Chapter 13
PNS
PNS
• Cranial nerves
• Spinal nerves
Sensory Receptors
• Specialized cells that monitor specific
conditions in the body or external
environment
• Activation of sensory receptors results in
depolarizations that trigger impulses to the
CNS
• Sensation: activation of sensory receptor
cells
Sensation vs. Perception
• Sensation:
– Activity in sensory cells
• Perception:
– Conscious awareness of a sensation (in
cortex)
Signaling
• Stimulation of a receptor produces
action potentials along the axon of a
sensory neuron
• Action potentials are all the same so:
• The frequency and pattern of action
potentials contains information about
the strength, duration, and variation of
the stimulus
Senses
• General senses:
temperature
–
–
–
–
–
pain
touch
pressure
vibration
proprioception
• Special senses
–
–
–
–
–
Olfaction (smell)
Vision (sight)
Gustation (taste)
Equilibrium (balance)
Hearing
General receptor types
• Exteroceptors
– Provide information about the external
environment
• Proprioceptors
– Report the positions of skeletal muscles and
joints
• Interoceptors
– Monitor visceral organs and functions
Modality
• Your perception of the nature of a stimulus
(its modality) depends on the path it takes
inside the CNS, especially, where in the
brain the information ends up.
Adaptation of Sensory
Receptors
• Adaptation occurs when sensory receptors
are subjected to an unchanging stimulus
– Receptor membranes become less
responsive
– Receptor potentials decline in frequency or
stop
• Receptors responding to pressure, touch,
and smell adapt quickly
• Pain receptors and proprioceptors do not
exhibit adaptation
Spinal Nerves
Figure 13–6
Spinal Nerves
• 31 pairs
• one per segment on each side of the spine
• dorsal and ventral roots join to form a spinal
nerve
• Carry both afferent (sensory) and efferent
(motor) fibers = mixed nerves
Spinal Nerve Organization
• Every spinal nerve is surrounded by 3
connective tissue layers that support structures
and contain blood vessels (just like muscles)
• Epineurium:
– outer layer
– dense network of collagen fibers
• Perineurium:
– middle layer
– divides nerve into fascicles (axon bundles)
• Endoneurium:
– inner layer
– surrounds individual axons
Peripheral Distribution
of Spinal Nerves
• Spinal nerves:
– start where dorsal and
ventral roots unite (just
lateral to the vertebral
column), then branch
and form pathways to
destination
Spinal Nerves: Rami
• The short spinal nerves branch into three
or four mixed, distal rami
– Small dorsal ramus
– Larger ventral ramus
– Rami communicantes at the base of the
ventral rami in the thoracic region
Nerve Plexuses
• All ventral rami except T2-T12 form interlacing
nerve networks called plexuses
• Plexuses are found in the cervical, brachial,
lumbar, and sacral regions
• Each resulting branch of a plexus contains fibers
from several spinal nerves
• Each muscle receives a nerve supply from more
than one spinal nerve
• Damage to one spinal segment (gray matter)
cannot completely paralyze a muscle
Peripheral Distribution of Spinal Nerves
Motor
fibers
Figure 13–7a
Motor fibers: First Branch
• From the spinal nerve, the first branch
(blue):
– carries visceral motor fibers to sympathetic
ganglion of autonomic nervous system (More
about this later)
Communicating Rami
• Also called Rami Communicantes, means
“communicating branches”
• made up of gray ramus and white ramus
together
Communicating Rami
• White Ramus:
– Preganglionic branch
– Myelinated axons (hence: white)
– Going “to” the sympathetic ganglion
• Gray Ramus
– Unmyelinated nerves (so: gray)
– Return “from” sympathetic ganglion
– Rejoin spinal nerve, go to target organ
Dorsal and Ventral Rami
Both are somatic and visceral outflow to
the body
• Dorsal ramus:
– contains somatic and visceral motor fibers
that innervate the back
• Ventral ramus:
– larger branch that innervates ventrolateral
structures and limbs
Peripheral Distribution of Spinal Nerves
Sensory
fibers
Figure 13–7b
Sensory Nerves
• Dorsal, ventral, and white rami (but not
gray) also carry sensory information in
addition to motor efferent outflow.
Dermatomes
• Bilateral region of skin
• Each is monitored by specific
pair of spinal nerves
Figure 13–8
Peripheral Neuropathy
• Regional loss of sensory or motor function
• Due to trauma, compression, or disease
Reflexes
Reflexes
• Rapid, automatic responses to specific
stimuli coordinated within the spinal cord
(or brain stem)
• Occurs via interconnected sensory, motor,
and interneurons
• Can be a movement, like a knee jerk, or
visceral, like pupil dilation or swallowing
Functional Organization
of Neurons in the NS
• Sensory neurons:
– about 10 million that deliver information to
CNS
• Motor neurons:
– about 1/2 million that deliver commands to
peripheral effectors
• Interneurons:
– about 20 billion that interpret, plan, and
coordinate signals in and out = information
processors
The Reflex Arc
• The wiring of a single reflex
– Begins at sensory receptor
– Ends at peripheral effector (muscle, gland,
etc)
– Generally opposes original stimulus (negative
feedback)
5 Steps in a Neural Reflex
• Step 1: Arrival of stimulus, activation of receptor
– physical or chemical changes
• Step 2: Activation of sensory neuron
– graded depolarization
• Step 3: Information processing by postsyn. cell
– triggered by neurotransmitters
• Step 4: Activation of motor neuron
– action potential
• Step 5: Response of peripheral effector
– triggered by neurotransmitters
5 Steps in a Neural Reflex
Figure 13–14
Classification of Reflexes
There are several ways to classify reflexes
but most common is by complexity of the
neural circuit: monosynaptic vs
polysynaptic
Monosynaptic Reflexes
• Have the least delay between sensory
input and motor output:
– e.g., stretch reflex (such as patellar reflex)
• Completed in 20–40 msec
• No interneurons involved
Monosynaptic Reflex
A stretch
reflex
Figure 13–15
Muscle Spindles
• The receptors in
stretch reflexes
• Bundles of small,
specialized muscle
fibers
• Sense passive
stretching in a muscle
Polysynaptic Reflexes
• More complicated than monosynaptic
reflexes
• Interneurons involved that control more
than 1 muscle group
• Produce either EPSPs or IPSPs
• Examples: the withdrawal reflexes
Withdrawal Reflexes
• Move body part away from stimulus (pain
or pressure):
– flexor reflex:
• pulls hand away from hot stove
– crossed extensor reflex
• Strength and extent of response depends
on intensity and location of stimulus
A Flexor Reflex
Figure 13–17
Key = Reciprocal Inhibition
• For flexor reflex to work:
– the stretch reflex of the antagonistic
(extensor) muscles must be inhibited
– reciprocal inhibition by interneurons in spinal
cord causes antagonistic extensors to be
inhibited
Reflex Arcs
• Crossed extensor reflexes:
– involves a contralateral reflex arc
– occurs on side of body opposite from the
stimulus
Crossed Extensor Reflexes
• Occur simultaneously and coordinated
with flexor reflex
• Example: flexor reflex causes leg to pull
up:
– crossed extensor reflex straightens other leg
to receive body weight
The Crossed
Extensor
Reflex
Figure 13–18
Integration and Control
of Spinal Reflexes
• Though reflex behaviors are automatic,
processing centers in brain can facilitate or
inhibit reflex motor patterns based in
spinal cord
Reinforcement of Spinal
Reflexes
• Higher centers can reinforce spinal
reflexes:
– by stimulating excitatory neurons in brain
stem or spinal cord
– creating EPSPs at reflex motor neurons
– facilitating postsynaptic neurons
Inhibition of Spinal Reflexes
• Higher centers can inhibit spinal reflexes:
– by stimulating inhibitory neurons
– creating IPSPs at reflex motor neurons
– suppressing postsynaptic neurons
Voluntary Movements and
Reflex Motor Patterns
• Higher centers of brain incorporate lower,
reflexive motor patterns
• Automatic reflexes:
– can be activated by brain as needed
– use few nerve impulses to control complex
motor functions
– e.g. walking, running, jumping