Chapter 13- The peripheral nervous system

Sensory Receptors

• pick up senses, respond to changes in environment (stimuli)

• involves GPs, stimuli, sensation, and perception

Graded Potential (GP)

a change in charge that doesn't hit threshold

Stimulis

anything that causes a graded potential that is strong enough to hit threshold and activate an action potential

Sensation

the graded potential, threshold, action potential, AP traveling through axon and into CNS, us becoming aware of it; awareness of stimulus; goes to somatosensory cortex in the postcentral gyrus

Perception

interpretation of the meaning of the stimulus; happens in the somatosensory association area

Classification of Sensory Receptors

stimulus they detect, location in body, structural complexity

Classification by Stimulus type

mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors nociceptors

Mechanoreceptors

mechanical force/physically moved; respond to mechanical forces such as touch, pressure, vibration, and stretch

Thermoreceptors

respond to temperature changes

Photoreceptors

respond to light

Chemoreceptors

respond to chemicals in solution (taste buds)

Nociceptors

respond to potentially damaging stimuli that result in pain

Classification by Location

exteroceptors, interoceptors, proprioceptors

Exteroceptors

• Stimuli outside of the body

• Touch, pain, pressure, temperature, and special senses

Interoceptors (visceroceptors)

• Stimuli within the body; chemical changes, tissue stretch, temp

• Stomach hurts, cramps

Proprioceptors

• Also respond to internal stimuli, their location is just more restricted

• Where body is in space

• Located in skeletal muscles

• Advice brain of body movements

• If body is stretched too much, overstretched, balance

• Aka stretch receptors

Classification by Receptor Structure

1. General Senses

2. Special Senses

General Senses

• Just dendritic endings with maybe some modifications

• Free nerve endings

Special Senses

• Housed in sense organs

• Sometimes purely neurons with a special structure

• Some are nonneuronal

Simple Receptors of General Senses

• tactile, temperature, proprioceptors, pain

• respond to different stimuli

• either encapsulated or nonencapsulated

Nonencapsulated Nerve Endings

• everywhere, mostly in epithelial and CT

• unmyelinated and has small diameter

• small knoblike swellings at sensory terminals increase SA

• pick up painful stimuli, temp, pressure caused by tissue movement

• cold nerve endings more superficial than heat

• many have vanilloid receptors

Reason for Small Diameter

prevent ions from running away and changing the charge and myelinations cannot prevent this from happening because they aren't there

Vanilloid Receptors

ion channel in the PM that is opened by heat, low pH, and various chemicals; this is why they can pick up more than one stimulus, they have more than one receptor

Encapsulated Nerve Endings

• terminals of sensory neurons enclosed in capsule of CT and improves SA

• all are mechanoreceptors b/c respond to forces, vary in function

• tactile, lamellar, bulbous corpuscles, muscle spindles, tendon organs, joint kinesthetic receptors

Tactile/Meissner's Corpuscles

• Contain egg shaped CT capsule around it

• Found at the top of the dermal papillae

• Discriminate touch

Lamellar Corpuscles

• Found deeper in the dermis and subcutaneous tissue underlying the skin

• stimulated by deep pressure

• Respond only when pressure is first applied

• Largest corpuscular receptors

Bublous Corpuscles

• Found in dermis, subcutaneous tissue, and joint capsules

• Respond to deeper pressures and deeper sensations, deep and continuous pressure

Muscle Spindles

• Fusiform (spindle-shaped) proprioceptors

• Found throughout the perimysium

• Detects if muscles are stretched too far, if this occurs, it initiates and reflex relaxation response

Tendon Organs

• Proprioceptors found right where skeletal muscle ends and tendon begins

• If the tendon is stretched too much, a reflex relaxation response occurs to relax the skeletal muscle and the tendon

Joint Kinesthetic Receptors

• Pick up what's going on in the joint

• Four receptor types: lamellar corpuscles, bulbous corpuscles, free nerve endings, and receptors resembling tendon organs

• Provide information on joint position and motion, a sensation of which we were highly conscious

Sensation

awareness of changes in the internal and external environments (picked up in primary somatosensory cortex)

Perception

conscious interpretation of those stimuli (happens in the somatosensory association area)

The Somatosensory System

• Part of the sensory system serving the body wall and limbs

• Receives inputs from exteroceptors, proprioceptors, an interoceptors

Processing at the Receptor Level

• Generating a signal

• Adaption

Generating a signal

• stimulus energy must match the specificity of the receptor

• Stimulus must be applied within the receptive field of the receptor

• Stimulus energy must be converted to the energy of a GP

• GP must hit threshold so that voltage-gated sodium channels on axon are opened and nerve impulses are generated and propagated to the CNS

Adaption

• receptors can adapt to constant stimuli

• info about stimulus (strength, duration, pattern) is encoded in fewquency of impulses: greater frequency, stronger stimulus

• Phasic or Tonic receptors

Phasic Receptors

fast adapting, often giving bursts of impulses at the beginning and the end of the stimulus

Tonic Receptors

provide a sustained response with little or no adaptation

The Circuit Level

• second level of integration

• give impulse to correct region of cerebral cortex to localize and percieve stimulus

• Different ascending pathways carry various types of info to different areas in the brain

Processing at the Perceptual Level

• sensory input is interpreted in cerebral cortex

• ability to ID and understand sensations depends on location of target neurons in sensory cortex

• remember the homunculus

Major Features of Sensory Perception

• Perceptual detection - has stimulus occurred?

• Magnitude estimation - how intense is the stimulus?

• Spatial discrimination - ID the site or pattern of stimulation (two-point discrimination test)

• Feature abstraction - tuning the neuron or circuit to one feature or property of a stimulus in preference to others

• Quality discrimination - what's being felt?

• Pattern recognition - recognizing familiar patterns, unfamiliar ones, or significant ones

Perception of Pain

• receptors are activated by extremes of pressures, temps, and some chemicals released by injured tissue

• Pain Producing Chemicals: histamine, K+, ATP, acids, bradykinin

• sensation of pain may occur but the processing and perception of pain may occur later (fight or flight)

Visceral Pain

• pain of internal organs

• Important stimuli for visceral pain are extreme stretching of tissue (such as what happens during inflammation), irritating chemicals, and muscle spasms

Referred Pain

Visceral neurons (afferent) travel along the same pathways as somatic pain fibers, go to similar locations in the brain and sometimes we can't segregate those out

Nerve Structure

consists of parallel bundles of peripheral axons (myelinated/nonmyelinated) enclosed by CT wrappings, also contains blood vessels

Endoneurium

• Surrounds each axon

• A delicate layer of loose CT that also encloses the fiber's associated Schwann cells

Perineurium

• coarser CT wrapping

• binds groups of axons into bundles called FASCICLES

Epineurium

A tough fibrous sheath, the encloses all the fascicles to form the nerve

Mixed Nerves

most nerves, both sensory and motor fibers, transmit impulses both to and from the CNS

Sensory (afferent) Nerves

carry impulses only toward CNS (found in the dorsal root)

Motor (efferent) nerves

carry impulses only away from the CNS (found in the ventral roots)

Damage to Axons

• if cell body or part close to cell body is cut, neuron and neurons connected to the cut neuron's axon may die

• if the damage is far enough away from body, PNS axons can regen but CNS axons can't

CNS Axon Regeneration

• can't do it

• Oligodendrocytes release growth-inhibiting proteins to prevent regeneration

• astrocytes at injury site form scar tissue and block axon regrowth

PNS Axon Regeneration

• can do this

• myelinations help regen

• will likely regen if damage isn't near cell body

Steps to PNS Axon Regeneration

1. distal end of cut dies b/c no access to cell body

2. Clean up, clean up by macrophages

3. Schwann cells divide

4. Axons sprout filaments (PM rebuilt by Schwann sheath - regeneration tube)

5. Sprouts grow and reconnect ends

6. Myelination occurs, won't work well without new wrapping and new nodes

Cranial Nerves

• 12 pairs

• first 2 attach to forebrain

• rest are associated with brainstem

• cranial nerves only serve head and neck structures (except vagus)

• OOOTTAFVGVAH

Spinal Nerves

• 31 pairs

• each have thousands of nerve fibers

• come from spinal cord

• supply all the body except head and some neck

• all are mixed b/c ventral + dorsal roots mix axons, both afferent and efferent

• get longer as they get more inferior (they are short 1-2 cm)

• branch into different parts

• have rami communicantes

Branching of Spinal Nerves

small dorsal ramus, large ventral ramus, tiny meningeal branch

Rami Communicantes

• contain autonomic nerve fibers

• attach to base of ventral rami of T spine nerves

Cervical Plexus

• branches are called rami

• crossing and intertwining of nerves create plexus

• contains Phrenic nerve, goes to diaphram and helps breathe

Brachial Plexus

• goes to arm

• major cords branch and intertwine to form plexus

Lumbosacral Plexus

goes to leg

Effectors of Autonomic Motor Endings

smooth muscle, cardiac muscle, glands

Innervation of Visceral Muscles and Glands

• either relax or contract muscle

• neurotransmitters (ACh, NE) cause secretion in glands

• Varicosities are present on autonomic nerve fibers

Varicosities

bulbous swellings on autonomic nerve fibers that store neurotransmitters

3 levels of motor control

Segmental, Projection, Precommand

Segmental Level

• lowest level

• reflexes and spinal cord circuits, things we do without thinking

• repeated motor activities (ex: walking, chewing)

Projection Level

• spinal cord control but also higher brain (motor) areas

• uses more thought

Precommand Level

• adds the cerebellum and basal nuclei in addition to cortex

• complex motor movements, maintain coordination and balance

Inborn Reflexes

• crying, breathing

• you are born with knowing how to do these

5 Components of Reflex Arcs

1. Step 1 Receptor: site of stimulus action

2. Step 2 Sensory Neuron: transmit afferent impulses to CNS

3. Step 3 Integration Center: simple = integration center is single synapse between sensory and motor neurons; complex = integration center is multple neurons, interneurons

4. Step 4 Motor Neuron: Conduct efferent impulses from center to effector organ

5. Step 5 Effector: Muscle fiber or gland cell responds to efferent impulses

Spinal Reflexes

• mediated by spinal cord

• stretch or tendon reflexes

• afferent signal goes to spinal cord to let nervous system know the length of muscle and tension in muscle and associated tendons

Intrafusal Muscle FIbers

modified skeltal muscle which have sensory stretch receptors

Patellar/Knee-Jerk Reflex

• hit tendon and causes it to stretch

• contract quads and cause extension of lower leg

• stimulus goes to spinal cord, through interneuron, into motor response

Flexor + Crossed Extensor Reflexes

• caused by pain, body part draws away

• Polysynaptic

• stimulus goes to neurons on other side of body to keep balance

Superficial Reflexes

• caused by gentle cutaneous stimulation

• abdominal reflex causes abs to contact and body to curl

• plantar reflex causes toes to curl