proprioception

Muscle spindles are the proprioceptors in skeletal muscles that monitor changes in the muscle length and participate in stretch reflexes

By adjusting how vigorously a muscle spindle responds to stretching of a skeletal muscle, the brain sets an overall level of muscle tone (the small degree of contraction that is present while the muscle is

at rest)

Each muscle spindle consists of several slowly adapting sensory nerve endings that wrap around 3 to 10 specialized muscle fibers. A connective tissue capsule encloses the sensory nerve endings and anchors the spindle to the endomysium

and perimysium

Muscle spindles are plentiful

in muscles that control fine

movements and much more

sparse in those that control

course or forceful movements

Proprioception

Free nerve endings and Ruffini corpuscles in the capsules of joints respond to pressure.

Pacinian corpuscles respond to acceleration and deceleration of joints during movement.

Somatic Sensory Pathways

No matter the type of receptor on the receiving end (where the generator potential is set up), first-order somatosensory neurons are unipolar in structure

This means that their cell body is located in the dorsal root ganglia (DRG) just outside the CNS

Their other end terminates

nearby in the posterior

gray horns of the

cord, usually at the

level where they enter

Mechanoreceptors provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. They also monitor the stretching of blood vessels and internal organs.

Second-order neurons conduct ascending impulses from the brain stem where their

axons decussate (cross over to

the opposite side) before

ascending to the thalamus

Thus, all somatic sensory

information from one side

of the body reaches the

thalamus on the opposite side

Somatic Sensory Pathways

Third-order neurons conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side.

Somatic Sensory Pathways

Somatic sensory neurons (and their axons that convey somatic sensations) are not distributed evenly in the body

The peripheral areas with the highest density are represented in the brain with the

largest amount of gray matter in the

sensory homunculus. The most

sensitive areas in the body are

therefore the tip of the tongue,

lips, and fingertips

There are two major spinocerebellar tracts in the spinal cord that carry proprioceptive impulses to the cerebellum

Although they are not

consciously perceived,

sensory impulses sent

to the cerebellum

along these two pathways

are critical for

posture, balance, and

coordination of skilled movements

Somatic Sensory Pathways

Motor activity begins in the primary motor areas of the precentral gyrus and other cerebral integrative centers

Any motor neuron that is not directly

responsible for stimulating target

muscles is called an upper

motor neuron (UMN)

UMNs connect the brain to

the appropriate level in the

spinal cord

Somatic Motor Pathways

The basal nuclei and cerebellum influence movement through their effects on upper motor neurons.

From there, all excitatory and inhibitory signals that control movement converge on second-order motor neurons known as lower motor

neurons (LMNs) that descend

to innervate skeletal muscle

Since only LMNs provide

output from the CNS to

skeletal muscle fibers they are also

called the final common pathway

Somatic Motor Pathways

Axons of LMNs extend through cranial nerves to the skeletal muscles of the face and head, and through spinal nerves to innervate

skeletal muscles

of the limbs and trunk

Two of the major

LMN tracts are the

lateral and anterior

corticospinal tracts

Somatic Motor Pathways

Only LMNs provide output from the CNS to skeletal muscle fibers. For this reason, they are also called the final common pathway.

Sensory and Motor Pathways
(Interactions Animation)

Somatic Sensory Pathways

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End of Chapter 16

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