Copyright © John Wiley & Sons, Inc. All rights reserved. Chapter 16 Sensory, Motor, and Integrative...

Copyright © John Wiley & Sons, Inc. All rights reserved.

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


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:

1. Stimulation of the sensory receptor - an

appropriate stimulus must occur within the

receptor’s receptive field

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

3. 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)

4. 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.)


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


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


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)


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


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


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 finemovements and much more sparse in those that controlcourse or forceful movements

Proprioception


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


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



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


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



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



Sensory and Motor Pathways(Interactions Animation)


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Copyright © John Wiley & Sons, Inc. All rights reserved. Chapter 16 Sensory, Motor, and Integrative...

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