Overview of the Peripheral Nervous System

Sensory Receptors

Respond to information from the environment- stimuli. Sensation & perception occur in the brain.

Mechanoreceptors

Respond to touch, vibration, stretch (force).

Thermoreceptors

Respond to temperature change.

Photoreceptors

Respond to light (retina).

Chemoreceptors

Respond to chemical changes (blood).

Nociceptors

Respond to damaging stimuli causing pain.

Exteroceptors

Stimuli outside the body.

Interoceptors

Stimuli inside the body.

Proprioceptors

Stimuli inside the body.

Perceptual detection

Ability to detect that stimulus has occurred.

Magnitude estimation

Ability to detect how intense the stimulus is.

Spatial discrimination

Ability to detect the site or pattern of stimulation.

Feature abstraction

Neurons tuned into one feature of a stimulus over all others.

Quality discrimination

Ability to differentiate the sub-modalities.

Pattern recognition

Ability to differentiate patterns in our world.

Pain receptors

Activated by histamine, ATP, acids, K+, etc.

Referred Pain

Visceral pain afferents along the same pathways as somatic pain.

Nerves

Bundles of axons.

Nerve Classification

Mixed- sensory & motor fibres. Conduct impulses both to and from CNS.

Afferent neuron cells

Contained in dorsal root ganglia.

Endoneurium

Connective tissue wrapping around individual axons.

Perineurium

Connective tissue wrapping around a bundle of axons.

Epineurium

Connective tissue wrapping around the entire nerve.

Afferent neuron

Neuron cells contained in dorsal root ganglia.

Autonomic motor neurons

Efferent neurons located in the ventral root.

Nerve Repair

Mature neurons do not generally repair; PNS axons may repair under right circumstances, while CNS axons are viewed as irreparable.

Cranial Nerves

12 pairs of cranial nerves are associated with the brain, with two attaching to the forebrain and the rest to the brain stem.

Cranial Nerves numbering

Each cranial nerve is numbered (I through XII) and named from rostral to caudal.

Cranial Nerves sensory vs motor

Most cranial nerves are mixed nerves, but two pairs are purely sensory.

Cranial Nerves mnemonic

Mnemonic: 'On occasion, our trusty truck acts funny—very good vehicle anyhow.'

Spinal Nerves

31 pairs of spinal nerves exist, including 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves.

Spinal Nerves connection

Spinal nerves connect to the spinal cord via dorsal/ventral roots.

Ventral roots

Ventral roots contain motor fibers that innervate skeletal muscles and autonomic functions.

Dorsal roots

Dorsal roots contain sensory fibers from dorsal root ganglia.

Nerve plexuses

All ventral rami join to form nerve networks called nerve plexuses, which occur in cervical, brachial, lumbar, and sacral regions.

Cervical Plexus

Located under the sternocleidomastoid muscle, contains nerves from C1-C4, with the phrenic nerve being the most important.

Phrenic nerve

Arises from C3-C5 and innervates the diaphragm.

Brachial Plexus

Situated in the neck and axilla, contains nerves from C5-C8 and T1.

Lumbar Plexus

Overlaps with the sacral plexus and arises from nerves L1-L4.

Femoral nerve

Innervates the quadriceps group (motor) and the skin of the anterior thigh and medial leg from knee to foot.

Sacral Plexus

Arises from nerves L4-S4 and innervates the buttock, lower limb, pelvis, and perineum.

Sciatic nerve

The thickest and longest nerve, supplies the entire lower limb except for the anteromedial upper leg, and divides into tibial and common fibular nerves.

Tibial Nerve

Supplies posterior leg muscles and skin of the posterior calf and sole of the foot.

Common Fibular Nerve

Divided into superficial and deep branches. Innervates knee joint. Innervates skin of anterior and lateral leg. Innervates dorsum of foot.

Dermatomes

Represents area innervated by single spinal nerve. Mapping dermatome allows clinician to determine exact nerve injury.

Reflex Classification - Intrinsic

Automatic. Pulling arm back from burning stove. Provides protection.

Reflex Classification - Acquired

Learned. Driving a car. Comes from practice and repetition.

Reflex Classification - Somatic

Activate skeletal muscle.

Reflex Classification - Visceral

Activate smooth/cardiac muscle or glands.

Stretch Reflex

Purpose: to aid NS in coordinating activity of skeletal muscles. Obtains information about muscle from muscle proprioceptors.

Autonomic Nervous System

Stabilizes internal environment via a system of motor neurons that innervate smooth and cardiac muscles as well as glands.

Sympathetic Nervous System (SNS)

Responsible for exercise, excitement, emergency, and embarrassment.

Parasympathetic Nervous System (PSNS)

Responsible for digestion, diuresis, and defecation.

Key Differences - Sympathetic

Nerve fibres originate in thoracic/lumbar regions of spinal cord. Short preganglion. Long postganglion. Ganglia are close to spinal cord. Long lasting and widespread effect.

Key Differences - Parasympathetic

Nerve fibres originate in brain/sacral region of spinal cord. Long preganglion. Short postganglion. Ganglia are close to effector sites. Localized and short lived effect.

Acetylcholine (ACh)

Released by all ANS preganglionic axons and all Parasympathetic ANS postganglionic axons (at synapses with their effectors).

Cholinergic Fibers

ACh-releasing fibers.

Norepinephrine (NE)

Released by sympathetic postganglionic axons. Classed as adrenergic fibers.

Adrenergic Fibers

Fibers that release norepinephrine.

Hypothalamus

Gives order to lower CNS structures for regulation (i.e. medulla oblongata). Regulates heart activity, blood pressure, body temperature, water balance, endocrine activity, and fear (with limbic system).

Dual Innervation

PSNS & SNS serve same organs but generally have opposite effects.