Chapter 15

Neural Integration I: Sensory Pathways and the Somatic Nervous System

An Introduction to Sensory Pathways and the Somatic Nervous System

Learning Outcomes

15-1 Specify the components of the afferent and efferent divisions of the nervous system, and explain what is meant by the somatic nervous system.

15-2 Explain why receptors respond to specific stimuli, and how the organization of a receptor affects its sensitivity.

15-3 Identify the receptors for the general senses, and describe how they function.

An Introduction to Sensory Pathways and the Somatic Nervous System

Learning Outcomes

15-4 Identify the major sensory pathways, and explain how it is possible to distinguish among sensations that originate in different areas of the body.

15-5 Describe the components, processes, and functions of the somatic motor pathways, and the levels of information processing involved in motor control.

An Introduction to Sensory Pathways and the Somatic Nervous System

An Introduction to:

Sensory receptors

Sensory processing

Conscious and subconscious motor functions

Focusing on the “general senses”

15-1 Sensory Information

Afferent Division of the Nervous System

Receptors

Sensory neurons

Sensory pathways

Efferent Division of the Nervous System

Nuclei

Motor tracts

Motor neurons

15-1 Sensory Information

Sensory Receptors

Specialized cells that monitor specific conditions

In the body or external environment

When stimulated, a receptor passes information to the CNS

In the form of action potentials along the axon of a sensory neuron

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Sensory Pathways

Deliver somatic and visceral sensory information to their final destinations inside the CNS using:

Nerves

Nuclei

Tracts

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Somatic Motor Portion of the Efferent Division

Controls peripheral effectors

Somatic Motor Commands

Travel from motor centers in the brain along somatic motor pathways of:

Motor nuclei

Tracts

Nerves

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Somatic Nervous System (SNS)

Motor neurons and pathways that control skeletal muscles

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General Senses

Describe our sensitivity to:

Temperature

Pain

Touch

Pressure

Vibration

Proprioception

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Sensation

The arriving information from these senses

Perception

Conscious awareness of a sensation

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Special Senses

Olfaction (smell)

Vision (sight)

Gustation (taste)

Equilibrium (balance)

Hearing

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The Special Senses

Are provided by special sensory receptors

Special Sensory Receptors

Are located in sense organs such as the eye or ear

Are protected by surrounding tissues

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The Detection of Stimuli

Receptor specificity

Each receptor has a characteristic sensitivity

Receptive field

Area is monitored by a single receptor cell

The larger the receptive field, the more difficult it is to localize a stimulus

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The Interpretation of Sensory Information

Arriving stimulus reaches cortical neurons via labeled line

Takes many forms (modalities)

Physical force (such as pressure)

Dissolved chemical

Sound

Light

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The Interpretation of Sensory Information

Sensations

Taste, hearing, equilibrium, and vision provided by specialized receptor cells

Communicate with sensory neurons across chemical synapses

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Adaptation

Reduction in sensitivity of a constant stimulus

Your nervous system quickly adapts to stimuli that are painless and constant

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Adaptation

Tonic receptors

Are always active

Show little peripheral adaptation

Are slow-adapting receptors

Remind you of an injury long after the initial damage has occurred

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Adaptation

Phasic receptors

Are normally inactive

Become active for a short time whenever a change occurs

Provide information about the intensity and rate of change of a stimulus

Are fast-adapting receptors

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Adaptation

Stimulation of a receptor produces action potentials

Along the axon of a sensory neuron

The frequency and pattern of action potentials contain information

About the strength, duration, and variation of the stimulus

Your perception of the nature of that stimulus

Depends on the path it takes inside the CNS

15-3 Classifying Sensory Receptors

Classifying Sensory Receptors

Exteroceptors provide information about the external environment

Proprioceptors report the positions of skeletal muscles and joints

Interoceptors monitor visceral organs and functions

15-3 Classifying Sensory Receptors

Proprioceptors

Provide a purely somatic sensation

No proprioceptors in the visceral organs of the thoracic and abdominopelvic cavities

You cannot tell where your spleen, appendix, or pancreas is at the moment

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General Sensory Receptors

Are divided into four types by the nature of the stimulus that excites them

Nociceptors (pain)

Thermoreceptors (temperature)

Mechanoreceptors (physical distortion)

Chemoreceptors (chemical concentration)

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Nociceptors (Pain Receptors)

Are common

In the superficial portions of the skin

In joint capsules

Within the periostea of bones

Around the walls of blood vessels

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Nociceptors

May be sensitive to:

Temperature extremes

Mechanical damage

Dissolved chemicals, such as chemicals released by injured cells

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Nociceptors

Are free nerve endings with large receptive fields

Branching tips of dendrites

Not protected by accessory structures

Can be stimulated by many different stimuli

Two types of axons - Type A and Type C fibers

15-3 Classifying Sensory Receptors

Nociceptors

Myelinated Type A fibers

Carry sensations of fast pain, or prickling pain, such as that caused by an injection or a deep cut

Sensations reach the CNS quickly and often trigger somatic reflexes

Relayed to the primary sensory cortex and receive conscious attention

15-3 Classifying Sensory Receptors

Nociceptors

Type C fibers

Carry sensations of slow pain, or burning and aching pain

Cause a generalized activation of the reticular formation and thalamus

You become aware of the pain but only have a general idea of the area affected

15-3 Classifying Sensory Receptors

Thermoreceptors

Also called temperature receptors

Are free nerve endings located in:

The dermis

Skeletal muscles

The liver

The hypothalamus

15-3 Classifying Sensory Receptors

Thermoreceptors

Temperature sensations

Conducted along the same pathways that carry pain sensations

Sent to:

The reticular formation

The thalamus

The primary sensory cortex (to a lesser extent)

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Mechanoreceptors

Sensitive to stimuli that distort their plasma membranes

Contain mechanically gated ion channels whose gates open or close in response to:

Stretching

Compression

Twisting

Other distortions of the membrane

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Three Classes of Mechanoreceptors

Tactile receptors

Provide the sensations of touch, pressure, and vibration

Touch sensations provide information about shape or texture

Pressure sensations indicate degree of mechanical distortion

Vibration sensations indicate pulsing or oscillating pressure

15-3 Classifying Sensory Receptors

Three Classes of Mechanoreceptors

Baroreceptors

Detect pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts

15-3 Classifying Sensory Receptors

Three Classes of Mechanoreceptors

Proprioceptors

Monitor the positions of joints and muscles

The most structurally and functionally complex of general sensory receptors

15-3 Classifying Sensory Receptors

Tactile Receptors

Fine touch and pressure receptors

Are extremely sensitive

Have a relatively narrow receptive field

Provide detailed information about a source of stimulation

Including its exact location, shape, size, texture, movement

15-3 Classifying Sensory Receptors

Tactile Receptors

Crude touch and pressure receptors

Have relatively large receptive fields

Provide poor localization

Give little information about the stimulus

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Six Types of Tactile Receptors in the Skin

Free nerve endings

Sensitive to touch and pressure

Situated between epidermal cells

Free nerve endings providing touch sensations are tonic receptors with small receptive fields

15-3 Classifying Sensory Receptors

Six Types of Tactile Receptors in the Skin

Root hair plexus nerve endings

Monitor distortions and movements across the body surface wherever hairs are located

Adapt rapidly, so are best at detecting initial contact and subsequent movements

15-3 Classifying Sensory Receptors

Six Types of Tactile Receptors in the Skin

Tactile discs

Also called Merkel discs

Fine touch and pressure receptors

Extremely sensitive to tonic receptors

Have very small receptive fields

15-3 Classifying Sensory Receptors

Six Types of Tactile Receptors in the Skin

Tactile corpuscles

Also called Meissner’s corpuscles

Perceive sensations of fine touch, pressure, and low-frequency vibration

Adapt to stimulation within 1 second after contact

Fairly large structures

Most abundant in the eyelids, lips, fingertips, nipples, and external genitalia

15-3 Classifying Sensory Receptors

Six Types of Tactile Receptors in the Skin

Lamellated corpuscles

Also called Pacinian corpuscles

Sensitive to deep pressure

Fast-adapting receptors

Most sensitive to pulsing or high-frequency vibrating stimuli

15-3 Classifying Sensory Receptors

Six Types of Tactile Receptors in the Skin

Ruffini corpuscles

Also sensitive to pressure and distortion of the skin

Located in the reticular (deep) dermis

Tonic receptors that show little if any adaptation

15-3 Classifying Sensory Receptors

Baroreceptors

Monitor change in pressure

Consist of free nerve endings that branch within elastic tissues

In wall of distensible organ (such as a blood vessel)

Respond immediately to a change in pressure, but adapt rapidly

15-3 Classifying Sensory Receptors

Proprioceptors

Monitor:

Position of joints

Tension in tendons and ligaments

State of muscular contraction

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Three Major Groups of Proprioceptors

Muscle spindles

Golgi tendon organs

Receptors in joint capsules

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Muscle Spindles

Monitor skeletal muscle length

Trigger stretch reflexes

Golgi Tendon Organs

Located at the junction between skeletal muscle and its tendon

Stimulated by tension in tendon

Monitor external tension developed during muscle contraction

15-3 Classifying Sensory Receptors

Receptors in Joint Capsules

Free nerve endings detect pressure, tension, movement at the joint

15-3 Classifying Sensory Receptors

Chemoreceptors

Respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid

Receptors exhibit peripheral adaptation over period of seconds

Central adaptation may also occur

15-3 Classifying Sensory Receptors

Chemoreceptors

Receptors that monitor pH, carbon dioxide, and oxygen levels in arterial blood are located in:

Carotid bodies

Near the origin of the internal carotid arteries on each side of the neck

Aortic bodies

Between the major branches of the aortic arch

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First-Order Neuron

Sensory neuron delivers sensations to the CNS

Cell body of a first-order general sensory neuron is located in dorsal root ganglion or cranial nerve ganglion

Second-Order Neuron

Axon of the sensory neuron synapses on an interneuron in the CNS

May be located in the spinal cord or brain stem

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Third-Order Neuron

If the sensation is to reach our awareness, the second-order neuron synapses

On a third-order neuron in the thalamus

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Somatic Sensory Pathways

Carry sensory information from the skin and musculature of the body wall, head, neck, and limbs

Three major somatic sensory pathways

The spinothalamic pathway

The posterior column pathway

The spinocerebellar pathway

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The Spinothalamic Pathway

Provides conscious sensations of poorly localized (“crude”) touch, pressure, pain, and temperature

First-order neurons

Axons of first-order sensory neurons enter spinal cord

And synapse on second-order neurons within posterior gray horns

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The Spinothalamic Pathway

Second-order neurons

Cross to the opposite side of the spinal cord before ascending

Ascend within the anterior or lateral spinothalamic tracts

The anterior tracts carry crude touch and pressure sensations

The lateral tracts carry pain and temperature sensations

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The Spinothalamic Pathway

Third-order neurons

Synapse in ventral nucleus group of the thalamus

After the sensations have been sorted and processed, they are relayed to primary sensory cortex

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Feeling Pain (Lateral Spinothalamic Tract)

An individual can feel pain in an uninjured part of the body when pain actually originates at another location

Strong visceral pain

Sensations arriving at segment of spinal cord can stimulate interneurons that are part of spinothalamic pathway

Activity in interneurons leads to stimulation of primary sensory cortex, so an individual feels pain in specific part of body surface

15-4 Sensory Pathways

Feeling Pain (Lateral Spinothalamic Tract)

Referred pain

The pain of a heart attack is frequently felt in the left arm

The pain of appendicitis is generally felt first in the area around the navel and then in the right, lower quadrant

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Posterior Column Pathway

Carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception

Spinal tracts involved

Left and right fasciculus gracilis

Left and right fasciculus cuneatus

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Posterior Column Pathway

Axons synapse

On third-order neurons in one of the ventral nuclei of the thalamus

Nuclei sort the arriving information according to:

The nature of the stimulus

The region of the body involved

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Posterior Column Pathway

Processing in the thalamus

Determines whether you perceive a given sensation as fine touch, as pressure, or as vibration

Ability to determine stimulus

Precisely where on the body a specific stimulus originated depends on the projection of information from the thalamus to the primary sensory cortex

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Posterior Column Pathway

Sensory information

From toes arrives at one end of the primary sensory cortex

From the head arrives at the other

When neurons in one portion of your primary sensory cortex are stimulated, you become aware of sensations originating at a specific location

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Posterior Column Pathway

Sensory homunculus

Functional map of the primary sensory cortex

Distortions occur because:

Area of sensory cortex devoted to particular body region is not proportional to region’s size, but to number of sensory receptors it contains

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The Spinocerebellar Pathway

Cerebellum receives proprioceptive information about position of:

Skeletal muscles

Tendons

Joints

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The Spinocerebellar Tracts

The posterior spinocerebellar tracts

Contain second-order axons that do not cross over to the opposite side of the spinal cord

Axons reach cerebellar cortex via inferior cerebellar peduncle of that side

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The Spinocerebellar Tracts

The anterior spinocerebellar tracts

Dominated by second-order axons that have crossed over to opposite side of spinal cord

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The Spinocerebellar Tracts

The anterior spinocerebellar tracts

Contain a significant number of uncrossed axons as well

Sensations reach the cerebellar cortex via superior cerebellar peduncle

Many axons that cross over and ascend to cerebellum then cross over again within cerebellum, synapsing on same side as original stimulus

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Sensory Information

Most somatic sensory information

Is relayed to the thalamus for processing

A small fraction of the arriving information

Is projected to the cerebral cortex and reaches our awareness

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Visceral Sensory Pathways

Collected by interoceptors monitoring visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities

These interoceptors are not as numerous as in somatic tissues

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Visceral Sensory Pathways

Interoceptors include:

Nociceptors

Thermoreceptors

Tactile receptors

Baroreceptors

Chemoreceptors

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Visceral Sensory Pathways

Cranial Nerves V, VII, IX, and X

Carry visceral sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, and associated vessels and glands

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Visceral Sensory Pathways

Solitary nucleus

Large nucleus in the medulla oblongata

Major processing and sorting center for visceral sensory information

Extensive connections with the various cardiovascular and respiratory centers, reticular formation

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The Somatic Nervous System (SNS)

Also called the somatic motor system

Controls contractions of skeletal muscles (discussed next)

The Autonomic Nervous System (ANS)

Also called the visceral motor system

Controls visceral effectors, such as smooth muscle, cardiac muscle, and glands (Ch. 16)

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Somatic Motor Pathways

Always involve at least two motor neurons

Upper motor neuron

Lower motor neuron

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Upper Motor Neuron

Cell body lies in a CNS processing center

Synapses on the lower motor neuron

Innervates a single motor unit in a skeletal muscle

Activity in upper motor neuron may facilitate or inhibit lower motor neuron

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Lower Motor Neuron

Cell body lies in a nucleus of the brain stem or spinal cord

Triggers a contraction in innervated muscle

Only axon of lower motor neuron extends outside CNS

Destruction of or damage to lower motor neuron eliminates voluntary and reflex control over innervated motor unit

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Conscious and Subconscious Motor Commands

Control skeletal muscles by traveling over three integrated motor pathways

Corticospinal pathway

Medial pathway

Lateral pathway

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The Corticospinal Pathway

Sometimes called the pyramidal system

Provides voluntary control over skeletal muscles

System begins at pyramidal cells of primary motor cortex

Axons of these upper motor neurons descend into brain stem and spinal cord to synapse on lower motor neurons that control skeletal muscles

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The Corticospinal Pathway

Contains three pairs of descending tracts

1. Corticobulbar tracts

2. Lateral corticospinal tracts

3. Anterior corticospinal tracts

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Corticobulbar Tracts

Provide conscious control over skeletal muscles that move the eye, jaw, face, and some muscles of neck and pharynx

Innervate motor centers of medial and lateral pathways

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Corticospinal Tracts

As they descend, lateral corticospinal tracts are visible along the ventral surface of medulla oblongata as a pair of thick bands, the pyramids

At spinal segment it targets, an axon in anterior corticospinal tract crosses over to opposite side of spinal cord in anterior white commissure before synapsing on lower motor neurons in anterior gray horns

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The Corticospinal Pathway

Motor homunculus

Primary motor cortex corresponds point by point with specific regions of the body

Cortical areas have been mapped out in diagrammatic form

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The Corticospinal Pathway

Motor homunculus

Homunculus provides indication of degree of fine motor control available

Hands, face, and tongue, which are capable of varied and complex movements, appear very large, while trunk is relatively small

These proportions are similar to the sensory homunculus

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The Medial and Lateral Pathways

Several centers in cerebrum, diencephalon, and brain stem may issue somatic motor commands as result of processing performed at subconscious level

These nuclei and tracts are grouped by their primary functions:

Components of medial pathway help control gross movements of trunk and proximal limb muscles

Components of lateral pathway help control distal limb muscles that perform more precise movements

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The Medial Pathway

Primarily concerned with control of muscle tone and gross movements of neck, trunk, and proximal limb muscles

Upper motor neurons of medial pathway are located in:

Vestibular nuclei

Superior and inferior colliculi

Reticular formation

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The Medial Pathway

Vestibular nuclei

Receive information over the vestibulocochlear nerve (VIII) from receptors in inner ear that monitor position and movement of the head

Primary goal is to maintain posture and balance

Descending fibers of spinal cord constitute vestibulospinal tracts

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The Medial Pathway

Superior and inferior colliculi

Are located in the roof of the mesencephalon, or the tectum

Colliculi receive visual (superior) and auditory (inferior) sensations

Axons of upper motor neurons in colliculi descend in tectospinal tracts

These axons cross to opposite side, before descending to synapse on lower motor neurons in brain stem or spinal cord

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The Medial Pathway

Reticular formation

Loosely organized network of neurons that extends throughout brain stem

Axons of upper motor neurons in reticular formation descend into reticulospinal tracts without crossing to opposite side

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The Lateral Pathway

Primarily concerned with control of muscle tone and more precise movements of distal parts of limbs

Axons of upper motor neurons in red nuclei cross to opposite side of brain and descend into spinal cord in rubrospinal tracts

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The Basal Nuclei and Cerebellum

Responsible for coordination and feedback control over muscle contractions

Whether contractions are consciously or subconsciously directed

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The Basal Nuclei

Provide background patterns of movement involved in voluntary motor activities

Some axons extend to the premotor cortex, the motor association area that directs activities of the primary motor cortex

Alters the pattern of instructions carried by the corticospinal tracts

Other axons alter the excitatory or inhibitory output of the reticulospinal tracts

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The Cerebellum

Monitors:

Proprioceptive (position) sensations

Visual information from the eyes

Vestibular (balance) sensations from inner ear as movements are under way

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Levels of Processing and Motor Control

All sensory and motor pathways involve a series of synapses, one after the other

General pattern

Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that preserve homeostasis over short term

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Levels of Processing and Motor Control

Cranial and spinal reflexes

Control the most basic motor activities

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Levels of Processing and Motor Control

Integrative centers in the brain

Perform more elaborate processing

As we move from medulla oblongata to cerebral cortex, motor patterns become increasingly complex and variable

Primary motor cortex

Most complex and variable motor activities are directed by primary motor cortex of cerebral hemispheres

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Levels of Processing and Motor Control

Neurons of the primary motor cortex

Innervate motor neurons in the brain and spinal cord responsible for stimulating skeletal muscles

Higher centers in the brain

Can suppress or facilitate reflex responses

Reflexes

Can complement or increase the complexity of voluntary movements