Sensory Receptors Miss Tagore A2 Biology

Learning Outcomes Outline the roles of sensory receptors in mammals in converting different forms of energy into nerve impulses. Describe, with the aid of diagrams, the structure and functions of sensory and motor neurones.

Sensory Receptors Changes in our surroundings are detected by specialised cells called sensory receptors. Sensory receptors convert one form of energy into another. E.g. a change in pressure in the skin. Change in energy levels in the environment is called a stimulus.

Light-sensitive cells (rods and cones) in the retina of the eye Light intensity and range of wavelengths (colour) Olfactory cells lining the inner surface in the nasal cavity Presence of volatile chemicals Taste buds in the tongue, hard palate, epiglottis and the first part of the oesophagus Presence of soluble chemicals Pressure receptors (Pacinian corpuscles) in the skin Pressure on skin Sound receptors in the inner ear (cochlea) Vibrations in the air

Summary Sensory receptors detect changes from surroundings; Sensory receptors transduce signals – they convert one type of energy into a form of electrical energy; Changes in energy levels in the environment are called stimuli; Stimuli are converted by sensory receptors into electrical energy called a nerve impulse.

Voltage gated ion channels Some of the proteins found on the surface of the cell membrane are channels that allow ions to move across. Open channels allow ions to diffuse from an area of high concentration to lower concentration until they are evenly spread out. Channel proteins found in neurons are more specific than this.

Voltage gated ion channels Protein channels in neurones are specific to either sodium or potassium ions; These protein channels have gates that open or close the channel. They are usually kept closed; When open, the permeability of the ions is increased and they flow through the channel. When closed, the permeability is reduced.

NOTE! These types of proteins simply let sodium or potassium ions to pass in or out of the cell

Summary There are a type of protein found on the cell surface called voltage-gated ion channels. When the channel/gate is open, proteins allow sodium or potassium ions to pass in to or out of the cell. Channels are normally kept closed.

Ion pump proteins Another type of protein in the cell membrane is one that actively pumps ions in or out of the cell. These pumps transport sodium (Na + ) and potassium (K + ) ions extra- and intracellularly respectively. This means lots of Na + is pumped out of the cell, while K + is pumped into the cell. This creates an overall negative charge inside the cell with respect to the outside.

Polarisation As the inside of the cell is “more negative” than outside, we say the cell membrane has become polarised. Negative charge Positive charge

Depolarisation Altering the permeability of sodium ions creates a nerve impulse. When sodium channels open, sodium ions flow down the concentration gradient from high to low, flowing back into the cell (These were the first proteins we talked about). The movement of positive Na ions going back into the cell makes the inside of the cell “less negative” than before. This creates a change in the potential difference, or charge, across the cell. This change of charges across the membrane of neurones is called depolarisation.

Sodium channel is open, allowing sodium ions to flow back into the cell. Sodium ions flowing back into the cell makes it “less negative” than before

Generator Potentials Receptor cells respond to changes in the environment. If a small number of Na + cross the channel, this is called a generator potential. The larger the stimulus (the change in energy levels in the environment) the more gated channels will open. If there are enough Na + entering the cell, the potential difference changes significantly and will initiate an impulse, known as an action potential.

Small stimuli create small changes in potential difference Large stimulus creates large change in potential difference

Different types of neurone

Transfer of information Stimuli detected have energy. This energy is converted in order to depolarise the membrane of a neurone (make the receiving neuron membrane “less negative”). Once this happens, an impulse is transmitted to other parts of the body. The impulse is transmitted through neurones as action potentials from one part of the body to the other.

Neurone Specialisations Very long axons – transmit over long distances Many gated ion channels on cell membrane surface to allow Na, K and Ca ions through Na/K pumps to facilitate active transport of ions into/out of cell Maintain a potential difference across cell membrane (+ve outside, -ve inside) Myelin sheath (Schwann cells) insulate neurone from electrical activity of nearby cells Cells have a nucleus, many mitochondria and ribosomes