Anatomy and Histology

Unencapsulated: 

1. 1)  Free nerve endings

2. 2)  Merkel cells/tactile discs

3. 3)  Root hair plexi

Papillary layer of the dermis Light touch, pressure, movement Tonic, small receptive field 

Unencapsulated: 

1. 1)  Free nerve endings 

2. 2)  Merkel cells/tactile discs 

3. 3)  Root hair plexi 

Papillary layer of the dermis Light touch, pressure, movement Tonic, small receptive field 

Unencapsulated: 

1. 1)  Free nerve endings 

2. 2)  Merkel cells/tactile discs 

3. 3)  Root hair plexi 

Papillary layer of the dermis Light touch, pressure, movement Tonic, small receptive field 

Encapsulated:

1) Meissner’s/tactile corpuscle

Eyelids, fingertips, lips, nipples, genitalia Coiled, interwoven dendrites covered by Schwann cells in a fibrous capsule (ø 40-60μm) Light touch, movement, vibration

Fast adaptation

2) Ruffini corpuscle

Dermis
Dendrites interwoven with collagenous fibres extending into the dermis
Pressure and distortion of the skin
Slow adaptation
3) Pacini/lamellated corpuscle
Dermis, fingers, breasts, genitalia, viscera Dendrites shielded in concentric cell layers Large, ca. 1mm long
Deep pressure, vibration and pulsing
Fast adaptation 

Encapsulated: 

2) Ruffini corpuscle 

Dermis
Dendrites interwoven with collagenous fibres extending into the dermis
Pressure and distortion of the skin
Slow adaptation

Encapsulated:

Pacini/lamellated corpuscle
Dermis, fingers, breasts, genitalia, viscera Dendrites shielded in concentric cell layers Large, ca. 1mm long
Deep pressure, vibration and pulsing
Fast adaptation 

• Monitor the position of joints, tendons, ligaments, muscles

• Integrated with vestibular system information

• Muscle spindles: muscle length

• Golgi organs: tendon tension

• Free nerve endings: joint tension,

• Pressure, movement, stretching 

• Stretch receptors in organ walls

• Free nerve endings branching within an elastic wall of a hollow organ

• Stretching distorts dendritic branches

• Alteration of action-potential generation 

Fungiform papillae: ca. 5 taste buds 

Circumvallate papillae: ca 100 taste buds 

Filiform papillae: no taste buds 

Types of Receptors

Classification by location

• Exteroceptors: located on or near body surface. Respond to stimuli that arise external to the body. Detect pressure, touch, pain and temperature

• Internoceptors or Visceroceptors: located internally. Receive stimuli from the internal body environment. Activated by pressure, stretching and chemical changes. Also mediate hunger and thirst.

• Proprioceptors: specialized type of visceroceptors. Located in skeletal muscle, joint capsules and tendons. Information of body movement, spatial orientation and muscle stretch 

1. Classification by structure

   • Free nerve endings

     • Simplest, most common

     • Terminates in dendritic knobs

   • Encapsulated nerve endings

     • Connective tissue capsule surrounding their terminal or dendritic end

     • Primarily mechanoreceptors 

Classification by stimulus detected

• Nociceptors: Pain

  • Location: skin, joints and bone periostea, blood vessels, some viscera

  • Free nerve endings, large receptive field (making localizing the source difficult)

  • Thermo, mechano and chemoreceptors

  • Fast pain: “stabing” pain perceived in primary sensory cortex, triggers somatic reflexes

  • Slow pain: “aching” pain activates the thalamus and reticular formation

• Thermoreceptors: Temperature

  • Location: skin dermis, skeletal muscle, liver, hypothalamus

  • Cold and warm receptors, phasic, i.e. adaptation

  • Same pathways as nociception to the primary sensory cortex, thalamus and reticular formation

• Chemoreceptors: Chemical compounds

  • Location: respiratory centres (pH and Pco2), carotid bodies (pH, Pco2, Po2),

    aortic bodies (pH, Pco2, Po2)

  • Water and lipid soluble compounds

• Mechanoreceptors: Touch

• Photoreceptors: Light - 

  • Only in the eye 

Hornhaut (Augen)

• Two components: sclera and cornea

• Provides: structure, protection, points of attachment

• Tough fibro-elastic capsule

• Sclera = opaque, dense

  fibrous connective tissue,

  the “whites” of the eye

• Provides attachment for 6

  extra-ocular muscles

• Nerves and vessels

  penetrate the anterior surface (i.e. blood-shot eyes)

• Corneal limbus: separates sclera and cornea 

Three components: choroid, ciliary body, iris 

Cuboidal epithelium

1. Pigmented epithelial cells –single layer

2. Photoreceptor layer - rod and cone processes

3. Outer limiting membrane - thin eosinophilic structure. Separates photoreceptor layer from outer nuclear layer. Line of intercellular junctions between Muller cells and photoreceptor cells

4. Outer nuclear layer - contains the cell bodies of the rod and cone photoreceptors.

5. Outer plexiform layer - contains synaptic connections between the short axons of the photoreceptor cells and integrating neurones

6. Inner nuclear layer - contains the cell bodies of the integrating neurons

7. Inner plexiform layer - synaptic connections of integrating neurons with

   dendrites of neurones whose axons form the optic tract.

8. Ganglion cell layer – contains the cells bodies of the optic tract neurons.

9. Optic nerve fibers layer - layer of afferent fibres passing towards the optic disc to form the optic nerve.

10. Inner limiting membrane - demarcates the innermost aspect of the retina from the vitreous body VB. Basement membrane of Muller cells. 

• Ca. 100 Million in humans

• Monochrome

• More sensitive to light

• Allow night vision

• Accumulate rhodopsin (purple)

• Absent from fovea and macula

• Increased density toward the

  periphery of the retina 

• Ca. 6 Million in humans

• Color

• Less sensitive to light

• Need more light for stimulation

• Accumulate other opsins: red (60%), green (30%), blue (10%)

• Most densely concentrated in the fovea