Peripheral Nervous System

difference between sensation and perception

sensation - awareness of change in the internal and/or external environment

perception - brains conscious interpretation and processing of sensory information

peripheral nervous system

PNS - “link to the outside world”, enables communication between CNS and body, coordinates actions in response to stimuli

• subdivisions:

  • autonomic nervous system - heart muscle, smooth muscle, glands

  • somatic nervous system - voluntary skeletal muscles

  • parasympathetic nervous system - rest and digest

  • sympathetic nervous system - fight or flight

components of PNS

sensory receptors - specialized cells that detect and respond to changes in the internal and external environment

• creates a graded potential that triggers a nerve impulse

• classified by:

  • type of stimulus

    • mechanoreceptors - detect mechanical forces

    • photoreceptors - respond to light

    • thermoreceptors - changes in temperature

    • nociceptors - react to harmful stimuli

    • chemoreceptors - react to chemical changes (food, air, environment)

  • body location

    • exteroceptors - found on the body surface, detect external stimuli

    • interoceptors - found inside body, detect internal stimuli (BP, O2)

    • Proprioceptors - found in muscles, provide info on body position + movement, sense where limbs are without looking

  • structural properties

    • general senses - detect basic sensations (touch, temp, pain, pressure, body position), simple receptors, found throughout body

    • special senses - detect specific sensations (vision, hearing, taste, smell), specialized receptors, dedicated to sensory organs

General structure of a nerve

nerve - cordlike organ composed of axons, some myelinated and some unmyelinated, which are responsible for transmitting electrical signals

• endometrium - thin connective tissue, surrounds each nerve fiber (axon)

• perineurium - course connective tissue, wraps bundles of nerve fibers (fascicles)

• epineurium - thick, outer layer of connective tissue, encases the entire nerve, consisting of many fascicles

cranial nerves

(I) Olfactory(sensory) - responsible for the sense of smell,

• originates in olfactory receptors in the nasal cavity, terminates in primary olfactory cortex

(II) Optic(sensory) - transmits visual information from retina to brain,

• originates in the retina of the eye, terminates in primary visual cortex

(III) Oculomotor(motor) - controls eye movement (up,down, medial), pupillary constriction and lens focusing

• originates in most extraocular muscles, the pupil and the ciliary muscle

(IV) Trochlear (motor) - controls downword and lateral movement

• originates in the superior oblique muscle of the eye

(V) Trigeminal (mixed) - provides sensory input from face (sensory), controls chewing muscles (motor)

• innervates the forehead, face, and jaw

(VI) Abducens (motor) - controls lateral eye movement

• lateral rectus muscle of the eye

(VII) Facial (mixed) - controls facial expressions (motor), taste buds on anterior 2/3 of tongue (sensory)

• five major branches: temporal(forhead+eyebrows), zygomatic (eyes+eyelids), buccal (cheek+upper lip), mandibular (lower lip +chin), Cervical(neck)

  • bells pasley - paralysis of the facial nerve on affected side and partial loss of taste sensation

(VIII) Vestibulocochlear (sensory) - responsible for hearing and balance

• cochlear division - the cochlea of the ear (hearing

• vestibular division - semicircular canal (balance)

(IX) Glossopharyngeal (mixed) - swallowing and stimulates salivation (motor), taste on posterior 1/3 of tongue (sensory)

• originates in medulla oblongata

• terminates in pharynx, posterior tongue and parotid salivary glands

(X) Vagus (mixed) - regulates autonomic functions (heart, lungs, digestion) (motor), provides taste controls muscles for swallowing and speaking (sensory)

• originates in the medulla oblongata

• terminates in various organs (pharynx, larynx, heart, lungs, digestive organs)

(XI) Accessory (motor) - controls muscles for head movement and shoulder shrugging

• originates in medulla oblongata

• terminates in the sternocleidomastoid and trapezius muscles

(XII) Hypoglossal (motor) - controls tongue movements for speech, swallowing, and eating

• originates in medulla oblongata

• terminates in musculature of the tongue

Major plexus and distribution + function of the peripheral nerves arising from each plexus

Nerve Plexus - ventral rami come together to form a complex network of nerves

• allow for flexible and coordinated nerve distribution

  • roots - the nerve exits the spinal cord

    • dorsal root - carries information to spinal cord

    • ventral root - carries information from spinal cord to muscles

  • rami - the nerve branches, once it has left the spinal cord

    • dorsal rami - go to back of body

      • innervate muscles and skin along back

    • ventral rami - go to front of body

      • innervate limbs and anterior trunk

1. Cervical plexus + neck - network of nerves fromed by the ventral rami of the first four cervical spinal nerves (c1-c4)

   • sensory - innervation to skin of neck, upper chest shoulders, and part of head

   • motor - control to muscles and the diaphragm

   • important nerve - phrenic nerve - innervates diaphragm, crucial for breathing

2. Brachial Plexus + upper limbs - network of nerves formed by the ventral rai of the c5-t1

   • motor - muscles of shoulder, arm, forearm, and hand

   • sensory - sensation to skin of shoulder, arm, forearm, and hand

   • important nerves

     • musculotaneous - controls muscles in upper arm

     • axillary - innervates the deltoid and teres minor (shoulder movement)

     • median - muscles in forearm and hand, important for grasping and fine motor skills

3. Lumbar Plexus + lower limbs - network of nerves formed by the ventral rami of the l1-l4

   • motor - muscles in anterior and medial thigh, parts of abdomin and pelvis

   • sensory - sensations to skin of lower abdomen, groin and upper thigh

   • important nerves

     • femoral - largest terminal nerve in lumbar plexus, innervates anterior thigh

     • obturator - innervates adductor muscles in th medial thigh

4. Sacral plexus - network of nerves formed by the ventral rami of the l3-s4

   • motor - muscles in buttocks, pelvis, and lower limbs

   • sensory - sensation to the skin of buttocks, genital regions and posterior thigh

   • important nerves

     • sciatic - largest nerve in the body

     • innervates posterior thigh

     • all muscles in lower leg/foot except those supplied by femoral and obturator

Components of a reflex arc and distinguish between autonomic and somatic reflexes

relfex arc - neural pathway that a reflex follows from stimulus to response

• involves a simple, involuntary circuit

1. receptor - detects stimulus

2. sensory neuron - transmits afferent signal to CNS

3. integration center - processing center where information is interpreted (brain or spinal cord)

4. motor neuron - conducts/carries efferent impulse to effector organ

5. effector - muscle or gland that responds and carries out response

autonomic vs. somatic:

• autonomic - involuntary, involve smooth muscle, cardiac muscle, or glands

  • ex. heart rate or digestion

• somatic - voluntary, involves skeletal muscle

  • ex. knee jerk

spinal reflexes

1. Monosynaptic reflex - involves one synapse between sensory and motor neurons

   • muscle is stretched causing an immediate contraction of the same muscle to resist the stretch

   • maintains posture and balance

2. Flexor reflex - “withdrawal reflex”, polysnaptic reflex, causes rapid contraction of flexor muscles to withdraw a body part from a painful stimulus

   • protects from harmful stimuli

   • avoiding injury

3. Superficial reflex - cutaneous reflex involving the skin/ mucous membranes as the receptor

   • protective reflexes

   • helps with coordinated movements