Chapter #13: Peripheral Nervous System Flashcards
Function of PNS
-The PNS links the CNS to the outside world
-Nerves extend from every area of the body: provide the brain with sensory input & allows for motor output to reach effector organs
General Structures of the PNS
-any nervous system tissue that is not the brain or spinal cord, including
1) Sensory receptors
2) Afferent fibers: carry sensory info (sends in)
3) Efferent fibers: carry motor info (sends out)
Two subdivisions of the PNS
- Afferent division
- Efferent division
Afferent division
-carry impulses from the body to the central nervous system
-concerned with sensory input
-Function: impulses allow CNS to interpret information & send out a response
Efferent division
-carry impulses from CNS to the effector organs
-concerned with motor output
-Function: impulses activate muscle or glands to carry out response
-Somatic division: skeletal muscle tissue is the effector
-Autonomic division: cardiac muscle, smooth muscle, & glands are effectors
Sensory receptors
Function: specialized structures that respond to changes in the environment (internal or external) called stimuli
How can sensory receptors be classified?
1) Stimulus type: what change in the environment activates the receptor
2) Location: either location in the body or the location of the source of the stimulus (inside or outside the body)
3) Receptor structure: nonencapsulated vs. encapsulated
Classification by Stimulus Type
1) Mechanoreceptors: respond to mechanical force (body-wide)
2) Thermoreceptors: respond to temperature changes (body-wide)
3) Photoreceptors: respond to light (localized; only found in eyes)
4) Chemoreceptors: respond to chemicals in solution (sense of smell, taste, blood pH, changing nutrient levels)
5) Nociceptors: respond to damaging stimuli (harm receptor; extreme pressure, temperature, pain)
Classification by Stimulus Location
1) Exteroceptor: mostly found at or near the body surface (close to surface of skin)
-Receptor is sensitive to stimuli that arise outside the body (ex. photoreceptors, mechanoreceptors)
2) Interoceptor: found deeper in the body
-Receptor responds to stimuli that arise deep in the body (ex. chemoreceptors, mechanoreceptors)
3) Proprioceptors: found in skeletal muscle, tendon, joints, ligaments (highly localized)
-Receptor responds to changes in body movement/position (mechanoreceptors)
Classification by Receptor Structure
-Receptors of the general senses
-Composed mostly of nerve endings: modified dendritic endings of sensory neurons; respond to change
-Two types of nerve endings:
1) Non-encapsulated (free) nerve ending
2) Encapsulated nerve ending
Non-encapsulated (free) Nerve Endings
-Dendritic ends of sensory receptors have no capsule or covering
-Mostly abundant in epithelia and connective tissue
-Function: respond mostly to pain (nociceptors) and temperature (thermoreceptors)
Other sensations allowed by free nerve endings
1) Itch (chemoreceptor): histamine release activates free nerve endings
2) Light pressure (mechanoreceptor): Merkel cells in the integument
3) Light touch specifically for hair (mechanoreceptor): free nerve endings wrap around hair follicle, respond to change in position of the hair & hair follicle
Encapsulated nerve endings
-Dendritic endings are enclosed in a capsule
-Most mechanoreceptors are encapsulated
1) Tactile corpuscles: found in dermal papillae of hairless skin
-Function: important for light touch sensation
2) Lamellar corpuscles: located deep in dermis
-Function: stimulated by onset of deep pressure
3) Bulbous corpuscles: located in dermis, subcutaneous tissue, and joint capsules
-Function: respond to deep, continuous pressure
4) Muscle spindles: proprioceptors found in tissue wrapped around skeletal muscle
-Function: detect muscle stretch & initiates reflexes to resist excessive stretching forces
5) Tendon organs: proprioceptors of tendons
-Function: initiates reflex that relaxes skeletal muscle to prevent damage to tendon
6) Joint kinesthetic receptor: proprioceptor monitoring stretch in joints
-Function: provide information about joint position and motion (prevent dislocation)
Sensation vs. Perception
-Sensation: awareness of a change in external/internal environment
-Perception: how we interpret the stimuli (do we like the change?)
Somatosensory system
-sensory system that specifically serves body wall and limbs
-Receives input from exteroceptors, proprioceptors, and interoceptors
-Three levels of integration operate to allow perception to occur:
1) Receptor level
2) Circuit level
3) Perceptual level
Processing at the Receptor Level
-sensation will only occur if a stimulus activates a receptor and if the action potentials reach the CNS.
-For this to happen:
1) Stimulus energy must match receptor specificity
-ex. mechanoreceptor will not respond to light
2) Stimulus must be applied within receptor’s receptive field
-dendrites only extend out so far; smaller receptor field is easier for your brain to figure out where it came from
3) The stimulus energy must be converted to a graded potential
4) Graded potentials must reach threshold for sensory receptor to generate an action potential
Processing at the Circuit Level
-delivering impulses to appropriate region of the cerebral cortex
Processing at the Perceptual Level
-sensory input is interpreted at the cerebral cortex
-Identifying and appreciating sensation does not depend on the message: it depends on the target neurons of the cerebral cortex (this is why some people have different perceptions of pain)
Perception of Pain
-No one likes to feel pain, but it is important
-pain is a warning to prevent you from damaging yourself more than you already have
-you can’t measure pain
-individuals interpret pain differently; different levels of pain tolerance
-What triggers pain?
-Excessive temperature & excessive pressure, chemicals released by damaged cells of injured tissue, histamine, K+, ATP, acids, bradykinin, etc.
Two types of pain
1) Sharp pain: usually occurs at onset of injury (shorter pain)
-Impulses carried by small, myelinated fibers
2) Burning pain: occurs after sharp pain (longer pain)
-Impulses carried by small, nonmyelinated fibers
Suppression of pain perception
-This occurs most often during times of stress: coping/escape mechanism (if you do not suppress the pain there will be a worse consequence)
-Endorphins and enkephalins suppress feeling of pain (ex. high stress-survival situations like car accidents, sprained ankles in times where you need to run)
-shut off the brain from responding to painful messages when the pain is too much
Pain arising from inside the body
1) Visceral Pain: noxious stimulation of receptors of the thorax and abdominal cavity
-Stimuli that result in visceral pain: extreme stretching of tissue, ischemia, muscle spasms/cramps, irritating chemicals (ex. over stretched stomach from eating too much)
2) Referred pain: pain stimuli arising from one part of the body are perceived as coming from another part
-Why does this occur? Visceral pain afferents travel along similar route as some somatic pain fibers (ex. heart attacks: first symptom is pain from the left arm)
-The brain is “confused” by the signal: cannot determine exact location of source
Nerves
-cordlike organ that is part of the peripheral nervous system consisting of parallel bundles of peripheral axons enclosed by connective tissue wrappings (reaches parts of the body in shorter distances
-Classification of nerves:
-Type of information the nerve carries: sensory or motor (some send both)
-Direction in which the information is carried:
-Afferent nerves: carry information toward CNS
-Efferent nerves: carry information away from CNS
-Mixed nerves: carry information to and from the CNS
Structure of a nerve
-Nerves are bundles of axons enclosed by connective tissue wrappings
-A single axon is surrounded by endoneurium
-Groups of axons are bundled together by perineurium
-Bundles are called fascicles
-Fascicles are bundled together by epineurium
-Bundles of fascicles form the actual nerve
-Also has blood vessels and lymphatic vessels (keeps all fascicles healthy; immune defenses)
Nerve Damage & Regeneration
-Remember: mature neurons do not divide!
-In the CNS: severed/damaged axons do not regenerate
-When damaged, the neuron dies (all nerves enclosed by bone)
-In the PNS: severed/damaged axons are capable of regeneration (small regenerative capacity; but do not replace)
-But for regeneration to occur:
1) The cell body must remain undamaged
2) The distance between severed ends must be short
-Regenerated axons do not ”behave” as before the injury (a regenerated neuron in the PNS is not as good as the original)
Axon Regeneration Steps
- Injury occurs
- Clean-up
- Axon regeneration begins
- Completion of regeneration
Step 1: Injury occurs
-The axon is damaged/torn during the injury
-Proximal portion of axon will seal itself off (so that cell body remains protected)
-The distal portion of the cell will degenerate
Step 2: Clean-up
-small amount of immune system access
-Macrophages invade area of damaged axon
-Dead/dying portion of neuron is destroyed
-if you do not clean up dead neurons, they would physically get in the way of regeneration
Step 3: Axon regeneration begins
-Schwann cells release growth factor to stimulate axon growth
-Proximal end of axon grows filaments
-Schwann cells form a “tube” (place limit on where the axon can grow; ensures the filaments to grow/sprout in the correct manner)
Step 4: Completion of regeneration
-Axon filaments continue to growforms complete axon
-Schwann cells form new myelin sheath around new axon
-Overall diameter of new axon is smaller than the original causing a lower conduction speed and slower messages
Cranial Nerves
-Twelve pairs of cranial nerves extend from the brain (24 nerves total)
-Cranial nerves serve head and neck structures (11 nerves)
-Exception: the vagus nerve (X) extends into the abdomen
I: Olfactory Nerve
-Function: nerves associated with olfaction (smell)
-Sensory nerve
II: Optic Nerve
-Function: nerves associated with vision
-Sensory nerve
III: Oculomotor Nerve
-Function: supplies four of the six extrinsic eye muscles the move the eyeball
-Motor Nerve
IV: Trochlear Nerve
-Function: innervates extrinsic eye muscle that depresses eye and turns it laterally and downward (called the superior oblique)
-Motor Nerve
V: Trigeminal Nerve
-Function: supplies sensory fibers to the face and motor fibers to the chewing muscles (largest of the cranial nerves)
-sensation of pain, touch, temperature on face
-if damaged, sensory deficits to face &. mouth & changes eating habits
-Both
VI: Abducens Nerve
-Function: controls extrinsic eye muscle (6th eye muscle) that abducts the eye (the lateral rectus muscle)
-Motor nerve
VII: Facial Nerve
-Function: innervates muscle for facial expression, contributes to taste (sends info to gustatory cortex)
-Both
VIII: Vestibulocochlear Nerve
-Function: contributes to hearing and balance (sends info to auditory & vestibular cortex)
-Sensory nerve
IX: Glossopharyngeal Nerve
-Function: innervates tongue for taste & general senses, innervates pharynx for general senses, innervates muscles of pharynx for swallowing
-Both
X: Vagus Nerve
-Longest cranial nerve and is the only cranial nerve that serves more than head and neck structures
-Function: fibers extend to and supply sensory & motor fibers to the organs of the thorax and abdomen
-Both
XI: Spinal Accessory Nerve
-Function: supplies motor fibers to muscles that move the head and neck
-Motor nerve
XII: Hypoglossal Nerve
-Function: innervates muscles of the tongue and under the tongue to allow movement of tongue for chewing, speech, swallowing
-Motor nerve
Mnemonic Mechanism for Nerve names
Oh oh oh to touch and feel a girl’s very soft hair
Mnemonic Mechanism for Sensory motor or both
Some say marry money but my brother says big brains matter more
Spinal Nerves
-31 pairs of spinal nerves branch from the spinal cord (62 total)
-Function: supply all parts of the body not served by the cranial nerves
-Remember: nerves connect to spinal cord via dorsal (sensory) and ventral (motor) roots
-all spinal nerves serve mixed functions
-Spinal nerves divide to form dorsal ramus and ventral ramus
-Each ramus provides fibers to specific parts of the body
Dorsal Ramus
-Function: provides sensory and motor fibers to the skin and muscles of the back
Ventral ramus (ventral rami)
-Function: provides sensory and motor fibers to lateral and ventral body walls (front and side of body) and to the upper & lower limbs
-structurally much more complex
-Ventral rami branch to form nerve networks called nerve plexuses
-Ventral serves most of the body
How is a nerve plexus formed?
-As ventral rami extend away from from the spinal cord, they branch several times
-Branches from neighboring rami join one another to form a “weaving network” of nerves
Effect of nerve plexus formation
1) Each branch of the plexus contains fibers from multiple spinal nerves
2) Fibers from each ramus travel to body via several routes
-if you suffer trauma to one part of your body and lose one plexus, you still have others; you can damage one part of your plexus & not have a complete lose of function to the limb overall
Nerve plexuses in body:
1) Cervical plexus
2) Brachial plexus
3) Lumbosacral plexus
Cervical Plexus
-Main function: cutaneous nerves that supply the neck, ear, back of the head, and shoulders
-only supplies skin
-C1-C4
Brachial Plexus
-Function: provides fibers that supply the upper body limbs
-C5-C8 and T1
Lumbosacral Plexus
-Lumbar plexus and sacral plexus have large degree of overlap
-Lumbar plexus
-Function: innervates parts of abdominal wall muscle, major branches innervate anterior & medial thigh
-Sacral plexus
-Some fibers of lumbar plexus contribute to sacral plexus via lumbosacral trunk
-Function: innervates buttocks and posterior aspect of lower limbs, pelvic structures and perineum
-ex. sciatic nerve (pain in lower back down leg)
Anterolateral Thorax and Abdominal Wall
-Ventral rami in the thorax do not form plexuses, but are arranged in segmental pattern
-Function: serve intercostal muscles between ribs, skin of anterolateral thorax, and most of abdominal wall
Back
-Dorsal rami innervate posterior body trunk in segmental pattern
-Function: each dorsal ramus innervates narrow strip of muscle & skin at same area where it emerges from spinal column
Skin (Dermatomes)
-Area of skin innervated by a single spinal nerve
-Function: provides sensory details (pressure, temperature, pain, etc.) of the skin to the brain
Reflex Arc
response that enables rapid and predictable responses by the body (automatic, response is almost the same each time; ex. losing your balance)
Types of reflexes
1) Intrinsic reflex: unlearned, unpremeditated, and involuntary: ”built-in” response
-Rapid, predictable motor response to a stimulus
-Can still be modified according to circumstance
-Ex: making fast adjustments to body posture after losing balance
2) Acquired reflex: results from practice or repetition: ”learned” responses
-Ex: driving a car
Components of a reflex arc
1) Receptor: site of stimulus action (afferent)
2) Sensory neuron: transmits afferent impulses to the CNS
3) Integration center: synapses found in the CNS (decides motor response)
4) Motor neuron: conducts efferent impulses from the integration center to the effector organ (travel to effector organ)
5) Effector: muscle fiber or gland that responds to the efferent impulse
Spinal Reflex
-any somatic reflex that is mediated by the spinal cord
-Most occur without any higher CNS involvement
Types of spinal reflexes
1) Stretch reflex
2) Tendon reflex
3) Flexor and Cross-Extension reflexes
Stretches reflexes
-ensure that a muscle stays at its optimal length
-Function: prevents damage to muscle and tendon by causing a muscle to contract when the muscle is stretched too far
All stretch reflexes are:
1) Monosynaptic: have only a single synapse (no interneurons involved) between the sensory neuron and a motor neuron
2) Ipsilateral: the stimulus and response occur on the same side of the body
All stretch reflexes are:
1) Monosynaptic: have only a single synapse between the sensory neuron and a motor neuron
2) Ipsilateral: the stimulus and response occur on the same side of the body
What is involved in stretch reflexes?
-Muscle spindles: provide information about the length of a particular muscle
-Muscle spindles function as proprioceptors
-When stretched, muscle spindle sends impulses at higher frequency to spinal cord
-Example: the knee-jerk reflex
-inhibitory; contracting quad and relaxing hamstring
Function of tendon reflexes
prevents damage to muscle by causing the muscle to relax when tension force (during muscle contraction) becomes so great it can damage the muscle
-almost the opposite of a stretch reflex
Tendon Organs
-provide information about the amount of tension in a muscle and its associated tendons
-Muscles relax and lengthen in response to tension
-Importance: helps prevent damage due to excessive stretch
-Tendon organs are involved in the tendon reflex
-Tendon reflexes are polysynaptic: involve multiple synapses with chains of interneurons
-excitatory; relax quad and contracting hamstring
Flexor & Crossed-Extensor Reflexes
-in many instances, these reflexes occur together
Flexor reflex (withdrawal reflex)
-initiated by painful stimuli
-Causes automatic withdrawal of body part from the stimulus source
-Ipsilateral and polysynaptic
-Importance: protection/survival mechanism
-this reflex can be overridden consciously; ex. piercings or tattoos, blood is drawn, vaccines, etc.
Crossed-Extensor Reflex
-often accompanies flexor reflex
-Prevalent in weight-bearing limbsmaintains balance
-Reflex is contralateral: the stimulus and response occur on opposite sides of the body
-Ex: stepping on a Lego
-The leg/foot that steps on the Lego has an ipsilateral withdrawal reflex; you pick your foot up off the floor
-Opposite leg has contralateral extensor reflex: you shift your weight to the opposite side of your body
