Chapter 13- The peripheral nervous system Flashcards
PNS general function
The PNS links the brain to the outside world as well as the body’s internal environment- includes all nerves that leave/enter the spinal cord. Without the PNS, the brain would have no real function.
How do sensory deprivation tanks work?
Sensory deprivation tank gets rid of all sensory input- the brain will start making stuff up to have something to do- causes massive hallucinations. If you’re nervous beforehand, the hallucinations can be bad/scary.
What are the general structures of peripheral nervous tissue? (3)
- Sensory receptors
- Afferent fibers
- Efferent fibers
Function of sensory receptors
respond to changes in the environment (stimuli)
How are sensory receptors classified? (3)
- Stimulus type- what change in the environment activates the receptor
- Location- either location in the body, or where the location of the source of the stimulus (inside or outside the body)
- Receptor structure- non encapsulated vs encapsulated
Receptor classifications by stimulus type (5)
- Mechanoreceptors
- Thermoreceptors
- Photoreceptors
- Chemoreceptors
- Nociceptors
Mechanoreceptors
Receptors that respond to mechanical force.
Ex- touch, vibration, stretch
Thermoreceptors
Receptors that respond to temperature changes
Photoreceptors
Receptors that respond to light
Chemoreceptors
Respond to chemicals in solution (will only respond if they’re in solution). Ex- smell, taste, changes in blood composition
Nociceptors
Respond to damaging stimuli- activation of this receptor usually results in painful sensation. Ex- extreme temperatures, excessive pressure/stretch
Receptor classifications by location (3)
- Exteroceptor
- Interoceptor
- Proprioceptors
Exteroreceptor
Mostly found at or near the body surface. Receptor is sensitive to stimuli that arise outside the body. Ex- vision, hearing, smell, taste, touch, pressure, pain, etc.
Interoceptor
Receptors that are found deeper in the body-responds to stimuli that arise deep in the body. Ex- chemical change, tissue stretch, internal temperature
Proprioceptors
Found in skeletal muscle, tendon, joints, ligaments. Receptor responds to changes in body movement/position. These receptors are technically interoceptors but are much more limited in their location.
Nerve endings
Dendritic endings of sensory neurons that compose the simple receptors of the general senses. Nerve endings can be non encapsulated or encapsulated.
Encapsulated nerve endings
Tissue covering wrapped around the nerve endings.
Non encapsulated (free) nerve endings
Exposed nerve endings- no tissue covering. They are essentially the peripheral end of a sensory axon.
Where are free (non encapsulated) nerve endings typically found?
Mostly abundant in epithelia and connective tissue
Non encapsulated nerve endings function
Respond mostly to pain (nociceptors) and temperature (thermoreceptors). Heat or cold outside the “range” of a thermoreceptor activates nociceptors. Extreme pressure (ex- pinching the arm) and chemicals are released from damaged tissue to activate nociceptors .
Which sensations are allowed by free nerve endings? (5)
- Temperature
- Pain
- Itch
- Light pressure
- Light touch
What causes the itch sensation?
Detected by a chemoreceptor. Histamine release activates free nerve endings
What causes the light pressure sensation?
Detected by a mechanoreceptor. Merkel cells in the integument are responsible for light touch, pressure, and vibration
What causes the light pressure sensation specifically for hair?
Detected by a mechanoreceptor. Free nerve endings wrap around the hair follicle and respond to change in position
What type of receptor are encapsulated nerve endings associated with?
Mechanoreceptors
What types of mechanoreceptors are composed of encapsulated nerve endings (6)?
- Tactile corpuscles
- Lamellar corpuscles
- Bulbous corpuscles
- Muscle spindles
- Tendon organs
- Joint kinesthetic receptors
Tactile corpuscles
Found in dermal papillae of hairless skin. Function- important for light touch sensation- are equivalent in function to hair follicle receptors in skin with hair
Lamellar corpuscles
Located deep in the dermis. Function- stimulated by initiation of deep pressure. If you continue to apply deep pressure, the receptors stop responding and bulbous corpuscles take over.
Bulbous corpuscles
Located in dermis, subcutaneous tissue, joint capsules. Function- respond to deep, continuous pressure
Muscle spindles location
Proprioceptors that are found in tissue wrapped around skeletal muscle
Muscle spindles function
Detect muscle stretch, initiates reflex to resist excessive stretching forces. This is protective, because excessive stretching can tear the muscle. Function is not necessarily voluntary.
Tendon organs
Proprioceptors of tendons. Function- activation of tendon organs by stretching of tendon initiates reflex that releases skeletal muscle- stops the tendon from contracting, forces it to relax. This is protective against muscle injury. Not necessarily voluntary
Joint kinesthetic receptor
Proprioceptor monitoring stretch in joints. Function- provide information about joint position and motion. This is protective against joint damage from excessive movement.
Sensation
Awareness of a change in external/internal environment
Perception
How we interpret stimuli
Somatosensory system
Sensory system that specifically serves body wall and limbs. Receives input from exteroreceptors, proprioceptors, and interoceptors, and allows perception of a change in the environment.
Levels of integration that allow perception to occur (3)
- Receptor level
- Circuit level
- Perceptual level
What two things must occur for sensation to occur with processing at the receptor level?
a stimulus must activate a receptor and the action potentials must reach the CNS.
What must happen for a stimulus to activate a receptor (4)
- Stimulus energy must match receptor specificity
- Stimulus must be applied within the receptor’s receptive field
- Transduction must occur
- Graded potentials must reach a threshold for action potential generation and propagation.
Transduction
conversion of stimulus energy to graded potential
Receptor specificity
Certain receptors can be more specific about their stimulus- this is especially true for special senses receptors. For example, the cells in your eye will only ever respond to light and that’s it.
A smaller receptive field=
greater ability to localize stimulus site
What occurs during processing at the circuit level?
Impulses are delivered to the appropriate region of the cerebral cortex for localization and perception. Must go to the correct region so the brain can process what is occurring.
What occurs during processing at the perceptual level?
sensory input is interpreted at the cerebral cortex
During processing at the perceptual level, what does identifying sensation depend on?
Identifying and appreciating sensation does not depend on the message. It depends on the target neurons of the cerebral cortex. As a result, the brain will always interpret activity of a specific type of sensory receptor as a specific sensation no matter how it is activated.
Why is pain important?
Pain warns us of impending damage or prevents us from further damaging ourselves
If you experience pain, you’re either about to seriously damage yourself or already are damaging yourself.
How is pain measured?
Pain is not measured, it is subjective- what is painful to one person may not be for another
What triggers pain? (8)
- Excessive temperature (cold or heat)
- Excessive pressures (pinching, etc.)
- Inflammatory chemicals from injured tissue (released from cuts, etc)
- Histamine (must be released in very large amounts to be perceived as pain)
- potassium, ATP, acids
- Bradykinin
Bradykinin
An inflammatory mediator- causes blood vessels to dilate. Will be perceived as pain in large amounts
Types of pain (2)
- Sharp pain
2. Burning pain
Sharp pain
Usually occurs at onset of injury, impulses are carried by small, myelinated fibers
Burning pain
Occurs after sharp pain. Can also be felt as a throbbing pain. Impulses are carried by small, nonmyelinated fibers
What 2 chemicals suppress pain perception?
Endorphins and enkephalins suppress feeling of pain
When does pain suppression typically occur?
This occurs most often during times of stress- coping/escape mechanism
Visceral pain
noxious stimulation of receptors of the thorax and abdominal cavity
Causes of visceral pain
- Extreme stretching of tissue (too much food in the stomach, etc.)
- Ischemia (loss of oxygen/circulation results in tissue death)
- Muscle spasms/cramps (deep organs, not skeletal muscle)
- Irritating chemicals (acidic foods that irritate the lining of the stomach).
Referred pain
Pain stimuli arising from one part of the body are perceived as coming from another part
Why does referred pain occur?
Visceral pain afferents travel along similar route as some somatic pain fibers
The brain is “confused” by the signal- cannot determine exact location of source so it guesses. Example- heart attack victims feel pain in their left arm. Fibers from the arm and heart run along the same route.
Nerve
Cordlike organ that is part of the peripheral nervous system consisting of parallel bundles of peripheral axons enclosed by connective tissue wrappings
How are nerves classified (2)?
- Type of information the nerve carries: sensory or motor
2. Direction in which the information is carried
Afferent nerves
carry information toward the CNS (sensory in nature)
Efferent nerves
carry information away from the CNS (motor)
Mixed nerves
Carry information to and from the CNS. Includes most of the nerves in the body, have both sensory and motor function.
Components of the structure of a nerve
Nerves consist of parallel bundles of axons enclosed by connective tissue wrappings with blood vessels (an artery and a vein) and lymphatic vessels
Endoneurium
Connective tissue that wraps around individual axons and schwann cells (if present). The axons may or may not be myelinated
Perineurium
Connective tissue that bundles groups of axons together
Fascicles
Bundles of groups of axons
Epineurium
The epineurium is the outermost tissue covering- bundles together individual fascicles
What forms a nerve?
Bundles of fascicles. Can be composed of anywhere from thousands to millions of axons.
Regeneration
Creating new parts of a damaged cell. This is different from reproduction (mitosis). Mature neurons DO NOT reproduce, but they can sometimes regenerate.
In what part of the nervous system can axons regenerate?
In the PNS, severed/damaged axons are capable of regeneration. This is advantageous, or else we would lose sensory/motor function early on in life. In the CNS, severed/damaged axons never regenerate. The entire neuron dies and may also cause the death of surrounding neurons
What must occur for axons in the PNS to regenerate?
Regeneration only occurs if the cell body is undamaged. Also, the greater the distance between severed ends, the less likely regeneration can occur .
How do regenerated axons behave?
Might not propagate action potentials as quickly, etc.. Therapy is often needed to train the nerve to respond appropriately.
Axon regeneration in the PNS (4 steps)
- The axon becomes fragmented at the injury site
- Macrophages clean out the dead axon distal to the injury
- Axon sprouts, or filaments, grow through a regeneration tube formed by Schwan cells
- The axon regenerates and a new myelin sheath forms
How many pairs of cranial nerves are there?
12
How many pairs of cranial nerves extend from the brain stem?
10
Cranial nerves
The nerves that extend from the brain. They serve the head and neck in general, but they each have their own specific function.
Cranial nerve 1
Olfactory nerve- sensory nerve. Function- sensory nerves associated with olfaction (smell).
Cranial nerve 2
Optic nerve- sensory. Function- sensory nerves associated with vision
Cranial nerve 3
Oculomotor nerve- motor nerve. Function- supplies 4 of the 6 extrinsic eye muscles that move the eyeball.
Cranial nerve 4
Trochlear nerve- motor nerve. Function- innervates extrinsic eye muscle (called the superior oblique) that loops through a pulley shaped ligament.
Cranial nerve 5
Trigeminal nerve- both sensory and motor. Function- supplies sensory fibers to the face and mucous membranes in the mouth and motor fibers to the chewing muscles
Trigeminal nerve damage
Damage affects how a person chews or if they are able to feel things in their mouth.
Cranial nerve 6
Abducens nerve- motor nerve. Function- controls extrinsic eye muscles that abducts the eye laterally (the lateral rectus muscle).
Cranial nerve 7
Facial nerve- both sensory and motor. Function- innervates muscle of facial expression, contributes to taste. It supplies the anterior two thirds of the tongue.
Cranial nerve 8
Vestibulocochlear nerve- sensory nerve. Function- contributes to hearing and balance (equilibrium).
Cranial nerve 9
Glossopharyngeal nerve- both sensory and motor. Function: Sensory- innervates the tongue for taste and general senses, innervates the pharynx for general senses.
Motor- innervates muscles of the pharynx for swallowing.
Cranial nerve 10
Vagus nerve- sensory and motor nerve. Function- fibers extend to and supply sensory and motor fibers to the thorax and abdomen.
Cranial nerve 11
Accessory nerve (spinal accessory nerve)- motor nerve. Function- supplies motor fibers to muscles that move the head and neck, also to the clavicular muscles.
Accessory nerve damage
Damage can affect the pectoral girdle.
Cranial nerve 12
Hypoglossal nerve- motor nerve. Function- innervates muscle found under the tongue to allow movement of tongue for chewing, speech, and swallowing.
Spinal nerves
Nerves that branch from the spinal cord. Supply all parts of the body except the head and neck (although some supply very specific structures on the head and neck).
How many pairs of spinal nerves are there?
31
How do nerves connect to the spinal cord?
Via dorsal (sensory) and ventral (motor) roots. Therefore, all spinal nerves have both sensory and motor functions (mixed nerves).
Dorsal roots influence which functions?
Sensory
Ventral roots influence which functions?
Motor
Spinal nerves divide to form
The dorsal ramus and ventral ramus. These are both sensory and motor in nature, in contrast to the roots
Dorsal ramus
Function- provides sensory and motor fibers to the skin and muscles of the back (your back, not the whole back of the body)
Ventral ramus
Function- provides sensory and motor fibers to lateral and ventral body walls and to the upper and lower limbs
Nerve plexus
Interlacing nerve networks formed by the ventral rami branching and joining.
Importance of nerve plexus formation
Each branch of the plexus contains fibers from multiple spinal nerves, and fibers from each ramus travel to body via several routes. This is advantageous because damage to one spinal segment or root can’t completely paralyze any limb muscle
Nerve plexuses in the body (3)
- Cervical plexus
- Brachial plexus
- Lumbosacral plexus
Cervical plexus
Formed from C1-C4 spinal nerves. Main function- cutaneous nerves that supply the skin of the neck, ear, back of the head, and shoulders
Brachial plexus
Formed from C5-C8, T1 spinal nerves, some fibers usually supplied by T2 and C4. Function- provides fibers that supply the upper body limbs. Contains 4 major groups
4 major groups of the brachial plexus
- Ventral rami (roots)
- Trunks
- Divisions (anterior and posterior)
- Cords- give rise to nerves that serve the upper limb
Lumbar plexus
Formed from L1-L4 spinal nerves. Function- innervates parts of abdominal wall muscle, major branches innervate anterior and medial thigh. The lumbar plexus and the sacral plexus have a large degree of overlap.
Sacral plexus
Formed from L4-S4 spinal nerves. Function- innervates buttocks and lower limbs, pelvic structures and perineum. The lumbar plexus and the sacral plexus have a large degree of overlap.
Lumbosacral trunk
A nervous band, joins the sacral spinal nerves to form the sacral plexus. Some fibers of lumbar plexus contribute to sacral plexus via the lumbosacral trunk.
Innervation of the anterolateral thorax and abdominal wall
Ventral rami in the thorax do not form plexuses- are arranged in segmental pattern. Arise from spinal nerves T1-T12. Function- serve intercostal muscles, skin of anterolateral thorax, and most of abdominal wall
Innervation of the back
Dorsal rami innervate posterior body trunk in segmental pattern. Function- each dorsal ramus innervates a narrow strip of muscle and skin at the same area where it emerges from the spinal column. Innervates the skin and muscle tissue immediately surrounding the dorsal rami
Dermatomes
Area of skin innervated by cutaneous branches of a single spinal nerve.
Dermatomes in the body trunk
In body trunk, dermatomes are mostly parallel and uniform in thickness
Dermatomes in the limbs
The upper limbs are supplied by ventral rami C5-T1. In lower limbs, lumbar serves most anterior surfaces, sacral serves most posterior surfaces. In the limbs, dermatomes may not be parallel. They might also be innervated by a single spinal nerve
Reflex arc
Response that enables rapid and predictable responses by the body. It is an automatic response by the body, not necessarily actively controlled. However, some are under some voluntary control.
Types of reflexes (2)
- Intrinsic reflex
2. Acquired reflex
Intrinsic reflex
Unlearned, unpremeditated, and involuntary, “built in” responses. You don’t think about the response before it’s initiated. This is a rapid, predictable motor response to a stimulus, but it can still be modified according to circumstance. For example, making fast adjustments to body posture after losing balance.
Which part of the nervous system sends out the message for an intrinsic reflex?
Generally, the spinal cord will send out the message and not the brain
Acquired reflex
This reflex results from practice or repetitions, “learned” responses. This is something you do over and over until it becomes automatic. Example- driving a car
Components of a reflex arc (5)
- Receptor
- Sensory neuron
- Integration center
- Motor neuron
- Effector
Reflex arc receptor
Site of stimulus action- where the stimulus is applied on the body
Reflex arc sensory neuron
Transmits afferent impulses to the CNS
Reflex arc integration center
Synapses found in the CNS, uses an interneuron. This is where the sensory impulse is interpreted and a motor (efferent) impulse is decided
Reflex arc motor neuron
Conducts efferent impulses from the integration center to the effector organ. Impulses could be excitatory or inhibitory
Reflex arc effector
Muscle fiber or gland that responds to the efferent impulse
Functional classification of reflexes (2)
- Somatic reflex
2. Autonomic reflex
Somatic reflex
Activate skeletal muscle tissue- most reflexes we will learn about are somatic reflexes
Autonomic reflex
Activate visceral effectors. Ex- cardiac muscle tissue, smooth muscle tissue, glands
Spinal reflex
Any somatic reflex that is mediated by the spinal cord. Most occur without any higher CNS involvement, but the brain is still aware it is happening
Types of spinal reflexes (3)
- Stretch and tendon reflexes
- Flexor and cross-extension reflexes
- Superficial reflexes
Stretch and tendon reflexes general function
Help the nervous system smoothly coordinate the activity of skeletal muscles during movement. Can be considered to be protective of damage in the body
Muscle spindles
Involved in stretch and tendon reflexes- provide information about the length of a particular muscle.
Proprioceptors (stretch and tendon reflexes)
Send information about the degree of muscle stretch. Composed of modified skeletal muscle fibers called intrafusal muscle fibers wrapped in a connective tissue fabric
What happens during a stretch and tendon reflex?
When stretched, muscle spindle sends impulses (action potentials) at a higher frequency to the spinal cord. During contraction, the impulses are sent at a slower frequency than they are at rest.
Stretch reflex
This reflex ensures that a muscle stays at its optimal length. Maintains muscle tone in large postural muscles and adjusts it reflexively. It does this by causing muscle contraction in response to increased muscle stretch
All stretch reflexes share which 2 features?
- Monosynaptic- have only a single synapse between the sensory neuron and a motor neuron, there is no interneuron
- Ipsilateral- located on the same side of the body
Knee jerk reflex
This is a type of stretch reflex. When one muscle contracts, the other relaxes- quadriceps strongly contract and tendon organs are activated, but efferent impulses to muscle with stretched tendon are dampened. This muscle relaxes, reducing tension
Tendon organs
Provide information about the amount of tension in a muscle and its associated tendons
In response to tension, muscles
Muscles relax and lengthen in response to tension. This helps prevent damage due to excessive stretch
Tendon reflex
A stretch reflex using tendon organs. Tendon reflexes are polysynaptic- involve multiple synapses with chains of interneurons. The leg at the knee will kick backward
Flexor reflex (withdrawal reflex)
Initiated by painful stimuli- causes automatic withdrawal of the body part from the stimulus source. Ipsilateral and polysynaptic.
Flexor reflex importance
Importance- protection/survival mechanism. However, the brain can override this reflex if we are expecting the pain (ex- painful prick to get a vaccine)
Crossed-extensor reflex
Often accompanies flexor reflex. Prevalent in weight bearing limbs- maintains balance as a result of the withdrawal reflex. Withdrawal reflex is ipsilateral, extensor reflex is contralateral
How does the crossed-extensor reflex occur?
Ex- stepping on a lego. The leg/foot that steps on the Lego has an ipsilateral withdrawal- you pick your foot up off the floor. Opposite leg has a contralateral extensor reflex- you shift your weight to the opposite side of your body. If the reflex wasn’t contralateral, you would probably end up falling over as a result of the withdrawal reflex
