peripheral nervous system - mount saint mary...

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Peripheral Nervous System

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Sensory Receptors and Sensation

• Sensory Receptors

– Respond to changes (stimuli) in the environment– Generate graded potentials that can trigger an action potential that is

carried to the CNS

• Sensation – awareness of the stimulus• Perception – interpretation of the stimulus

• Classification by Stimulus Type

– Mechanoreceptors– Thermoreceptors– Photoreceptors– Chemoreceptors– nociceptors

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• Classification by Location

– Exteroceptors– Interoceptors– Proprioceptors

• Classification by Structural Complexity

– Simple Receptors

• Modified dendritic endings• Found throughout the body• Monitor general sensation

– Complex Receptors

• Sense organs (special senses)

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• Simple Receptors of the General Senses

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• Sensation to Perception – General Organization of the Somatosensory System – three major levels

– Receptor Level Processing – sensory neurons

• For a sensation to occur a stimulus must excite a receptor

– Receptor specificity– Stimulus must be applied to the receptor’s receptive field (the smaller

the field the better the brain can localize the stimulus)– Energy of the stimulus must be converted to a graded potential

(receptor potential); called transduction– Generator potential of the first order neuron must reach threshold to be

transmitted– Strength of stimulus is encoded in frequency (the faster the stronger)– Terminology

» Tonic receptors – usually firing at a particular rate constantly; stimulus changes the rate

» Phasic receptors – usually off; stimulus turns them on» Adaptation – reduction in sensitivity to a stimulus

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– Circuit Level Processing – ascending (sensory) pathways (tracts)

• Job is to deliver impulses the correct area of the cerebral cortex• Impulses first goes from the a nerve to the spinal cord, ultimately to

either the cerebellum or thalamus (second order neuron)• If the information is going to the cerebral cortex a third order neuron

carries it from the thalamus to the appropriate area of the cerebral cortex

– Perceptual Level Processing – cerebral cortex, but also along the way – perception is defined by the region of the brain interpreting it not necessarily the stimulus – the brain interprets and projects where in the body the stimulus is coming from

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– Main Aspects of Perception

• Perceptual Detection• Magnitude Estimation• Spatial Discrimination – two-point discrimination test• Feature Abstraction – interpreting several features of a perception

to determine the actual “make-up” of the stimulus• Quality Discrimination – some modalities have different qualities, ex.

taste; bitter, sour, sweet, salty• Pattern Recognition – misspelled sheet I handed out

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Transmission Lines: Nerves and Their Structure and Repair

• Nerves and Associated Ganglia

– Structure

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– Classification

• Sensory (afferent) nerves• Motor (efferent) nerves• Mixed nerves

– Ganglia

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– Regeneration of Nerve Fibers

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Cranial Nerves

• Naming

– Use of Roman numerals I – XII

– Named according to structures served or function(s)

On occasion our trust truck acts funny – very good vehicle anyhow

Some say marry money but my brothers say bad business marry money

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Spinal Nerves

• General Features

– 31 pairs– Naming

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Spinal Nerves

– Anatomy

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Spinal Nerves

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Spinal Nerves

• Innervation of Specific Body Regions

– Dorsal Rami – supply skin and muscles of posterior trunk– Ventral Rami – anterior trunk and limbs

• All but T1 – T12 form plexuses

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Cervical Plexus

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Brachial Plexus

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Dermatomes

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Motor Endings and Motor Activity

• Peripheral Motor Endings

– Innervation of Skeletal Muscle - neuromuscular junctions

– Innvervation of Visceral (Smooth) Muscle and Glands

• Possess varicosities that typically contain either acetylcholine or norepinephrine

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Reflexes

• Reflex Arc

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Spinal Reflexes

• Stretch and Golgi Tendon Reflexes

– Functional anatomy

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Spinal Reflexes

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Spinal Reflexes

• Stretch Reflex

– Demonstrates reciprocal inhibition– Example is the knee-jerk or patellar reflex

• Monosynaptic• Ipsilateral• Gives us information concerning

– The status of both the sensory and motor pathway– The status of the spinal cord

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Spinal Reflexes

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Spinal Reflexes

• Golgi Tendon Reflex

– Polysynaptic– Results in reciprocal

activation

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Spinal Reflexes

• Flexor (Withdrawl) Reflex/Crossed Extensor Reflex

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Spinal Reflexes

• Superficial Reflexes

– Plantar Reflex – blunt object stroked along lateral side of the plantar surface

• Test spinal cord segment (L4 – S2)• Integrity of corticospinal tracts• Normal response downward deflection of toes• Abnormal – Babinski’s sign – toes dorsiflex, smaller toes flare laterally – this

is normal in children

– Abdominal Reflexes - stroke skin on lateral side of abdomen to the side, above or below the umbilicus

• Normal – umbilicus should move towards stroked side• Test cord segments (T8 – T12)• Integrity of corticospinal tracts

peripheral nervous system - mount saint mary...

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