Chapter 13 Flashcards
The PNS consists of all neural structures outside…
Brain and spinal cord (CNS)
PNS is divided into two categories…
Sensory (afferent) division and Motor (efferent) division
Motor (efferent) division is divided into two categories…
Somatic nervous system and autonomic nervous system (ANS)
Autonomic nervous system (ANS) is divided into two categories…
Sympathetic and parasympathetic division
Sensory receptors:
- specialized to respond to changes in environment (stimuli)
- activation results in graded potentials that trigger nerve impulses
- awareness of stimulus (sensation) and interpretation of meaning of stimulus (perception) occur in brain
Three ways to classify receptors:
- types of stimulus
- body location
- structural complexity
Mechanoreceptors
respond to touch, pressure, vibration, and stretch
Thermoreceptors
sensitive to changes in temperature
Photoreceptors
respond to light energy (ex: retina)
Chemoreceptors
respond to chemicals (ex: smell, taste, changes in blood chemistry)
Nociceptors
sensitive to pain-causing stimuli (ex: extreme heat/cold, pressure, inflammatory chemicals)
Exteroceptors
- respond to stimuli arising outside body
- receptors in skin for touch, pressure, pain and temperature
Interoceptors (viscerorecptors)
- respond to stimuli arising in internal viscera and blood vessels
- sensitive to chemical changes, tissue stretch, and temperature changes
Proprioceptors
- respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles
- inform brain of ones movements
(these are how we know where our body is in space)
Majority of sensory receptors belong to one of the two categories:
Simple receptors of general senses
- modified dendritic endings of sensory neurons
- are found throughout body and monitor most types of general sensory information
Receptors for special senses
- vision, hearing, equilibrium, smell, and taste
- all are housed in complex sense organs
Survival depends upon…
Sensation: the awareness of changes in the internal and external environment
Perception: the conscious interpretation of those stimuli
Somatosensory system:
part of sensory system serving body wall and limbs
Somatosensory system receives inputs from:
- exteroceptors
- proprioceptors
- interoceptors
input is relayed toward head, but processed along the way
Three levels of neural integration in sensory systems
- ) receptor level: sensory reception and transmission to CNS (muscle spindles and kinesthetic receptors)
- ) circuit level: processing in ascending pathways (on the way ‘up’ the spinal cord and through the brainstem, midbrain, and cerebellum)
- ) perceptual level: processing in cortical sensory centers (particularly the somatosensory)
Processing at the receptor level: Generating a Signal
for sensation to occur, the stimulus must excite a receptor, and the action potential must reach CNS
- stimulus energy must match receptor specificity
- stimulus must be applied within Receptive Field
- Transduction must occur, energy of stimulus is converted into graded potential called Generator Potential or Receptor Potential
- graded potentials must reach threshold… Action Potential
- strength of stimulus is encoded by Action Potential frequency
Three types of Neurons:
- multipolar
- bipolar
- unipolar
Multipolar
many processes extend from the cell body. all are dendrites except for a single axon.
Bipolar
two processes extend from the cell body. one is a fused dendrite, the other is an axon.
Unipolar
one process extends from the cell body and forms central and peripheral processes, which together comprise an axon.
Multipolar, bipolar, and unipolar have different “trigger zones”
These zones are essentially the regions of the neuron where generator/receptor potentials summate to determine if an action potential will be fired by that neuron.
Adaptation:
change in sensitivity in presence of constant stimulus
- receptor membranes become less responsive
receptor potentials decline in frequency or stop
Phasic receptors: (fast-adapting) send signals at beginning or end of stimulus, report changes in environment (ex: receptors for pressure, touch, smell)
Tonic receptors: adapt slowly or not at all (ex: nociceptors and most prorioceptors)
How many neurons does it take to “sense” something?
Most sensory pathways involve 3 neurons
Processing at the Circuit Level (ascending pathways)
- first-order sensory neurons
- second-order sensory neurons
- third-order sensory neurons
First-order sensory neurons
conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS (cell body in DRG, synapse in spinal cord)
Second-order sensory neurons
transmit impulses to third-order sensory neurons (spinal cord to thalamus)
Third-order sensory neurons
conduct impulses from thalamus to the somatosensory cortex (perceptual level)
Processing at the Perceptual Level
interpretation of sensory input depends on specific location of target neurons in sensory cortex
Aspects of sensory perception:
perceptual detection: ability to detect a stimulus (requires summation of impulses from multiple receptors)
magnitude estimation: intensity coded in frequency of impulses
spatial discrimination: identifying site or pattern of stimulus (studied by two-point discrimination test)
Perception of Pain
- warns of actual or impending tissue damage so protective action can be taken
- stimuli include extreme pressure and temperature, or chemicals (histamine, K+, ATP, acids, bradykinin)
- some pain impulses are blocked by inhibitory endogenous opioids (ex: endorphins and enkephalins)
Perception of Pain: Pain tolerance…
- all begin to perceive pain at the same stimulus intensity
- pain tolerance varies
- “sensitive to pain” means low pain tolerance, not low pain threshold
- genes help determine pain tolerance as well as response to pain medications
Perception of Pain: Visceral and referred pain…
- Visceral pain results from stimulation of visceral organ receptors (felt as aching, gnawing, burning / activated by tissue stretching, ischemia, chemicals, muscle spasms)
- Referred pain: pain from one body region perceived as coming from different region (visceral and somatic pain fibers travel along same nerves, so brain assumes stimulus comes from common somatic region / ex: left arm pain during heart attack)
Nerve:
- cordlike organ of PNS
- bundle of myelinated / unmyelinated peripheral axons enclosed by connective tissue
- TWO TYPES of NERVES: spinal or cranial, depending on where they originate
Endoneurium vs. Perineurium
- Endoneurium wraps around individual axons / forms outer covering of the nerve
- Perineurium wraps around bundles of axons called fascicles
Most nerves are mixtures of …
afferent and efferent fibers and somatic (motor) and autonomic (visceral) fibers
Nerves are classified according to…
the direction they transmit impulses
Mixed nerves:
contain both sensory and motor fibers (impulses travel to and from CNS)
Sensory (afferent) nerves:
impulses only TOWARD CNS
Motor (efferent) nerves:
impulses only AWAY from the CNS
Pure sensory (afferent) or pure motor (efferent) nerves are…
RARE, most nerves are mixed
Types of fibers in mixed nerves:
- Somatic afferent (muscle to brain)
- Somatic efferent (brain to muscle)
- Visceral afferent (organs to brain)
- Visceral efferent (brain to organs)
Ganglia:
contain neuron cell bodies associated with nerves in PNS
- ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons (dorsal root ganglia, sensory/somatic)
- ganglia associated with efferent nerve fibers contain autonomic motor neurons (autonomic ganglia, motor/visceral)
How many pairs of spinal nerves are there?
31 pairs
- 8 pairs of cervical nerves
- 12 pairs of thoracic nerves
- 5 pairs of lumbar nerves
- 5 pairs of sacral nerves
- 1 pair of tiny coccygeal nerves
- all are mixed nerves named for point of issue from spinal cord
- supply all body parts except head and part of neck
Major nerves of Cervical…
Phrenic
Major nerves of Brachial…
Axillary, musculocutaneous, median, radial, ulnar
Major nerves of Lumbar…
Femoral, obturator
Major nerves of Sacral…
Sciatic (composed of tibial and common fibular)
Peripheral Motor Endings (Reflex Activity)
- Inborn (intrinsic) reflex: rapid, involuntary, predictable motor response to stimulus
- ex: maintain posture, control visceral activities
- Learned (acquired) reflexes result from practice or repetition
- ex: driving skills
Five basic components of all reflex area:
- ) receptor
- ) sensory neuron
- ) integration center
- ) motor neuron
- ) effector
1.) receptor
site of stimulus action
2.) sensory neuron
transmits afferent impulses to CNS
3.) integration center
either monosynaptic or polysynaptic region within CNS
4.) motor neuron
conducts efferent impulses from integration center to effector organ
5.) effector
muscle fiber or gland cell that responds to efferent impulses by contracting or secreting
Reflexes are classified functionally as:
- Somatic Reflexes: activate skeletal muscle
- Autonomic (visceral) reflexes: activate visceral effectors (smooth or cardiac muscle or glands)
Ex: of a stretch reflex
The patellar (knee-jerk) reflex
- A stretch reflex helps keep muscles at the appropriate length, in this case the quadriceps to prevent knee buckling upon standing
