what are examples of encapsulated dendritic endings
all mechanoreceptors in connective tissue capsule
tactile corpuscles: discriminative touch
lamellar corpuscles: deep pressure and vibration
bulbous corpuscles: deep continuous pressure
muscle spindles: muscle stretch
tendon organs: stretch in tendons
joint kinesthetic receptors: joint position and motion
survival depends on what
sensation: awareness of changes in the internal and external environment
perception: the conscious interpretation of those stimuli
sensory integrations
somatosensory system: part of the sensory system serving body wall and limbs
receives inputs from exteroceptors, proprioceptors, and interoceptors
input relayed toward head, but processed along way
what are the levels or neural integration in sensory systems
receptors level: sensory receptors
circuit level: processing in ascending pathways
perceptual level: processing in cortical sensory areas
processing at the receptors level
to produce a sensation
receptors have specificity for stimulus energy
stimulus must be applied in receptive field
transduction occurs (stimulus changed to graded potential; generator or receptor potential)
graded potentials must reach threshold for action potential
processing at receptor level for general sense receptors
graded potential called generator potential
stimulus -> generator potential in afferent neuron -> action potential
processing at receptor level in special sense organs
stimulus -> graded potential in receptor cell called receptor potential -> affects amount of neurotransmitter released -> neurotransmitters generate graded potentials in sensory neuron
adaptation of sensory receptors
adaptation is change in sensitivity in presence of constant stimulus
receptor membranes become less responsive
receptor potentials decline in frequency of stop
what are the two types of adaptation of sensory receptors
phasic (fast adapting) receptors: signal beginning or end of stimulus; e.g. receptors for pressure touch, smell
tonic receptors: adapt slowly or not at all; e.g. nociceptors and most proprioceptors
processing at the circuit level
pathways of three neurons conduct sensory impulses forward to appropriate cortical regions;
first order sensory neurons, second order sensory neurons, and third order sensory neurons
first order sensory neurons
conduct impulses from receptors level to spinal reflexes or second order neurons in CNS
second order sensory neurons
transmit impulses to third order sensory neurons
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, magnitude estimation, spatial discrimination
perceptual detection
ability to detect a stimulus (requires summation of impulses)
magnitude estimation
intensity coded in frequency of impulses
spatial discrimination
identifying site or pattern of stimulus (studied by two point discrimination test)
what are the main aspects of sensory perceptions
feature abstraction: identification of more complex aspects and several stimulus properties
quality discrimination: ability to identify submodalities of a sensation (e.g. sweet or sour tastes)
pattern recognition: recognition of familiar or significant patterns in stimuli (e.g moldy in piece of music)
perception of pain
warns of actual or impending tissue damage (protective action)
stimuli include extreme pressure and temp, histamine (inflammation), K+, ATP, acids, and bradykinin
impulses travel on fibers that releases neurotransmitters glutamate and substance P
some pain impulses are blocked by inhibitory endogenous opioids (endorphins)
pain tolerance
all perceive paint at same stimulus intensity
pain tolerance varies
sensitive to pain means low pain tolerance, not low pain threshold
genes help determine pain tolerance, response to pain medications
three types of connective tissue coverings in a nerve
endoneurium: loose connective tissue that encloses axons and their myelin sheaths
perineurium: coarse connective tissue that bundles fibers into fascicles
epineurium: tough fibrous sheath around a nerve
classified nerves according ot direction transmit impulses
mixed nerves: both sensory and motor fibers; impulses both to and from CNS
sensory (Afferent): impulses only toward CNS
motor (efferent) nerves: impulses only away from CNS
what are the four different types of fibers in mixed nerves
somatic afferent
somatic efferent
visceral afferent
visceral efferent
peripheral nerves classified as
cranial or spinal nerves
ganglia associated with afferent nerve fibers
dorsal root ganglia
ganglia associated with efferent nerve fibers
autonomic ganglia
the process if peripheral axon is damaged
axon fragments spreads distally form injury
macrophages clean dead axon; myelin sheath intact
axon filaments grow through regeneration tube
axon regenerates; new myelin sheath forms
which type of CNS cell bears growth inhibiting proteins that prevent CNS fiber regneration
oligodendrocytes
which type of cell blocks axonal regrowth
astrocytes
ventral roots
contain motor (efferent) fibers from ventral horn motor neurons. fibers innervate skeletal muscles
doral roots
contain sensory (afferent) fibers from sensory neurons in dorsal root gnaglia and conduct impulses form peripheral receptors
doral root and central roots unite to form what
spinal nerves
which emerge form vertebral column via intervertebral foramina
dermatome
area of skin innervated by cutaneous branches of single spinal nerve
hilton's law
any nerve serving a muscle that produces movement at joint also innervates joint and skin over joint
review of innervation of skeletal muscle
takes palce at neuromuscular junction
neurotransmitter acetylcholine released when nerve impulse reaches axon terminal
ACh binds to receptors, resulting in: movement of Na+ and K+ across membrane, depolarization of muscle cell, an end plate potential, which triggers an action potential (muscle contraction)
steps when a nerve impulse reaches a neuromuscular junction (6 steps)
action potential arrives at axon terminal of motor neuron
voltage gated Ca2+ channels open. Ca2+ enters axon terminal moveing down its electrochemical gradient
Ca2+ entry causes ACh to be released by exocytosis (a neurotransmitter)
ACh diffuses across the sympatic cleft and binds to its receptors on the sarcolemma
ACh binding opens ion channels in the receptors that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions exit, which produces a local change in the membrane potential called the end plate potential
ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction
which is slower, visceral motor responses or somatic responses
visceral motor
acetylcholin and norepinephrine act indirectly via second messengers during
innervation of visceral muscle and glands
branches form syanpses en passant via varicosities during
innervation of visceral muscle and glands
levels of motor control
segmental level
projection level
precommand level
what are the main parts of motor control
cerebellum and basal nuclei are the ultimate planners and coordinators of complex motor activities
segmental level
lowest level of motor hierarchy
reflexes and automatic movements
central pattern generators: segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles
controls locomotion, and specific, often repeated motor activity
projection level
consists of upper motor neurons that initiate direct pyramidal system to produce voluntary skeletal muscle movements and the brain stem motor areas that oversee indirect extrapyramidal system to control reflex and CPG controlled motor actions
projection motor pathways send info to lower motor neurons and keep higher command levels informed of what is happening
precommand level
neurons in cerebellum and basal nuclei
regulate motor activity
precisely start or stop movements
coordinate movements with posture
block unwanted movements
monitor muscle tone
perform unconscious planning and discharge in advance of willed movements
cerebellum: acts on motor pathways through projection areas of brain stem; acts on motor cortex via thalamus to fine tune motor activity
basal nuclei: inhibit various motor centers under resting conditions
spinal cord is all about reflexes, true or false
true
two types of reflexes
inborn (intrinsic) and learned (acquired)
inborn (intrinsic) reflexes
rapid, involuntary, predictable motor response to stimulus
e.g. posture, control visceral activities
can be modified by learning and conscious effort
learned (acquired) reflexes
result form practice of repetition
e.g. driving skills
components of a reflex arc
receptor: site of stimulus action
sensory neuron: transmits afferent impulses to CNS
integration center: either monosynaptic of polysynaptic region within CNS
motor neuron: conducts efferent impulses from integration center to effector organ
effector: muscle fiber or gland cell that responds to efferent impulses by contracting or secreting
functional classification of reflexes
somatic reflexes: activate skeletal muscles
autonomic (visceral) reflexes: activate visceral effectors (smooth or cardiac muscle or glands)
spinal reflexes
integration center in spinal cord
effectors are skeletal muscles
stretch and tendon reflexes
to smoothly coordinate skeletal muscle nervous system must receive proprioceptor input regarding:
length of muscle: form muscle spindles
amount of tension in muscle: from tendon organs
functional anatomy of muscle spindles
composed of 3-10 modified skeletal muscle fibers (intrafusal muscle fibers) wrapped in connective tissue capsule
intrafusal fibers
noncontractile in central regions (lack myofilaments)
two types of afferent endings: anulospiral endings and flower spray endings
anulospiral enedings
primary sensory endings
endings wrap around spindle; stimulated by rate and degree of stretch
flower spray endings
secondary sensory endings
small axons at spindle ends; respond to stretch
what are the two ways muscle spindles are excited
external stretch of muscle and muscle spindle
internal stretch of muscle spindle
what does a stretch cause
it causes increased rate of impulses to spinal cord
the stretch reflex
maintains muscle tone in large postural muscles and adjusts it reflexively
causes muscle contraction in response to increased muscle length (stretching)
how stretch reflexes work
stretch activates muscle spindle
sensory neurons synapse directly with motor neurons in spinal cord
motor neurons cause stretched muscle to contract
all stretch reflexes are monosynaptic and ipsilateral
recipricol inhibition
fibers synapse with interneurons that inhibit motor neurons of antagonistic muscles
e.g. patellar reflex, stretched muscle (quads) contract and antagonists (hamstrings) reflex
positive reflex reactions indicate
sensory and motor connections between muscle and spinal cord intact
strength of response indicates degree of spinal cord exciteability
stretch reflexes are hypoactive or absent if
peripheral nerve damage or central horn injury
stretch reflexes are hyperactive if
there are lesions of corticospinal tract
events of which muscle stretch is damped
1. when stretch activates muscle spindles, the associated sensory neurons transmit afferent impulses at higher frequency to the spinal cord
2. the sensory neurons synapse directly with alpha motor neurons, which excite extrafusal fibers of the stretched muscle. sensory fibers also synapse with interneurons that inhibit motor neurons controlling antagonistic muscles
3a. efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists of reverses the stretch
3b. efferent impulses of alpha motor neurons to antagonist muscles are reduced (recipricol inhibition)
the tendon reflex
polysnaptic reflexes
helps prevent damage due to excessive stretch
important for smooth onset and termination of muscle contraction
produces muscle relaxation (lengthening) in response to tension
steps in the tendon reflex
1. quadriceps strongly contracts. tendon organs are activated
2. afferent fibers synapse with interneurons int he spinal cord
3a. efferent impulses to muscle with stretched tendon are damped. muscle relaxes, reducing tension
3b. efferent impulses to antagonist muscle cause it to contract
flexor reflex
initiated by painful simulus
causes automatic withdrawal of threatened body part
ipsilateral or polysnaptic
protective; important
brian can override (e.g. stick for blood)
crossed extensor reflex
occurs with flexor reflexes in weight-bearing limbs to maintain balance
consists of ipsilateral withdrawal reflex and contralateral extensor reflex
e.g. step barefoot on broken glass
superficial reflexes
elicited by gentle cutaneous stimulation
depend on upper motor pathways and cord level reflex arcs
plantar reflex and abdominal reflex
plantar reflex
superficial reflex
test integrity of cord from L4-S2
stimulus: stroke lateral aspect of sole of foot
response: downward flexion of toes
damage to motor cortex or corticospinal tracts > abnormal response =babinski's sign
normal in infants til 1 year due to incomplete myelination
abdominal reflexes
superficial reflex
test integrity of cord from T8-T12
cause contraction of abdominal muscles and movement of umbilicus in response to stroking of skin