Peripheral nervous system

• Sympathetic neurons are preganglionic, while parasympathetic neurons are postganglionic.
• AFFERENT division of the PNS:

  sends information about the external and internal environment to the CNS.
• Sensory afferent: sensory neurons that carry information from the sensory organs to the CNS.

  1)Somatic sensation

  2)Special senses (vision, hearing, equilibrium, taste, smell)

  = concious input
• Visceral afferent: Information about the internal environment ( blood pressure, CO2 presence in the body fluids) = Subconscious input
• Iris: controls the amount of light entering the eye

  •Thin, pigmented, smooth muscle

  •Forms a visible ringlike structure within the aqueous humor

  Iris muscles are controlled by the autonomic nervous system.
• The eye’s refractive structures

   are the cornea and the lens:

  The refractive ability of a cornea remains constant

  The refractive ability of the lens can be adjusted by changing its curvature as needed for near or far vision, mediated by the ciliary muscle
• Dark - retinal in 11-cis form- High conc of cGMP - depolaristion of photoreceptor- synaptic terminal- opens ca2+ channels in the synaptic terminal- neurotransmitter released- hyperpolarisation/depolarisation of ganglion cells- propagation to visual cortex
• light- retinal becomes all-trans form, activating photopigment- activates transducin - activates phosphodiesterase- decreases conc of cGMP- na+ channels in outer segment close- hyperpolarisation of the photoreceptor- Ca2+ channels close in synaptic terminal- depolarisation/hyperpolarisation-propagation to visual cortex.
• Rods (night vision) Outer segment is rod shaped 120 million/retina, more concentrated in the periphery so used for peripheral vision.

  • High sensitivity
  • Monochromatic vision (shades of grey)
• light stimulus - light sensors - electrical signals - optic nerve - lateral geniculate nucleus - visual cortex
• Optic Disc: The point where the optic nerve exits the eyeball. It contains no photoreceptors, so it appears white when viewed directly.
• Retina has two layers of neurons that receive input from photoreceptors: bipolar cells and ganglion cells. Bipolar cells have dendrites that branch out into the outer plexiform layer and axons that extend through the inner nuclear layer to the inner plexiform layer. Ganglion cells have large cell bodies located in the inner nuclear layer and send their axons through the optic disc to the brain via the optic nerve.
• Macula Lutea: A small yellow spot located at the center of the posterior surface of the retina. It contains high concentrations of cones and is responsible for central vision.
• Primarily a protective mechanism triggered on stimulation of nociceptors (pain receptors)
• 3 categories of pain receptors:  

  1. mechanical nociceptors
  2. thermal nociceptors
  3. polymodal nociceptors
• Pain Pathway:

  Noxious stimuli -> Nociceptors -> dorsal horn excitatory neurons -> reticular formation/ Thalamus -> somatosensory cortex

  Thalamus/ reticular formation -> hypothalamus: limbic system
• Endogenous opioid system


  The body's own pain relief system where endorphins, enkephalins, and dynorphins are released to reduce or mitigate pain
• Pain signal transmission


  1. Nociceptors stimulated

  2. Signal enters spinal cord through dorsal nerve root

  3. Synapses at dorsal grey horn

  4. Crosses to opposite side in spinothalamic tract

  5. Ascends to reticular formation, hypothalamus, thalamus, amygdala
• Role of brain regions

  • Reticular formation - conscious awareness of pain
  • Hypothalamus - fight-or-flight response
  • Thalamus - sensory integration
  • Amygdala - emotional response to pain
• Descending pain inhibition


  1. Cerebral cortex, amygdala, hypothalamus send signals to periaqueductal grey matter

  2. Periaqueductal grey matter stimulates raphe nuclei

  3. Raphe nuclei release serotonin

  4. Serotonin inhibits pain transmission neuron

  5. Enkephalin released to bind opioid receptors and block pain
• Endogenous opioids


  Enkephalins, endorphins, dynorphins - natural pain-relieving chemicals produced by the body
• Endorphins bind to mu opioid receptors, enkephalins bind to delta opioid receptors, and dynorphins bind to kappa opioid receptors
• The fovea is a small, central pit in the retina of the eye where visual acuity is highest.
• The macula lutea is a yellowish spot located near the center of the retina responsible for sharp vision.
• When light enters the eye and passes through the cornea, pupil, and lens, it reaches the retina where it is detected by the photoreceptor cells (Rods and cones).
• The iris contains muscles that control the size of the pupil in response to varying light conditions and other factors.
• The primary afferent pain fibers, also known as nociceptors, relay information to the central nervous system (CNS) by synapsing with neurons in the dorsal horn of the spinal cord.
• Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS). It acts on postsynaptic receptors, such as NMDA receptors and AMPA receptors, to depolarize neurons and increase their excitability.