The Peripheral Nervous System

Chapter 13-14

Nervous System Structural Overview

Peripheral Nervous System

• All neural structures outside of the brain and spinal cord.

• Includes sensory receptors, peripheral nerves, ganglia, and efferent motor endings.

Sensory Receptors• Respond to specific changes in their environment called stimuli

• General Sensory Receptors:

• Free nerve endings

• Tactile (Merkel) discs

• Hair follicle receptors

• Tactile (meissner’s) Corpuscles

• Lamellar (pacinian) corpuscles

• Ruffini Endings

• Muscle Spindles

• Tendon organs

Somatosensory Neural Integration

• Three main levels of neural integration in the sensory system

• Receptor Level

• Circuit Level

• Perceptual Level

Processing at the Receptor Level

• Step 1: Stimulus excites receptor and action potentials reach the CNS

• Stimulus must match the specificity of the receptor

• stimulus must be applied within the receptor field

• stimulus must be converted into a graded potential (transduction)

• Adaptation

Processing at the Circuit Level

• Step 2: Impulses must be delivered to the appropriate region of the cortex for localization and perception of the stimulus.

Processing at the Perceptual Level

• Step 3: Sensory input is interpreted in the cerebral cortex

Perception of Pain

• Activated by extremes of pressure or temperature

• Sharp, then aching/burning. A delta, c fibers

• Based on situation

• Pain threshold versus pain tolerance

Nerves

• Cord-like organs composed of bundles of axons.

• Connective tissue wrappings:

• endoneurium, perineurium (fascicles), epineurium.

• Nerves classified by transmission direction:

• Sensory nerves, motor nerve, mixed nerves.

Peripheral Motor Endings

• The Neuromuscular Junction

Motor Integration

• Levels of Motor Control:

• The Segmental Level (reflexes, CPG’s)

• The Projection Level (initiate voluntary movement, oversee the segmental level).

• The Precommand Level (Cerebellum, basal nuclei: coordination, timing, start/stop)

Reflex Activity

• The Reflex Arc

• inborn or learned

Components of a Reflex Arc1. Receptor

2. Sensory Neuron

3. Integration Center

4. Motor Neuron

5. Effector

• Can be either somatic or autonomic

Spinal Reflexes

• Stretch Reflex

Spinal Reflexes

• The Tendon Reflex

Spinal Reflexes

• The Flexor and Crossed Extensor Reflexes

Superficial Reflexes

The Autonomic Nervous System

ANS Divisions and Roles

• Parasympathetic Division

• Rest and Digest Division

• Keeps body energy as low as possible

• Sympathetic Nervous System

• Fight or Flight System

• Prepares body for emergency situations

Comparison of Somatic NS and ANS

ANS Anatomy• Dual Innervation

• Sites of Origin

• Relative lengths of their fibers

• Location of their ganglia

ANS Anatomy• Parasympathetic (craniosacral)

Division

• Cranial part

• Occulomotor N.

• Facial N.

• Glossopharyngeal N.

• Vagus N. (Multiple Plexi)

• Sacral Part

• Pelvic Splanchic n.

• Inferior hypogastric plexus

ANS Anatomy

• Sympathetic (Thoracolumbar division)

• Preganglionic fibers arise from Lateral Horns of segments T1-L2

ANS Anatomy

• Sympathetic trunks and pathways

3 Pathways of Synaptic Innervation

• Synapse at the Same Level

3 Pathways of Synaptic Innervation

• Synapse at a higher or lower level

3 Pathways of Synaptic Innervation

• Synapse in a distant collateral ganglion anterior to the vertebral column

ANS Physiology• Neurotransmitters and

Receptors

• Acetylcholine

• Released by all ANS Preganglionic axons and all parasympathetic postganglionic axons (cholinergic fibers)

• Norepinephrine

• Most sympathetic postganglionic axons (adrenergic fibers)

Cholinergic Receptors

• Nicotinic Receptors

• Found on sarcolemma of skeletal fibers, all postganglionic neurons, adrenal medulla

• Muscarinic Receptors

• Parasympathetic target organs

Adrenergic Receptors

• alpha and beta receptors

• alpha 1, 2, Beta 1, 2, 3.

• can be either excitatory or inhibitory

Pharmacological Effects

• Atropine (anticholinergic)

• Neostigmine (anticholinesterase)

• B2 activators (dilate airways)

• Beta Blockers (slow heart rate, lower BP)

Interactions of the Autonomic Divisions

• Dual Innervation

• Antagonistic Interactions

• Sympathetic and Parasympathetic Tone

• Unique roles of the Sympathetic Division

• thermoregulatory response to heat, renin release, metabolic effects

• Localized Versus diffuse effects

Control of Autonomic Function

• Brain Stem and Spinal Cord controls

• Hypothalamic Controls

• Cortical Controls