Chapter 13 PNS and Reflex Activity

Question 1

Peripheral nervous system

Answer 1

Peripheral nervous system: all neural structures outside the brain and spinal cord

Sensory receptors, peripheral nerves, ganglia and efferent motor endings

Question 2

Sensory Receptors

Answer 2

Specialized to respond to certain stimuli – changes in their environment

• *sensation = awareness of the stimulus
• *perception = interpretation of the meaning of the stimulus

Question 3

Sensory Receptors:

**Type of stimulus they detect

Answer 3

Mechanoreceptors – respond to mechanical force – e.g. pressure, vibration, stretch

Thermoreceptors – detect temperature changes

Photoreceptors – respond to light

Chemoreceptors – respond to chemicals in solution – e.g. smell, taste, blood composition

Nociceptors – respond to potentially damaging stimuli that produce pain – searing heat, extreme cold, excessive pressure, inflammatory chemicals

Question 4

Sensory Receptors:

Location in the body

Answer 4

• Exteroceptors – respond to stimuli arising outside of the body – most at, or near, surface
• Interoceptors – respond to stimuli within the body – e.g. tissue stretch - blood pressure, fullness; chemical composition - osmolarity, pH, O2 and CO2 levels; internal temperature
• Proprioceptors – located in skeletal muscles, tendons and joints – provide information about movement and body’s position in space based on stretch of the receptors

Question 5

Sensory Receptors:

Structural complexity

Answer 5

**Non-encapsulated (free) nerve endings – abundant in epithelia and connective tissue
most non-myelinated group C fibers
temperature, pain, itch (histamine activates itch receptor)
modified free nerve endings = touch receptors: Merkel discs (nerve ending and enlarged epidermal cells), hair follicle receptors (nerve plexus wrapped around hair follicle)

• *Encapsulated nerve endings = ending from one or more sensory neurons enclosed in a connective tissue capsule
  mechanoreceptors: tactile (Meissner’s) corpuscles, lamellar (Pacinian) corpuscles, bulbous (Ruffini) corpuscles , muscle spindles, tendon organs, joint kinaesthetic receptors

Question 6

Sensory Receptors:

General Senses

Answer 6

General senses = majority of modified dendritic endings – distributed throughout the body and monitor general sensory information – touch, stretch, pain, temperature…

Question 7

Sensory Receptors:

Special Senses

Answer 7

Special senses = specially modified nerve ending housed in complex sense organs located in the head that provides the senses of vision, hearing, equilibrium, smell and taste.

Question 8

Somatosensory system

Answer 8

Somatosensory system – receives input from receptors serving the body wall and limbs (skin, skeletal muscle, tendons and joint)

Question 9

Somatosensory system

Neural integration occurs at:

Answer 9

**Neural integration occurs at:
Receptor level
Circuit level in the ascending pathways
Perceptual level in the cortical sensory areas

Question 10

Somatosensory system

Receptor level processing

Answer 10

Receptor level processing

Stimulus must match specificity of the receptor (light to photoreceptor, touch to mechanoreceptor)

Stimulus must be applied within the receptive field of the sensory receptor

**Transduction = conversion of the stimulus energy into change in membrane potential

**Generator potential = change in membrane potential
may lead to action potential if threshold is reached

**frequency of nerve impulses = measure of strength, duration and pattern of stimulus
the higher the frequency – the stronger the stimulus

**adaptation = change in the sensitivity of the sensory receptors (and nerve impulse frequency) in the presence of a constant stimulus intensity:

*Phasic receptors = fast adapting – give off a burst of impulses at the beginning and end of the stimulus application
important for signalling changes in the internal or external environment

*Tonic receptors = slow adapting – provide a sustained response with little or no change in impulse frequency
Nociceptors and proprioceptors

Question 11

Somatosensory system

Circuit level processing

Answer 11

Circuit level processing Relay pattern of sensory neurons from first to third-order neurons

Central processes of first-order neurons branch when they enter the central nervous system
may activate spinal cord reflexes, 2nd-order sensory neurons, or both

Different ascending pathways carry various types of information to different areas of the brain

Question 12

Somatosensory system

Perceptual level processing

Answer 12

Perceptual level processing: occurs at in the cerebral cortex

*Perceptual detection – ability to detect that a stimulus has occur = simplest level
generally input from several receptors must be summed for perceptual detection to occur

*Magnitude estimation – how intense the stimulus is = frequency of arriving action potential

*Spatial discrimination – ability to identify site or pattern of stimulation
two-point discrimination= smallest distance at which two stimuli applied simultaneously can be detected as separate entities

*Feature abstraction – neuron or circuit is tuned to one feature or property of a stimulus in preference to others
sensation involves integration of several stimulus features – e.g. ice cube – cold, smooth, cube shape, has consistent weight

*Quality discrimination = ability to differentiate sub-modalities of a particular sensation – e.g. each sensory modality (the thing the sensory receptor responds to: light, temperature, taste…) has several qualities, or sub-modalities
taste modality – has five qualities: sweet, salty, sour, bitter, umami

*Pattern recognition = ability to take in scene around us and recognize a familiar pattern, and unfamiliar patter, or a pattern with special significance
words, music, art, faces

Question 13

Pain perception

Answer 13

Warns of actual or impending tissue damage
Motivates protective action
Cannot be measured objectively = intensely personal experience

**pain threshold = ability to perceive pain – occurs at roughly same stimulus intensity
tolerance to pain varies widely:
A number of genes determine pain tolerance and response to pain medications
Low pain tolerance = sensitive to pain

Question 14

Pain perception

Answer 14

Nociceptors – activated by extremes of pressure and temperature, chemicals released from damaged tissue
Histamine, K+, ATP, acids, and bradykinin = potent pain inducing chemicals

Neurotransmitters released by primary nociceptive neurons= glutamate and substance P
Activate 2nd order neurons in pain pathway

Endogenous (built-in) analgesic (pain suppression) system: endorphins and enkephalins

Question 15

Long-lasting or very intense pain

Answer 15

Long-lasting or very intense pain inputs activates NMDA receptors (strengthens neural connections during learning) that can lead to:

**hyperalgesia – pain amplification
Chronic pain
**Phantom limb pain – pain perceived in tissue that is no longer present

Question 16

Visceral pain

Answer 16

**Visceral pain – sensation usually dull aching, gnawing, or burning arising from extreme stretching of tissue, ischemia (low blood flow), chemicals, and muscle spasms of organs

Referred pain – travels along same pathways as somatic pain – so perceived as coming from somatic or cutaneous region

Question 17

Nerve

Answer 17

Nerve = cordlike organ containing parallel bundles of peripheral axons (myelinated and non-myelinated bundles), blood vessels, lymph vessels and connective tissue

Endoneurium = loose connective tissue enclosing each axon - also Schwann cells for myelinated axons

Perineurium = coarser connective tissue wrapping bundles of axons into fascicles

Epineurium - outermost covering = tough fibrous sheath enclosing all fascicles

Question 18

Nerve types

Answer 18

**Sensory (afferent) nerves – carry information into the CNS

**Motor (efferent) nerves – carry information away from the CNS to the effectors (muscle, glands)

**Mixed nerves – carry both sensory and motor fibers

Most mature neurons do not divide (post-mitotic)
damage if severe or close to the cell body can kill entire neuron
Other neurons stimulated by the killed neuron may also die
If cell bodies remain intact – axons of peripheral neurons can regenerate
Axons of CNS neurons cannot – function of inhibitory proteins in oligodendrocytes and scar tissue from astrocytes

Question 19

Cranial nerves

Answer 19

12 pairs of nerves that arise from brain that, except for the vagus (X), serve head and neck only
I = olfactory – sensory for smell
II – optic – sensory for vision
III – oculomotor – motor to four of the muscles of the eye
IV – trochlear – motor to superior oblique muscle of the eye
V – trigeminal – mixed nerve – largest cranial nerve – main general sensory information from face and motor to chewing muscles
VI – abducens – motor to the lateral rectus muscle of the eye
VII – facial – mixed nerve – sensory for taste (anterior 2/3 of tongue), chief motor nerve of face
VIII – vestibulocochlear – sensory for hearing and balance
IX – glossopharyngeal – mixed nerve –taste and general sensation of tongue, swallowing, gag
**X – vagus – mixed nerve- sensory from thoracic and abdominal viscera, major nerve for the **parasympathetic motor division
XI – accessory – motor to trapezius and sternocleidomastoid muscles that move the head and neck
XII – hypoglossal – motor to muscles of the tongue

Question 20

Spinal nerves

Answer 20

31 pairs of mixed nerves arising from spinal cord – supply all parts of body except head and some of neck

Connects to spinal cord by **ventral (motor/efferent) root and dorsal (sensory/afferent) root

Question 21

Spine Nerve branches

Answer 21

Branches almost immediately into:
*dorsal ramus – supplies dorsal skin and muscles
*ventral ramus – larger – supplies muscles and skin of the lateral and anterior portions of the body
rami communicantes = special connections from the autonomic nerve fibers to the ventral rami
all ventral rami branch and join one another lateral to the vertebral column

Question 22

Nerve Plexuses

Answer 22

Nerve plexuses = interlacing nerve networks formed by connections between ventral rami
Cervical plexus – most = cutaneous branches that supply skin
Phrenic nerve (C3, C4, C5) = most important motor nerve that supplies diaphragm

*Brachial plexus – gives rise to almost all nerves that innervate the upper limb
injury to brachial plexus – when upper limb pulled hard (stretching the plexus) or by blows to the top of the shoulder that force the humerus inferiorly - common and can weaken or paralyze entire upper limb
gives rise to the **axillary, musculocutaneous, median, ulnar and radial nerves

**Lumbosacral plexus – formed from the extensive overlap of the lumbar and sacral plexi
Lumbar plexus*– its major branches descend anteriorly and medially to **supply the thigh
**femoral (largest), obturator nerves

Sacral plexus* – lies caudal to the lumbar plexus – branches serve buttock, lower limb, pelvic structures and perineum
Sciatic (largest) = two fused nerves – **tibial and common fibular (peroneal) nerves
when transected – leg is nearly useless – leg cannot be flexed because hamstrings are paralyzed, and foot and ankle cannot move at all - footdrop
sciatica – characterized by stabbing pain radiating over the course of the sciatic nerve
branch to sural and medial and lateral plantar nerves
Superior and inferior gluteal and pudendal nerves

Question 23

Dermatomes

Answer 23

Dermatomes = area of skin innervated by the cutaneous branches of a single spinal nerve

In patients with spinal cord injury – damaged nerves and extent of the injured region of the spinal cord can be determined by assessing which dermatomes are affected

Question 24

Joint innervation

Answer 24

Joint innervation – Hilton’s law = any nerve which serves a muscle that produces movement at a joint also innervates the joint and the skin over the joint

Question 25

Cerebral cortex

Fig. 13.14

Answer 25

Cerebral cortex is at highest level of **conscious motor output

Question 26

Cerebellum and basal nuclei

Answer 26

Cerebellum and basal nuclei = planners and coordinators of complex motor output

Question 27

Reflex Arcs

Answer 27

Reflex arcs – mediate motor output at lower levels

**Intrinsic (inborn) reflex = rapid, predictable motor response to stimulus
unlearned, unpremeditated, and involuntary – but can be modified by learning
help in maintaining posture, avoiding pain, and control visceral activities

**Acquired (learned) reflex – results from practice and repetition

Question 28

Components of reflex arc

Fig. 13.15

Answer 28

1. Receptor
2. Sensory (afferent) neuron
3. Integration center – connection within the central nervous system
4. Motor (efferent) neuron
5. Effector – muscle or gland that responds to efferent signalling

Question 29

Somatic reflexes

Answer 29

Somatic reflexes – activate skeletal muscle

Question 30

Autonomic (visceral) reflexes

Answer 30

Autonomic (visceral) reflexes – activate smooth muscle, cardiac muscle or glands

Question 31

Spinal reflexes

Answer 31

Spinal reflexes – occur without direct involvement of higher brain centers

Proprioceptors play important role in spinal reflexes and provide feedback to cerebral cortex and cerebellum

• muscle spindles – present in skeletal muscle – firing increases with the amount of stretch on muscle (change in muscle length)
• Golgi tendon organs – present in tendons associated with muscles and firing increases in response to increased muscle tension

Question 32

Stretch reflex (Focus Fig. 13.1)

Answer 32

Stretch reflex – initiated in response to muscle stretch - e.g. knee-jerk reflex
helps keep knees from buckling when standing upright

As knees begin to buckle – stretch on quadriceps muscle lengthens muscle and activates muscle spindle

Sensory input **monosynaptic - activates motor neurons that contracts ipsilateral (same side) quadriceps muscle (knee extensors) – muscle from which the afferent information came from

Branches of the afferent neuron act on interneurons that inhibit antagonistic muscle contraction (knee flexors) = reciprocal inhibition

Allows straightening of the leg

Question 33

Stretch reflex 
(Golgi) tendon reflex (Fig. 13.18)

Answer 33

(Golgi) tendon reflex – **polysynaptic reflex – initiated in response to muscle tension

Produce opposite effect to stretch reflex – muscles relax and lengthen in response to increased tension

Question 34

Stretch reflex

Flexor and crossed extensor reflexes (Fig. 13.19)

Answer 34

Flexor and crossed extensor reflexes – initiated in response to pain (noxious stimulus)

Protective and important and override spinal pathways and prevent other reflexes from occurring at the same time

Can be influenced by anticipation

**flexor or withdrawal reflex – removes body part from stimulus

**crossed-extensor reflex – accompanies flexor reflex in weight-bearing limbs – causes contralateral limb to extend