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Human Biology > Receptors
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Receptors

  • Stimulus: environmental change of an organism → change in energy
  • Transduce (→convert) a stimulus into a nerve impulse
    • Stimulus above threshold
    • Detected by receptors
    • Changes its membrane potential
    • Causes generator potential
    • Action potential along sensory neurone
  • Receptors respond to specific stimuli only

Pacinian Corpuscles

  • Found in dermis of skin, joints, tendons, external genitalia, internal organs
  • Structure
    • Layers (maellae) of connective tissue surround
    • Myelinated sensory neurone (nerve fibre ending) that have
    • Stretch-mediated Na+ channels
  • Round pacinian corpuscle has a resting potential
  • Stretch-mediated sodium channels restrict movement of ions
  • PRESSURE OPENS SODIUM CHANNELS
  • Entry of sodium ions
  • Causes depolarisation / membrane potential / generator potential
  • Threshold potential reached
    • Slight pressure / small generator potential / no depolarisation
    • Great pressure / more channels open / larger generator potential

The Eye

Table 16-8-1: The structure and function of the mammalian eye


Conjunctiva

Protection of cornea

Sclera

- Protection
- Attachment for eye muscles

Cornea

Refracts (→focuses) and allows passage of light

Choroid

Pigment prevents light reflection within the eyeball by absorbing light

Ciliary body

- Accommodation
- Secretion of humour

Iris

Regulates passage of light

Lens

Refracts light

Retina

Contains light receptors

Fovea

Contains only cone cells

Blind spot

Optic nerve (sensory nerve fibres) leave the eyeball

Humour

Maintains shape of the eyeball

Transmissive And Refractive Properties Of The Eye In Focusing An Image On The Retina

  • Light/photons travel through transparent media in a light ray
    • Rays reflect at a predictable angle when they strike an object
    • Rays passing through mediums of different density refract (change angle)
  • Accommodation → focus of rays from near/distant objects by changing shape of lens
  • Light rays form an image in the retina [EXAM]
    • Refraction / by lens or cornea / shape of lens changes

NEAR ACCOMMODATION

DISTANT ACCOMMODATION

CILIARY MUSCLES

CONTRACT

RELAX

TENSION IN SUSPENSORY LIGAMENTS

REDUCED

INCREASED

SHAPE OF LENS

FAT, ROUNDED

THIN, FLAT

RESULT

LIGHT BENDS

LIGHT BENDS LESS

FOCUSES

DIVERGING LIGHT RAYS

PARALLEL LIGHT RAYS

Role of Rod Cells and Cone Cells in Effecting Monochromatic and Trichromatic Vision

  • Retina contains 4 layers → synapse between them
    • Cone and rod cells (light-sensitive receptor)
      • Inner segment → nucleus, mitochondria, ribosomes, synaptic region
      • Outer segment → membranous disks containing pigments
    • Bipolar neurones (relay neurone)
    • Ganglion cells (sensory neurones)
    • Axon of ganglion cells → optical nerve
      • Send impulses to the brain
  • Light passes through neurones before it strikes the retina
  • There are no cone and rod cells where the optic nerves pass through the retina; this point is called the blind spot

Table 16-8-2: Features of rod cells and cone cells


FEATURE

ROD CELLS

CONE CELLS

Number in retina

More

Fewer

Distribution

- Evenly throughout the retina
- Absent from the fovea
- Only type of light receptor at the periphery of the retina

Present in the fovea

Shape of outer segment

Rod shaped

Cone shaped

Sensitivity to

Dim light

Bright light

Visual acuity

Poorly resolved images

Well-resolved images

Light-sensitive pigments

- Only rhodopsin
- Monochromic vision

- Iodopsin
- Sensitive to blue, green, blue light
- Trichromatic vision (combination)

Synapse with relay cells

Several rod cells synapse with same relay cell

Each cone cell synapses with just one relay cell

Table 16-8-3: Absorption of light by rhodopsin creates a generator potential in rod cells (AP = action potential)


In the dark (rod cell)

In light (rod cell)

Opsin + Cis-Retinal → Rhodopsin

Rhodopsin → Opsin + Trans-Retinal

Causes sodium channels to open

Causes sodium channels to close

Membrane depolarised

Membrane hyperpolarised

Neurotransmitter released into inhibitory synapse [rod → bipolar cell]

No neurotransmitter released into inhibitory synapse

Bipolar neurone hyperpolarised → no impulse

Bipolar neurone depolarised → AP

No neurotransmitter released into excitatory synapse [bipolar → ganglion cell]

Neurotransmitter released into excitatory synapse

No action potential

Action potential along ganglion neurone

Resynthesis of rhodopsin

  • TRANS-RETINAL + OPSIN → RHODOPSIN ATP → ADP + PI
  • Mitochondria in inner segment synthesis ATP
  • Slow reaction compared to rhodopsin breakdown by light
  • Bright light into dim light conditions → poor vision until rhodopsin is resynthesised
  • Retinal is a derivative of vitamin A

The Connection Between Sensory Cells and The Neurone of the Optic Nerve

  • Rod cells are working in dim light conditions
    • Several rod cells synapse with one relay cell → retinal convergence
    • Impulse by summation \ rod cells collectively cause generator potential
    • Poor visual acuity but high sensitivity to dim light
  • Cone cells are working in bright light
    • Each cone cell synapses with each individual relay cell
    • Several impulses pass along the optic nerve to the brain
    • High visual acuity (ability of the brain to resolve images)