Stimuli and Somatic Reflexes

Reflex

• quick (involves few neurons), involuntary, stereotyped (same way ea. time) reaction from glands, organs, or skeletal muscles that respond to
  stimuli

• all ___ require stimulation to respond

Somatic Reflexes

• involves muscles of body (skeletal)

  • cranial reflexes pass through brain

  • spinal reflexes pass through the spinal cord

Reflex Arcs

Have the following structures (sometimes may have 3):

• Receptor (in skin, muscle, tendon, bone)

• Afferent (sensory) nerve

• integrating center (brain or spinal cord and one or more interneurons)

• Efferent (motor) nerves

• Effector

  • f the effector is skeletal muscle, then it is a somatic reflex

  • If the effector is smooth or cardiac muscle or a gland, then it is a visceral reflex

Monosynaptic Reflexes

Sensory neurons bypass interneurons & quickly
exit SC and synapse directly with motor fibers in a reflex arc (1 synapse)

Polysynaptic Reflexes

Sensory neurons synapse with at least one
interneuron before synapsing with motor fibers (2+ synapses)

Muscle spindles

• stretch receptors that maintain tension in skeletal muscles

• type of proprioceptor → sense organs that monitor position of body and movement of limbs -→ sends info to cerebellum

• when muscle is stretched, primary afferent fibers from the muscle spindles send a strong signal to the spinal cord causing contractions in muscle that was stretched

• abundant in muscle that requires control

Reciprocal inhibition

• prevents muscles from working against each other (prevents contraction of antagonist)

  • This requires multiple interneurons to coordinate this movement with motor signals being sent

  • Some must communicate with the opposite side of the body
    to do this

  • These are polysynaptic pathways that involve at least one interneuron (two or more synapses)

Stretch (Myotatic) Reflex

• When a muscle or tendon is stretched, it fights back.

  • Muscles responds by contracting to maintain
    increased tonus/ tone (a stretch reflex)

  • Tendons do not contract but prevent excess
    contraction of muscle it is attached to

• Both help to protect the muscle from potential damage
  • Helps maintain equilibrium & posture

  • head starts to tip forward →muscles contract in back of neck to raise the head upright

• Stabilize joints by balancing tension in extensors & flexorsto smooth the skeletal muscle actions

Tendon Reflex

very sudden muscle stretch causes a ____

Knee-jerk (patellar) reflex

• monosynaptic reflex (no interneuron in pathway)

Phasic Receptor

• burst of activity and neurons quickly adapt (smell and hair receptors)

Tonic Receptor

• neurons adapt slowly, generate impulses continually (proprioceptor or pain signals)

Sensory Adaptation

• all receptors demonstrate this property with enough elapsed time, with prolonged stimulation, all receptors will fire more slowly over time

• decline in firing frequency and conscious sensation (brain ignores sensation)

Free Nerve Endings

• detects warm, cold, and pain of epithelium and CT

• bare dendrites

Tactile (Merkel) discs

• tonic receptors associated with cells at base of epidermis

• for fine touch and superficial pressure

• bare dendrites

Hair (peritrichial) receptors

• phasic dendrites wrapped around base of pillus that monitor the movement also detect movement on skin surface

• bare dendrites

Tactile (Meissner) corpuscles

• phasic receptors in dermal papillae for light touch and texture

• dendrites wrapped by glial cells or CT

Lamellar (Pacinian) corpuscles

• phasic receptors for deep pressure, stretch, tickle, and vibrations

chemoreceptors

• detect chemicals; taste, smell, and chemical changes in body fluids like pH

Thermoreceptors

• heat and cold; relative and stable

Nociceptors

• pain receptors; respond to tissue damage, ischemia, or extremes in temperatures

Mechanoreceptors

• detect pressure changes; there are several subtypes

  • light and deep touch in skin

  • baroreceptors

  • vibration for hearing

  • proprioceptors

  • stretch receptors in viscera, GI Tube, blood vessels, and joints

baroreceptors

• detect pressure changes in vessels

Proprioreceptors

• aid in balance and equilibrium

Photoreceptors

• detect photons of light

Punctate Distribution

• refers to the unequal distribution of receptor densities in the skin (vary number from one area to another)

  • touch receptors on the back are further apart than on the fingers

Temperature receptors

• vary between cool and warm

• in the forearm, 28 cool receptors for each warm receptor

• warm receptors are closer to the surface but cool receptors axons are myelinated (warm are not)

Referred pain

• misinterpreted visceral pain (brain thinks its somatic)

• brain assumes pain is coming from skin or superficial sites even though it is coming from visceral organs

• sensory information from skin or muscle follow same ascending pathway in SC as information coming from organs so organ pain may feel like its coming from skin