14 Reflex arc & Receptors

• CNS - central nervous system, brain and spinal cord
• PNS - peripheral nervous system, pairs of nerves originating from the brain or spinal cord
  • Sensory neurones - Receptors to CNS
  • Motor neurones - CNS to effectors
    • Voluntary nervous system - body muscles under conscious control, voluntary
    • Autonomic nervous system - glands, smooth muscle and cardiac muscle, subconscious and involuntary

• Rapid, short-lived, localised & involuntary
• Doesn't go to the brain
• Just 3 neurons making it fast
• Relay neurone is in the spinal cord so it is also called a spinal reflex

• Stimulus - e.g. heat
• Receptor - temperature receptors generate an action potential in sensory neurone
• Sensory neurone - passes nervous impulse to the spinal cord
• Coordinator - (intermediate neurone) links the sensory neurone to the motor neurone in the spinal cord
• Motor neurone - carries nerve impulse to the effector such as the bicep
• Effector - muscle is stimulated to contract
• Response - the hand is pulled away from the hot object

• Aids survival
• Involuntary - no decision making power is needed from the brain so the brain can do more complex responses. (Some responses are still sent to the brain as well so it can sometimes override the reflex if needed
• Don't have to be learnt - effective from birth
• Fast - short neurone pathway with typically only 2 synapses between neurones
• Fast - due to no decision making is done by the brain

Specific

• Different receptors respond to different stimuli
• e.g. pressure, heat, light, sound

Acts as a transducer producing a generator potential

• Energy from the stimuli is converted into nerve impulses
• A generator potential is created
• This is all created by the intake of od sodium ions

• They respond to pressure
• Deep in the skin
• Most abundant on fingers, soles of the feet and external genitalia
• Also in ligaments, tendons and joints
• Has an appearance similar to an onion's layers
• Has stretch-mediated sodium channels
• Permeability changes when deformed by stretching
• In the resting state, they are too narrow for sodium ions to pass through
• Stretching allows sodium ions to diffuse through into the neurone
• Membrane becomes depolarised
• This produces a generator potential
• This creates an action potential that passes along the neurone

• Cannot distinguish between different wavelengths of light
• Only black and white
• 120 million per eye
• Many rod cells are connected to a single sensory neurone in the optic nerve
• Rod cells detect low light intensity
• A generator potential is created in the bipolar cell which is connected to rod cells
• Spatial summation with retinal convergence
• Allows us to see in low light intensity
• Rhodopsin (a pigment) is broken down by energy from light, even in low-intensity light
• The brain cannot distinguish between the different rod cells leading to low visual acuity and poor resolution

• 3 different types, due to responding to different wavelengths
• 6 million per eye
• Each has its own bipolar cell which is connected to the optic nerve
• This causes cone cells to only react to high light intensity as there is only one cone cell to stimulate a bipolar cell to produce a generator potential
• Contain different pigments than rod cells - iodopsin which requires more light to be broken down in the cone cells
• 3 different types of iodopsin depending on the type of cone cell which respond to different wavelengths of light
• Better resolution and good visual acuity as each bipolar cell stimulated is one cone cell

• Fovea - more cone cells, no rod cells. Peripheries - no cone cells, lots of rod cells
• Due to the different intensities of light found
• Leads to good all-round vision in mammals during the day and night