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Chapter 15
Sensory Pathways and the Somatic Nervous System
82
Anatomy
Undergraduate 2
11/30/2017

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Term
Sensory receptors
Definition
Specialized cells or cell processes that monitor specific conditions in the body or the external environment.
Term
transduction
Definition
A sensory receptor detects an arriving stimulus and translates it into an action potential that can be conducted to the CNS.

Begins when stimulus changes transmembrane potential receptor cell
Term
Labeled line
Definition
The link between peripheral receptor and cortical neuron, consists of axons carrying information about one type (modality) of stimulus.
Term
Sensory coding
Definition
The translation of complex sensory information into meaningful patterns of action potentials.
Term
adaptation
Definition
A reduction in sensitivity in the presence of a constant stimulus.

-Nervous system quickly adapts to stimuli
that are painless and constant
Term
Peripheral adaptation
Definition
When the level of receptor activity changes.
-Reduces the amount of information that reaches CNS
Term
Tonic receptors
Definition
Sensory neurons that are always active. Also called slow-adapting receptors.

Provide information on duration of stimulus
Term
Phasic receptors
Definition
Sensory neurons that are usually not active, also called fast-adapting receptors.

Provide information on intensity and rate of stimulus.
-Pressure receptor or temperature receptor
Term
3 types of receptors for the general senses
Definition
1. Exteroceptors: provide information about the external environment

2. Proprioceptors: report the positions and movements of skeletal muscles and joints. No proprioceptors in the visceral organs of the thoracic and abdominopelvic cavities.

3. Interoceptors: monitor visceral organs and functions.
Term
4 types of natures of stimulus
Definition
1. nociceptors (pain)
2. thermoreceptors (temperature)
3. mechanoreceptors (physical distortion)
4. chemoreceptors (chemical concentration)
Term
Sensation
Definition
arriving information from these senses
-Sensory information interpreted by frequency of action potentials
-Harder pressure-high frequency of action potentials
Term
Perception
Definition
Conscious awareness of a sensation
Term
Receptor sensitivity
Definition
Each receptor has a characteristic sensitivity
-taste receptor: dissolved chemicals, not pressure
Term
Free nerve endings
Definition
Branching tips of sensory neurons
-simplest receptors, little receptor sensitivity
-respond to tissue damage, sensation Pain-chemical, pressure, temperature, trauma
Term
Afferent division
Definition
-receptors
-sensory neurons
-sensory pathways
Going TO CNS
Term
Efferent division
Definition
-nuclei (cell body)
-motor tracts (bundles of axons)
-motor neurons
Exiting FROM CNS
Term
Receptive field
Definition
The area monitored by a single receptor cell
-the larger the receptive field, the more difficult it is to localize a stimulus
-general body surface: larger field
-fingertip: smaller field
Term
Detection of stimuli
Definition
Arriving stimulus may take many forms:
physical force (such as pressure), dissolved chemical, sound, light

Form doesn't matter: information always sent as action potential (electrical event)
Term
Receptor potential
Definition
"Change", graded depolarization or graded hyperpolarization in receptor cell
Term
generator potential
Definition
A depolarizing receptor potential in a neural receptor
Term
Central adaptation
Definition
-Smell receptors accustomed to scent
-Involved inhibition of nuclei along sensory pathway
-Restrict amount of detail arriving at cerebral cortex
Term
Nociceptors
Definition
-Common in superficial portions of skin, joint capsules, within periostea of bones, around walls of blood vessels

-Few nociceptors in deep tissue, visceral organs

-Sensitive to: temperature extremes, mechanical damage, dissolved chemicals, ie chemicals released by injured cells

Nociceptors are free nerve endings with large receptive fields
2 types of axons, type A (fastest) and type C (slowest)
Term
Myelinated Type A fibers
Definition
Carry sensations of FAST pain (prickling), i.e. deep cut or injection

Sensations reach CNS quickly and often trigger somatic reflexes

Also relayed to primary sensory cortex (cerebrum) and receive conscious attention
Term
Type C fibers
Definition
Carry sensations of SLOW pain, burning and aching

Cause a generalized activation of reticular formation and thalamus

Aware of the pain but have only a general idea of the area affected
Term
Nociceptors-tonic receptors
Definition
-Peripheral adaptation does not occur
-Receptors continue to respond as long as painful stimulus occurs
-The sensory neurons that bring pain sensations into the CNS release glutamate and/or substance P as neurotransmitters
-hyperexcitable interneurons can continue to generate pain sensations
Term
Thermoreceptors
Definition
Free nerve endings located in: dermis, skeletal muscles, liver, hypothalamus

Sent along the same pathways as pain sensations: reticular formation, thalamus, primary sensory cortex (to a lesser extent)

Phasic receptors: active if temperature changes, quickly adapt to a stable temperature
Term
3 classes of mechanoreceptors
Definition
1. Tactile receptors
2. Baroreceptors
3. Proprioceptors
Term
Mechanoreceptors
Definition
-Sensitive to stimuli that distort the plasma membrane
-Contain mechanically gated ion channels whose gates open/close in response to:
stretching
compression
twisting
other distortions of the membrane
Term
Tactile receptors
Definition
Provide sensations of touch, pressure, vibration
-touch: shape or texture
-pressure: indicates degree of mechanical distortion
-vibration: indicate pulsing or oscillating pressure
Term
Baroreceptors
Definition
-Detect pressure changes in walls of blood vessels, and in digestive, reproductive, and urinary tracts
Term
Proprioceptors
Definition
Monitor positions of joints and muscles, most structurally and functionally complex
Term
Tactile receptors, fine touch and crude touch
Definition
Fine touch and pressure receptors-extremely sensitive, relatively narrow receptive field
-provide detailed information: exact location, shape, size, texture, movement

Crude touch and pressure receptors-provide poor localization, relatively large receptive fields
-Give little information about the stimulus
Term
6 types of tactile receptors in skin
Definition
1. Tactile discs (Merkel discs)
2. Free nerve endings
3. Root hair plexus nerve endings
4. Tactile corpuscles (Meissner corpuscles)
5.Lamellated corpuscles (Pacinian corpuscles)
6. Ruffini corpuscles
Term
Free nerve endings
Definition
-Sensitive to touch and pressure

-Situated between epidural cells

-Tonic receptors with small receptive fields
Term
Root hair plexus nerve endings
Definition
-Monitor distortions and movements across body surface wherever hairs are located

-Adapt rapidly,best at detecting initial contact and subsequent movements
Term
Tactile discs (Merkel discs)
Definition
-Fine touch and pressure receptors

-Extremely sensitive to tonic receptors

-Have very small receptive fields
Term
Tactile corpuscles (Meissner corpuscles)
Definition
-Perceive sensations of fine touch, pressure, low-frequency vibration

-Adapt to stimulation within 1 second after contact

-Fairly large structure

-Most abundant in the eyelids, lips, fingertips, nipples, external genitalia
Term
Lamellated corpuscles (Pacinian corpuscles)
Definition
-Sensitive to deep pressure

-Fast adapting receptors

-Most sensitive to pulsing or high-frequency vibrating stimuli
Term
Ruffini corpuscles
Definition
-Sensitive to pressure and distortion of skin

-Located in the reticular (deep) dermis

-Tonic receptors, show little if any adaptation
Term
Baroreceptors
Definition
-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

-Trigger urination and defecation reflexes, as well as monitor lung expansion
Term
Proprioceptors
Definition
-Monitor position of joints, tension in tendons and ligaments, state of muscular contraction

-Muscle spindles: monitor skeletal muscle length, trigger stretch reflexes

-Golgi tendon organs: located at junction between skeletal muscle and its tendon, stimulated by tension in tendon, monitor external tension developed during a muscle contraction

-Receptors in joint capsules: free nerve endings detect pressure, tension, movement at the joint
Term
Chemoreceptors
Definition
Respond only to water-soluble and lipid-soluble substances that are dissolved in body fluids (interstitial fluid, blood plasma, and CSF)

Exhibit peripheral adaptation over a period of seconds; central adaptation may also occur

Located in the carotid bodies (near origin of internal carotid arteries on each side of neck) and in aortic bodies (between major branches of aortic arch)

Receptors monitor pH, carbon dioxide, and oxygen levels in arterial blood
Term
Sensory pathways
Definition
First-order neuron: sensory neuron that delivers sensations to the CNS
-cell body located in dorsal rot ganglion or cranial nerve ganglion

Second-order neuron:what the 1st order sensory neuron connects to in the CNS
-located in spinal cord or brain stem

Third-order neuron: if sensation is to reach our awareness, the 2nd-order neuron synapses on a 3rd-order neuron in the thalamus
Term
Decussation
Definition
The crossing of an axon from the left side to the right side, or from the right side to the left side
Term
Somatic sensory pathways
Definition
Carry sensory information from the skin and musculature of the body wall, head, neck, and limbs

3 major somatic sensory pathways:
Posterior column pathway
Spinothalamic pathway
Spinocerebellar pathway
Term
Tract names give clues to their function
Definition
Starts "spino", it originates in the spinal cord and is heading TO CNS (ascending tract)

Ends "spinal" it originates in brain and is heading TO the spinal cord (descending tract)
Term
Posterior column pathway
Definition
Carries sensations of highly localized touch, pressure, vibration, proprioception
-Begin peripheral receptor, end primary sensory cortex cerebral hemispheres
-Spinal tracts involved:
left and right fasciculus gracilis
left and right fasciculus cuneatus
Term
Posterior column pathway-first order neurons
Definition
1st-order axons reach CNS in dorsal roots of spinal nerves and sensory roots of cranial nerves

Axons ascending within the posterior column (2nd-order interneurons
-inferior body half: ascend within fasciculus gracilis, synapse in gracilis in medulla oblongata
-Superior body half: ascend within the fasciculus cuneatus, synapse in the nucleus cuneatus
Term
Posterior column pathway-2nd order neurons
Definition
decussation-once on opposite sides of brain axons enter tract-medial lemniscus

Medial lemniscus axons synapse on 3rd order neurons in one of ventral nuclei of thalamus

Nuclei sort the arriving information according to:
-nature of the stimuli
-region of the body involved
Term
Posterior column pathway-processing
Definition
Processing in the thalamus determines whether you perceive a given sensation as fine touch, as pressure, or as vibration Ability to determine where the stimulus originated depends on the projection of information from the thalamus to the primary sensory cortex Sensory information from the toes arrives at one end of primary sensory cortex, from 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
Term
Sensory homunculus
Definition
Functional map of the primary sensory cortex

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

Lips, tongue, hand-large areas
back-little area
Term
Spinothalamic pathway
Definition
Provides conscious sensations of poorly localized (crude) touch, pressure, pain, and temperature

Axons of 1st-order sensory neurons enter spinal cord and synapse on 2nd-order neurons within the posterior gray horns

2nd-order neurons cross to opposite side of spinal cord before ascending within anterior or lateral spinothalamic tracts.

-anterior tracts carry crude touch and pressure sensations
-lateral tracts carry pain and temperature sensations
Term
Spinothalamic pathway-2nd order
Definition

2nd-order neurons synapse to 3rd-order neurons in ventral nucleus group of thalamus. After sensations have been sorted and processed, they are relayed to primary sensory cortex.

Perception of stimulus (cold? hot? vibration?) depends on 2nd and 3rd order neurons that are stimulated.

Ability to localize the sensation depends on area of primary sensory cortex that is stimulated

Term
Injury/abnormality along spinothalamic pathway
Definition
Results in inappropriate sensations or inaccurate localization of the source.
Term
Phantom limb pain
Definition
Caused by activity in the sensory neurons or interneurons along the spinothalamic pathway.
Term
Referred pain
Definition
A person can feel pain in an uninjured part of the body when the pain actually originates in another part of the body.
Ex. heart attack: felt in left arm

Ex. appendicitis: felt first in area around the navel, then in the RLQ
Term
Spinocerebellar pathway
Definition
Cerebellum receives proprioceptive information about position of skeletal muscles, tendons, and joints.
-This information does NOT reach our awareness!
Axons of 1st-order neurons synapse on interneurons in dorsal gray horns of spinal cord

Axons of 2nd-order neurons ascend in 2 tracts:
posterior spinocerebellar tracts
anterior spinocerebellar tracts
Term
Posterior spinocerebellar tracts
Definition
2nd-order axons do NOT cross over to opposite side of spinal cord
Axons reach cerebellar cortex via inferior cerebellar peduncle of that side
Term
Anterior spinocerebellar tracts
Definition
Most 2nd-order axons cross over to opposite side of spinal cord, contain significant number of uncrossed axons as well

Sensations reach cerebellar cortex via superior cerebellar peduncle

Many axons that cross over, ascend to cerebellum then cross over again within the cerebellum, synapsing on the same side as original stimulus
Term
Visceral sensory pathways
Definition
Collected by interoceptors monitoring visceral tissues and organs, primarily within thoracic and abdominopelvic cavities
Interoceptors not as numerous as in somatic tissues, include:
nociceptors, thermoreceptors, tactile receptors, baroreceptors, and chemoreceptors

Usually 1st-order neurons travel with autonomic motor fibers in same structures
Term
Visceral sensory pathways-cranial nerves
Definition
Carry visceral sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, associated vessels and glands to SOLITARY NUCLEUS (large nucleus in medulla oblongata)
Major processing and sorting center for visceral sensory information
Extensive connections with various cardiovascular and respiratory centers and reticular formation
1st-order neurons deliver visceral sensory information to interneurons who ascend within spinothalamic pathway.
We remain unaware of these sensations
Term
SNS (Somatic motor system)
Definition
controls contractions of skeletal muscles
Term
ANS (visceral motor system)
Definition
Controls visceral effectors, such as smooth muscle, cardiac muscle, and glands
Term
Somatic motor pathways
Definition
Always involve at least 2 motor neurons:
-upper motor neuron, whose cell body lies in a CNS processing center
-lower motor neuron, whose cell body lies in a nucleus of the brain stem or spinal cord
Term
Upper motor neuron
Definition
Synapses on lower motor neuron which innervates a single motor unit in a skeletal muscle

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

Destruction of or damage to a lower motor neuron eliminates voluntary and reflex control over the innervated motor unit
Term
3 integrated motor pathways
Definition
Corticospinal pathway
Medial pathway
Lateral pathway

Conscious and subconscious motor commands control skeletal muscles by traveling over them

Activity within these motor pathways adjusted by basal nuclei and cerebellum

Output stimulate/inhibit activity of
1. motor nuclei or
2. primary motor cortex
Term
Corticospinal pathway
Definition
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

Contains 3 pairs of descending tracts:
Corticobulbar tracts
Lateral corticospinal tracts
Anterior corticospinal tracts
Term
Corticobulbar tracts
Definition
Provide conscious control over skeletal muscles that move the eye, jaw, face, some muscles of the neck and pharynx

Innervate motor centers of medial and lateral pathways
Term
Corticospinal tracts (lateral and anterior)
Definition
Axons synapse on lower motor neurons in the anterior gray horns of the spinal cord.

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 decusses in anterior white commissure before synapsing on lower motor neurons in anterior gray horns.
Term
Motor homunculus
Definition
Primary motor cortex corresponds point by point with specific regions of the body
Cortical areas have been mapped out in diagrammatic form
Homunculus provides indication of degree of fine motor control available:
hands, face, and tongue, which are capable of varied and complex movements are very large, while the trunk is relatively small.

Proportions similar to sensory homunculus.
Term
Medial pathway
Definition
Help control gross movements of trunk and proximal limb muscles.

Upper motor neurons of medial pathway are located in:
vestibular nuclei
superior and inferior colliculi
reticular formation
Term
Lateral pathway
Definition
Helps control distal limb muscles that perform more precise movements
Term
Medial and lateral pathways
Definition

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

These nuclei and tracts are grouped by their primary functions:

medial pathways-gross movements of proximal limb muscles and trunk

lateral pathway-distal limb muscles that perform more precise movements

Term
Medial pathway-vestibular nuclei
Definition
Receive information over 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
Term
Medial pathway-superior and inferior colliculi
Definition
-located in roof of mesencephalon (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.
-detect reflexive changes in position of the head, neck, and upper limbs in response to bright lights, sudden movements, or loud noises
Term
Medial pathway-reticular formation
Definition
A loosely organized network of neurons that extends throughout the brain stem. Axons of upper motor neurons in reticular formation descend into reticulospinal tracts without crossing to the opposite side. -Effects of reticular formation stimulation determined by region stimulated.
Term
Lateral pathway
Definition
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.
Term
Basal nuclei and cerebellum
Definition
Responsible for coordination and feedback control over muscle contractions, whether contractions are consciously or subconsciously directed.
Term
Basal nuclei
Definition
Provide background patterns of movement involved in voluntary motor activites.
ex. may direct rhythmic cycles of movement as in walking or running

-Do not exert direct control over lower motor nuclei; they adjust the activities of upper motor neurons in the various motor pathways based on input from all portions of the cerebral cortex, as well as from the substantia nigra
The basal nuclei adjust or establish patterns of movement by 2 major pathways:
1.one group synapses on thalamic neurons, whose axons extend to the premotor cortex. This arrangement creates a feedback loop that changes the sensitivity of the pyramidal cells and alters the pattern of instructions carried by the corticospinal tracts.
2. A second group of axons synapses in the reticular formation, altering the excitatory or inhibitory output of reticulospinal tracts.
Term
2 distinct groups of neurons exist in basal nuclei
Definition
1.one that stimulates neurons by releasing acetylcholine
2. one that inhibits neurons by releasing
gamma aminobutyric acid (GABA).

Normal conditions, excitatory interneurons kept inactive, tracts leaving basal nuclei have an inhibitory effect on upper motor neurons.
Term
Parkinson's disease
Definition
the excitatory neurons become more active, leading to problems with the voluntary control of movement.
Term
Cerebellum
Definition
Monitors:
Proprioceptive (position) sensations-from spinocerebellar tracts.
Visual information from the eyes-from superior colliculi
Vestibular (balance) sensations from inner ear as movements are underway-from vestibular nuclei

The output of the cerebellum affects upper motor neuron activity in the corticospinal, medial, and lateral pathways.
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