Ch. 13 The PNS

The Spinal nerve has two branches:

Posterior Ramus: Travels to posterior side of body Anterior Ramus: Travels to anterior side of body and/or to an upper or lower limb

What are Reflexes?

Programmed, automatic responses to a stimulus

What are muscle spindles?

Receptors found scattered among regular contractile muscle fibers

Golgi Tendon Organs

Receptors located within tendons near muscle-tendon junction

What do Secondary Afferents respond to?

To both static length of a muscle and position of a limb

What are Receptive Fields?

Areas served by a particular neuron and neurons with more branches innervate larger receptive fields -Body regions whose primary function is sensing environment (fingertips) contain many neurons with smaller receptive fields -Body regions that are not as involved in sensing environment (skin of forearm) have fewer neurons with larger receptive fields

Epineurium

Outermost layer of connective tissue that holds motor and sensory axons together

Superficial somatic pain

Pain that arises from stimulation of receptors in the skin

There are 31 pair of spinal nerves

-8 pairs of cervical nerves -12 pairs of thoracic nerves -5 pairs of lumbar and sacral nerves -1 pair of coccygeal nerves *Anterior Rami of cervical, lumbar, sacral spinal nerves each merge to form complicated networks of nerves called nerve plexuses (plexi)

Visceral Sensory divison detects:

-Internal information (i.e. blood pressure) from organs of abdominopelvic and thoracic cavities

*Motor Cranial Nerves Five Cranial nerves contain primarily axons of motor neurons:

-Oculomotor (III) - Moves eyeball, opens the eye, constricts the pupil, changes lens shape -Trochlear (IV) - Moves the eye up & down -Abducens (VI) - Moves the eyeball laterally -Accessory (XI) - Used in muscles involved in speech, controls SCM & trapezius muscles -Hypoglassal (XII) - Moves the tongue

*Sensory Cranial Nerves Three cranial nerves contain axons of only sensory neurons:

-Olfactory (I) - Sense of smell -Optic (II) - Sense of vision -Vestibulocochlear (VIII) - Senses of hearing and balance

How does the PNS work with the CNS?

-PNS detects sensory stimuli and delivers information to the CNS as sensory input. -CNS processes input and transmits impulse through PNS to muscle cells and glands as motor output *Remember that Cranial and Spinal nerves are part of the PNS

What is the Autonomic (visceral) Nervous system responsible for?

-Responsible for maintaining many aspects of homeostasis by controlling involuntary motor functions in the body. -Neurons innervate cardiac muscle cells, smooth muscle cells, and secretory cells of glands

What is the Somatic nervous system responsible for?

-Responsible for voluntary motor functions -Composed of lower motor neurons (somatic motor neurons) which directly trigger skeletal muscle contractions

*Mixed Cranial Nerves Four cranial nerves contain axons of both sensory and motor neurons:

-Trigeminal (V) - Facial sensations/Muscles of chewing -Facial (VII) - Taste/Facial expressions and salivation -Glossppharyngeal (XI) - Taste/Swallowing and salivation -Vagus (X) - Taste/Speaking and swallowing, etc.

Somatic Sensory division detects:

-internal and external stimuli -General sensory receptors detect stimuli from skin -Special sensory receptors detect stimuli from special sense organs

Steps to Sensory Transduction (3)

1. Before any stimulus arrives, the ion channels in the axolemma of the somatic sensory neuron are closed. 2. When a stimulus such as pressure is applied, mechanically gated Na+ channels open and Na+ enter the axoplasm, generating a receptor potential 3. If enough Na+ enter that the membrane potential reaches threshold, voltage-gated Na+ channels open. This triggers an action potential, which will be propagated along the axon to the spinal cord.

What are the three steps of a Reflex Arc?

1. PNS detects and delivers stimulus to CNS 2. CNS integrates stimulus 3. PNS delivers motor response from CNS to effectors -Reflexes begin with a sensory stimulus and finish with a rapid motor response

Steps of Sensation (4)

1. Stimuli (inside or outside body) are detected by sensory receptors and are converted into action potentials (sensory transduction) 2. Action potentials are propagated to the CNS by nerves 3. Nerve tracts convey action potentials to the cerebral cortex and other areas of the CNS 4. The cerebral cortex "translates" the signal and makes the individual aware of the stimulus

What is the Two-point discrimination threshold?

A method for measuring relative size of receptive fields

Mixed Nerves contain...

BOTH sensory and motor neurons

Peripheral Nervous System links the __________ to the rest of the body and to the external environment.

Central Nervous system (CNS)

What is the Motor (efferent) Division responsible for?

Consists of motor (efferent) neurons that carry out motor functions of the nervous system using the Somatic nervous system and the Autonomic (visceral) nervous system (ANS)

What is the Sensory (afferent) Division responsible for?

Consists of sensory neurons that detect and transmit sensory stimuli to the CNS using the somatic sensory division and the visceral sensory division.

What is a Polysynaptic Reflex?

Involved multiple synapses and usually involve the brainstem (i.e. gag reflex)

What is a Monosynaptic Reflex?

Involves only a single synapse within the spinal cord between a sensory and motor neuron (i.e. patellar reflex)

What are Peripheral Nerves?

Main organs of PNS that consists of axons of many neurons bound together by connective tissue -Nerves of PNS innervate (contact) the majority of structures in the body -3 types of nerves (Sensory, Motor, and Mixed)

Mechanoreceptors can be further classified into...

Merkel Disc- Detect discriminative touch stimuli (object form and texture) in fingertips Meissner (Tactile) Corpuscles- Detect discriminative touch stimuli in dermal papillae Ruffini Corpuscle- Respond to stretch and movement from dermis, hypodermic and ligaments Pacinian Corpuscles- Detect deep pressure Hair Follicle Receptors- Cause hair to bend Proprioceptors- Detect movement and position of a joint or body part

Motor Nerves contain...

Mostly motor neurons with a few sensory neurons involved in muscle stretch and tension

Sensory Nerves contain...

ONLY sensory neurons

Spinal Nerves

Originate from the spinal cord and innervate structures below the head and neck -Anterior Root consists of motor neurons -Posterior Root consists of sensory neurons -Posterior root includes the posterior root ganglion that contains the cell bodies of sensory neurons -Posterior and anterior roots fuse to form the 31 pairs of mixed spinal nerves

How are Sensory Receptors classified?

Sensory Receptor forms: -Encapsulated nerve endings are surrounded by specialized supportive cells -Free nerve endings lack supportive cells Sensory Receptor locations: -Exteroceptors: detect stimuli from outside body -Interoceptors: detect stimuli from within body -Mechanoreceptors: Respond to anything that mechanically deforms tissue including vibration, light tough, stretch, and pressure -Thermoreceptors: Respond to hot or cold temperature changes -Chemoreceptors: Responde to specific chemical -Photorecpetors: Respond to light -Nociceptors: Respond to noxious stimuli (pain)

Fascicles

Small groups of bundled axons surrounded by connective tissue called perineurium -Each individual axon within a fascicle is surrounded by its own connective tissue called endoeurium

Deep somatic pain

Stimulation of receptors in skeletal muscles, joints, tendons, and fascia causes...

What do Primary Afferents respond to?

Stretch when it is first initiated

What are the divisions of the PNS?

The PNS is divided into the Sensory (afferent) and the Motor (efferent) divisions. -The Sensory division is further divided into the somatic sensory and visceral sensory divisions. -The Motor division is further divided into the somatic nervous and autonomic nervous systems. -The Autonomic nervous system is further divided into the Sympathetic, Parasympathetic and Enteric Nervous system.

Sensation (Perception)

The conscious awareness of the effects of stimuli received by sensory receptors -Sensations are the result of action potentials from sensory receptors reaching the cerebral cortex

How many pair of cranial nerves are there?

There are 12 pair -They are part of the PNS, but attach to the inferior surface of the brain

Lumbar Plexuses:

innervate pelvic structures and the lower extremities

Sacral Plexuses:

innervate structures of pelvis gluteal region, and the lower extremities

Cervical Plexuses:

innervate the neck and sections of the head, chest, and shoulders and the diaphragm

Brachial Plexuses:

innervate the upper limbs

Rapidly Adapting Receptors...

respond rapidly with high intensity to stimuli but stop sending signals after a certain time period (called Adaptation)

Slowly Adapting Receptors...

respond to stimuli with constant action potentials that don't diminish over time