Nerves, Sensation, and Sensory Receptors

• A nerve contains bundles of neuron axons.
• A nerve is enclosed by connective tissue: endoneurium, perineurium, and epineurium.
• Most nerves are mixed nerves, containing both sensory and motor components.
• Sensation involves the activation of receptors.
• Perception involves interpretation, analysis, and the understanding of sensory information, a process that occurs in the brain.
• Special senses include sight, sound, taste, and smell.
• Somatic sensory receptors respond to stimuli from skin, muscles, and joints.
• Visceral receptors respond to stimuli from internal organs.

Receptor Types and Functions

• Mechanoreceptors respond to mechanical force.

• Thermoreceptors respond to temperature changes.

• Photoreceptors respond to light changes.

• Chemoreceptors respond to changes in blood chemistry.

• Nociceptors respond to potentially damaging stimuli (pain).

• Exteroceptors respond to stimuli arising outside the body.

• Interoceptors respond to stimuli within the body.

• Proprioceptors respond to stretch and position.

• Non-encapsulated receptors include general free nerve endings, tactile (Merkel) discs, and hair follicle receptors.

• General free nerve endings are located primarily in the skin and respond to temperature and painful stimuli; some are thermoreceptors, and some are nociceptors.

• Specific receptors that respond to touch or light touch include tactile (Merkel) discs, hair follicle receptors, and tactile (Meissner's) corpuscles.

• Specific receptors that respond to deep pressure and vibration include lamellar corpuscles (Pacinian corpuscles) and bulbous corpuscles (Ruffini endings).

• Specific receptors that respond to stretch include muscle spindles, found in skeletal muscle, and tendon organs, found in tendons.

• Adaptation is when sensory nerves do not experience adaptation; there is change in sensitivity in the presence of a constant stimulus.

• The brain focuses less on a constant stimulus, which allows it to focus on other stimuli and does not occur with pain sensations.

• The biological step of feeling pain includes pain receptor activation and pain signals being sent to the brain.

• The neurological step of feeling pain is the brain interpreting the pain signals.

• Pain threshold is the point at which pain begins to be felt and occurs when nociceptors are activated.

• Pain tolerance is the maximum level of pain a person is able to bear and the point at which they try to stop the pain; it varies from person to person and is associated with genetics.

• Nociceptive pain is caused by activation of nociceptors.

• Neuropathic pain is caused by damage or disease to the sensory nervous system and is described as burning, tingling, stabbing, or "pins and needles."

• Superficial somatic pain is sharp and well-defined because nociceptors in the skin are activated.

• Deep somatic pain is dull, aching, and poorly localized because nociceptors in ligaments, tendons, and bones are activated.

• Visceral pain is dull, aching, and poorly localized because nociceptors are located in internal organs are activated.

• Visceral pain is caused by stretch, ischemia, and inflammation.

• Referred pain is pain perceived at a location other than the site of the painful stimulus.

• Visceral pain fibers travel along the same pathways as somatic pain fibers.

• The brain first thinks the pain is coming from the somatic source (skin).

• As more pain sensations come from the organ, the brain later realizes where the pain is actually coming from.

• Structures involved in motor control are the cerebellum and basal nuclei, premotor cortex, primary motor cortex, and spinal cord.

• An inborn (intrinsic) reflex is a rapid, predictable motor response to a stimulus that helps maintain posture, avoid pain, and control visceral activities.

• Spinal reflexes do not involve the brain (the brain will be made aware after the reflex has occurred).

• Spinal reflexes allow the muscle to contract or relax very quickly and help avoid danger.

• A stretch reflex causes muscle CONTRACTION in response to increased muscle LENGTH and is initiated by MUSCLE SPINDLE receptors.

• As the initial muscle relaxes, the muscle is STRETCHED.

• The stretch activates SENSORY information to the spinal cord.

• This causes motor neurons to fire and cause the initial muscle to CONTRACT; this action resists or reverses the stretch.