The Peripheral Nervous System

Dr. Anderson GCIT

Overall Organization

• Sensory Receptors

• Peripheral Nerve tracts

• Ganglia

• Efferent Motor Nerves

Sensory Receptors

• Can be classified by:

• Stimulus Type

• Body Location

• Structural Complexity

Classification – Stimulus Type

• Mechanoreceptors – • Thermoreceptors –

• Photoreceptors –

• Chemoreceptors –

• Nocireceptors -

Receptor Classification - Location

• Exteroreceptors

• Interoreceptors

• Proprioreceptors – advise the brain of stretch in joints, muscles, tendons and ligaments (relay body position)

Receptor Classification - Structure

• Simple receptors – modified dendritic ending of sensory neurons• Nociceptors• Proprioreceptors

• Complex receptors – actually make up sensory organs • eye, inner ear, taste buds, olfactory receptors

Simple Receptor Structure

• Unencapsulated Dendritic Endings • Located in dermis and epithelial tissue

• Respond to heat, cold, pain, itch

• Merkel cells – associate with large epidermal cells and detect light touches

• Hair follicle receptors – activate upon movement of hairs on the body• How is this adaptive?

Simple Receptor Structure

• Encapsulated Dendritic Endings• A connective tissue capsule surrounds the nerve fiber terminal• Most are mechanoreceptors

• Examples• Meissner’s corpuscles – very sensitive touch receptors in hairless skin (fingertips,

soles of feet)• Pacinian Corpuscles – respond to pressure deep in the skin and subcutaneous

tissue• Muscle Spindles – detect stretch in muscles

Dermal receptors

Sensation to Perception

1. The proper stimulation (degree and proximity) is needed to get the receptor to depolarize

2. Once depolarized, the nerve will “fire” relative to the strength of the stimulus

Higher frequency for stronger stimuli

3. The action potential is then transmitted to second-order neurons, eventually leading to the hypothalamus and the somatosensory cortex

Perception of Impulses• Perception of a stimulus

depends solely on the area of the brain that is stimulated• E.g. stimulating the visual

cortex – light is perceived

• Any stimulation that can activate a sensory area may result in a perceived “sensation”

Factors Affecting Perception• Perceptual Detection – ability to detect a stimulus (minimum stimulation

needed)

• Magnitude Estimation – ability to detect the magnitude of the stimulus

• Spatial Descriminiation – sensory “acuity”, being able to determine the difference between different aspects of a sensation

• Quality discrimination – differentiating the modalities of a sense (e.g. does this taste salty or sweet?)

• Pattern recognition – arranging shapes into something familiar (e.g. pointalism, recognizing notes in a melody)

Nerve Structure

• Made of parallel bundles of axons, wrapped by connective tissue

• Groups of fibers (fascicles)• Fascicles wrapped by perineurium• Groups of fascicles wrapped by epineurium

Spinal Nerve Structure

Nerve Function

• Afferent – sensory impulses only travel to brain

• Efferent – Motor impulses travel from brain to muscles or glands

• Mixed – may contain nerves that contain both afferent and efferent nerves

Dermatomes

• Area of skin innervated by the cutaneous branch of a spinal nerve

• Used to assess the level of spinal injury

Motor Activity

• Muscles are innervated by neuromuscular junctions

• Glands and smooth muscles are innervated by nerve endings called varicosities

• Neurotransmitters that are released act as secondary signals

Levels of Motor Activity• Segmental Level – spinal cord circuits • Crude movements (walking, running, etc.) are regulated by central pattern

generators which are made of a network of oscillating neurons

• Projection Level – upper motor neurons (motor cortex) to lower motor neurons (spinal and peripheral efferent nerves)

• Precommand Level – Cerebellum and basal nuclei • Precisely start, stop and coordinate movements that are planned in the

cerebrum

Levels of Motor Control

Reflex Activity

• Stretch reflex• Keeps muscles at a

certain length

• Flexor reflexes• Initiated by pain

The Autonomic Nervous System• Instead of influencing skeletal muscles, the ANS influences smooth

muscle and glands

• ANS neurons release neurotransmitters in the target tissue• Acetylcholine (excitatory)• Norepinephrine (inhibitory)• However, the effects above are highly dependent on certain cell receptors of

the target tissue

Parasympathetic Division

• Responsible for maintaining the resting “housekeeping” rate of metabolism and organ activity

• Examples?

Sympathetic Division

• The “fight or flight” system

• Generally serves to increase smooth, cardiac muscle and gland activity• Heart rate• Vasoconstriction• Liver releases glucose into blood• Airways of lungs dilate

• Why is this adaptive?

Special Senses

• Vision• Hearing• Taste• Smell• Touch

• All of the sensory organs (besides skin receptors) are located in the head

The Eye• Entire structure has evolved for:

• Capturing light• Focusing light• Converting light into action

potentials• Transmitting these action

potentials to the visual cortex to be interpreted by the visual association area

External Anatomy

• Eyes must be kept clean and moist to facilitate transmission of light

• Eyebrows and eyelashes prevent debris from injuring the eye

• Tears lubricate the eye to facilitate movement and prevent infection

Light Transmission

• The iris is part of the eye containing pigments and alters the aperture (the amount of light coming in)

• The pupil is the aperture made by the iris

• Pupil size will vary due to amount of available light

Conjunctiva• Layer of epithelial cells that line the inside of the eyelids and covers

the sclera (white) of the eye

• Like many exposed mucus membranes, are prone to infection!

Normal Infection (Conjunctivis)

Trachoma

• Caused by Chlamydia trachomatis bacteria

• Spread by flies in arid environments!

• Leading cause of blindness in children under 10 worldwide

• WHY?

Eye Internal Anatomy

Cornea

• Clear covering of epithelial tissue that covers the front of the eye (surrounded by sclera)

• The curved nature of this structure helps to focus light

The Iris• Composed of fibrovascular tissue and epithelial tissue (both

pigmented)

• The periphery of the iris is attached to sphincter muscles (the ciliary muscles or ciliary body)• Controls pupil diameter by the autonomic nervous system

• Try it!

• Problems with dilation or contraction are often signs of neurological impairment

Iris Coloration

• Almost solely due to differences in melanin expression

Eye Shape

• In order for the eye to keep it’s shape• The arrangement of connective tissue in the sclera (white)• The fluid inside the eye

• This dictates how light will pass through to the retina to be focused

Light Focusing

Retina• Layer of photosensitive cells • Cones – sensitive to bright light and colors• Rods – Sensitive to dim light (no color)

• Impulses are sent to the optic nerve, and eventually travel to the brain (visual cortex and visual association area)

• Under the retina is the choroid the vascular layer of the eye

Taste and Smell

• Based on chemoreceptors that are triggered by compounds in the• Air (smell)• Saliva (taste)

Olfaction (Smell)

• Chemicals dissolved in the air are detected by the olfactory epithelium in the roof of the nasal cavity

• How do we determine the difference between the chemicals that we smell?

Taste• Made possible by gustatory

receptors on the tongue (taste buds)

• Located on the papillae (bumps) on the tongue

• Microvilli (small cellular projections) of receptor cells extend through a pore to the surface of the tongue

Taste• Taste (proper) is divided into

discrete zones on the tongue

• What causes these areas to be sensitive to these different chemical properties?

• Why are these specific taste centers important?

Trickery•Why might we want to “trick” our olfactory/gustatory receptors?

Hearing• Collects and converts sound into

action potentials

• Inner ear is also responsible for the sense of balance and equilibrium

Eardrum and Ossicles• The eardrum is located at the

“bottom” of the ear canal

• Sound entering the ear vibrates the eardrum, transferring vibrations to the ossicles

• The 6 ossicles (the smallest bones in the body) transfer sound to the middle ear