Chap 13

Classification of sensory receptors: stimulus type, location, structural complexity


Sensory receptors: specialized to respond to changes in environment (stimuli)


Classification by stimulus type: mechanoreceptors, thermoreceptors, photoreceptors, chemorecptors, nociceptors


  • Mechanoreceptors: respond to mechanical force such as touch, pressure (including BP), vibration and stretch


  • Thermoreceptors: respond to temperature changes


  • Photoreceptors: respond to light (such as those in retina)


  • Chemoreceptors: respond to chemicals in solution (molecules smelled or tasted, or changes in blood or interstital fluid chemistry)


  • Nociceptors: respond to potentially damaging stimuli that result in pain (ex: searing heat, extreme cold, excessive pressure, and inflammatory chemicals are all interpreted as painful)


Classification by location: exteroceptors, interoceptors, proprioceptors


  • Exteroceptors: sensitive to stimuli arising outside the body (touch, pressure, taste, pain, near or at the body surface)


  • Interoceptors: respond to stimuli w/in body (monitor a variety of stimuli: chemical changes, tissue stretch, etc)(activity sometime cause pain, hunger, thirst, etc)


  • Propriceptors: respond to internal stimuli (occurs in skeletal muscles, tendons, joints, etc)


Classification by structural complexity: receptors for special senses (vision, hearing, equilibrium, smell, taste)


Tactile sensations: (a mix of touch, pressure, stretch, and vibration), temperature monitoring and pain as well as the “muscle sense” provided by proprioceptors


  • Two types: nonencapsulated = free nerve endings, encapsulated = nerve endings


Nonencapsulated nerve endings: present nearly everywhere in body, abdunat in epithelia and connective tissue, mostly nonmyelinated and small diameter

  • Thermoreceptors: cold receptors (10-40C); in superficial dermis / heat receptors (32-48C); in deeper dermis

  • Nociceptors: temperatures outside the range of thermoreceptors(perceived as pain), pinching, chemicals from damaged tissue, itch


Nonencapsulated nerve endings: light pressure receptors (tactile (merkel discs) and hair follicle receptors (wrap around hair follicles)


Encapsulated nerve endings: meissner’s (tactile) corpuscles, pacinian (lamellar) corpuscles, bulbous (ruffini endings) corpuscles, muscle spindles, tendon organs, joint kinesthetic receptors


Survival depends on: sensation and perception


Somatosensory system: receives input from exteroreceptors, proprioceptors, and interoceptors (transmit info)


3 levels of neural integration: receptor level = sensory receptors, circuit level = processing in ascending pathways, perceptual level = processing in cortical sensory areas


Perception of pain: warns of actual or impending tissue damage so protective action can be taken


Pain threshold: perception of pain at roughly the same stimulus intensity


Pain tolerance: varies widely


Visceral pain: results from noxious stimulation of receptors in organs of thorax and abdominal cavity


Referred pain: pain stimuli arising in one part of the body are perceived as coming from another part


Nerves are: cordlike bundles(myelinated and nonmyelinated) of axons that conduct sensory and moter impulses


Two types of nerves: spinal and cranial


Most nerves are mixtures of: afferent and efferent fibers and somatic and autonomic (visceral) fibers


Nerves are classified by: according to the direction they transmit impulses


Mixed nerves: contain both sensory and motor fibers and transmit impulses both to and from CNS


Sensory (afferent) nerves: carry impulses only toward CNS

Motor (efferent) nerves: carry impulses only away from CNS


CNS axons: most CNS fibers never regenerate after injury


PNS axons: PNS axons can regenerate if damage is not severe


cranial nerves: 12 pairs of nerves associated with the brain, most are mixed in function; two pairs are purely sensory, each nerve is identified by a number and a name (I through XII)


Spinal nerves: 8 cervical (C1-C8), 12 Thoracic (T1-T12), 5 Lumbar (L1-L5), % Sacral (S1-S5), 1 Coccygeal (C0)


Spinal nerves divide into three branches: dorsal ramus, ventral ramus, and meningeal branch


Rami communicates: contain autonomic (viscera) nerve fibers; attach to base of ventral rami of thoracic spinal nerves  


Spinal nerve rami and their main branches supply entire: somatic region of body from neck down


  • Dorsal rami supply: posterior body trunk

  • Ventral rami supply: rest of trunk and limbs


Difference between roots and rami: roots lie medial to and form spinal nerves, each root is purely sensory or motor. Rami lie distal to and are lateral branches of spinal nerves, can carry both sensory and motor.


All ventral rami except T1-T12 form interlacing nerve networks called nerve plexuses (ex. Cervical, brachialm, lumbar, sacral


Cervical plexus: formed from ventral rami of 1st 4 cervical nerves, most branches are cutaneous nerves that supply only skin (transmit sensory impulses from skin of neck, ear arena back of head, and shoulder), phrenic nerve (supplies both motor and sensory fibers to diaphragm, chief muscle causing breathing movements) 


Brachial plexus and upper limb: situated in neck and axilla, gives rise to virtually all nerves that innervate upper limb, roots to trunks to divisions to cords, important nerves are axillary/musculocutaneous/median/ulnar/radial.


Lumbar plexus: arises from spinal nerves L1-L4, lies ithin psoas major muscle, important nerves are femoral and obturator


Sacral plexus: arises from spinal nerves L4-S4, important nerves are sciatic (tibial and common fibular)

Ventral rami of T2-T12 are intercostal nerves  that supply muscles of ribes, anterolateral. Thorax, and abdominal wall. 


Two special thoracic nerves: Tiny t1(most fibers enter brachial plexus) and T12( lies inferior to 12th rib; making it a subcostal nerve


Hilton’s law: any nerve serving a muscle that produces movement at a joint also innervates the joint and skin over the joint


Peripheral motor nerve endings connect nerves to their effectors


Innveration of skeletal muscles: neuromuscular junction


Innervation of visceral muscle and glands: varucisities


The basic mechanisms of motor systems operate at three levels: segmental level, projection level, and precommand level


Segmental level: reflexes and automatic movements

Projection level: executing voluntary movements 

Precommand level: consist of controls the output of brain


Inborn (intrinsic reflex): a rapid, predictable motor response to a stimulus. Unlearned, unpremeditated and involuntary.


Learned (acquired) reflexes: results from practice or repetition 


Reflexes are classified functionally as: somatic reflexes: activate skeletal muscles and autonomic (visceral) reflexes


Somatic reflexes: activate skeletal muscles


autonomic reflexes: activate visceral effectors (smooth or cardiac muscle or glands)


To smoothly coordinate skeletal muscle, the nervous system must receive proprioceptor input regarding: length of muscle and amount of tension in muscle and its associated tendon.


Length of muscle: information sent from muscle spindles


Amount of tension in muscle and its associated tendon: information sent from tendon organs


How a stretch reflex works: Stretch activates the muscle spindle, Sensory neurons synapse directly with α motor neurons in the spinal cord, α motor neurons cause the stretched muscle to contract

Reciprocal inhibition: Branches of afferent fibers also synapse w/ interneurons that inhibit motor neurons controlling antagonistic muscles. Consequently, the stretch stimulus causes the antagonists to relax so that they cannot resist the shortening of the stretched muscle caused by the main reflex arc


All stretch reflexes are monosynaptic and ipsilateral (motor activity is on same side of body)


Positive reflex reactions provide two pieces of info: Proves that the sensory and motor connections b/w that muscle and the spinal cord are intact. The vigor of the response indicates the degree of excitability of the spinal cord


Tendon reflexes: Polysynaptic reflexes, Help to prevent damage due to excessive stretch, Important for smooth onset and termination of muscle contraction, Produces muscle relaxation (lengthening) in response to tension 


Reciprocal activation: motor neurons in spinal cord circuits supplying the contracting muscle are inhibited and antagonist muscles are activated ( As a result, contracting muscle relaxes as its antagonist is activated)


Flexor (withdrawal) reflex: initiated by a painful stimulus


Crossed extensor reflex: maintain balance


Superficial reflexes: elicited by gentle cutaneous stimulation. Depend on upper motor pathways and cord-level reflex arcs (plantar reflex and abdominal reflexes)