Sensory Receptors

Free Nerve Endings

Free nerve endings are just the dead end of nerves.  They are very common but they also are not specific to the type of sensory input they receive.  They can determine pain, temperature, touch, and some of the other general sensesSensory modalities distributed throughout the body: touch, pressure, temperature, pain, etc. that don’t require specific anatomyThe study of the structure of the human body..  Free nerve endings are everywhere in your body. However, they are more common in somatic (skinThe body’s largest organ, providing protection and regulation., bone, muscle) tissue than in visceral tissue.

 In factA statement based on direct observation that is repeatedly confirmed., you have more sensory endings in your somatic tissues than you do in your visceral tissues overall.  Free nerve endings, for us, will be sensory neuronsNeurons that carry information from sensory receptors to the CNS. bringing information to the brain.  We already discussed free nerve endingsUnencapsulated receptors that detect pain, temperature, and crude touch.. Information travels from the brain to a skeletal muscle. We covered this when we talked about the neuromuscular junctionThe connection between a motor neuron and a muscle fiber..

Receptor Fields

Have you ever scratched someone’s back for them and it’s like they don’t know where the itch is?  This is because free nerve endings have big receptorA structure that detects stimuli. fields or areas that they monitor.  On your back, these receptor field are really big but they also overlap.  Someone scratches your back in one place. This inadvertently activates another receptor field. Consequently, they tell you to move to the left.  Then you hit another field and they tell you to move up.  And it keeps going.  On your fingers, you have very tiny receptor fields.  This allows you to have very precise sensation for touching things. 

Nociceptors

Nociceptors are pain receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter.  You have more nociceptorsPain receptors that respond to tissue damage or potentially harmful stimuli. in your somatic tissue than you do in your visceral tissue.  This is why issues with squishy organs usually don’t present symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue). until the condition is really bad.  You have very few nociceptors in your pancreasA gland that produces digestive enzymes and hormones like insulin and glucagon..  This is why when pancreatitis presents itself, it is usually at the point where it is quite painful. 

Nociceptors can span very large receptor fields, making pain a bad indicator or injury location.  Also complicating things is that pain can feel like different sensations depending on the type of nerve that is damaged.  If I turn off my spinal cordThe central nervous system structure that relays signals between the brain and body. stimulator, a sharp slicing pain sets in on the back of my thighs. A burning aching pain sets in on my entire left leg. Intense pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. appears on my ankles and knees.  These are all pain sensations that my spinal cord stimulator interrupts. More on that in the section A cut or slice of the body or an organ for study. on the spinal cord. 

Thermoreceptors

Thermoreceptors are more common in the somatic tissue than in visceral tissue.  Most thermoreceptorsSensory receptors that respond to temperature changes. are in the skin and are concerned with sensing the temperature of the external environment.  A sensory neuron is triggered by heat or cold. It sends an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. to the thalamusThe brain’s relay center, sending sensory information to the cerebral cortex. of the brain. The thalamus then sends another action potential to the somatosensory cortex. This part of the brain places meaning to the action potential.

The hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. is the brain’s center that senses and controlsVariables that remain constant to ensure a fair test. body temperature. It manages overall body temperature for things like a fever or hot flash.  There are these proteinsLarge molecules made of amino acids with various functions in the body. called pyrogens.  When you run a temperature, these pyrogens circulate and stimulate the hypothalamus.

Chemoreceptors

Your body also possesses chemoreceptors that sense different moleculesGroups of atoms bonded together. such as carbon dioxide and oxygen.  Right now, chemoreceptors are located in your aorta and carotid arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr. These lead to your brain. These chemoreceptors are sensing oxygen and carbon dioxide in the blood, especially the blood going to the brain.  These sensors can influence the lungs. They try to increase or decrease the rate and depth of breathing.  Actually, these chemoreceptors are more concerned about the carbon dioxide than they are about the oxygen.  You can go pretty low on oxygen before it becomes a problem, but carbon dioxide is poisonous at high levels.  It is acidicA solution with a pH below 7, having a higher concentration of H⁺ ions. and it makes the blood acidic.  Oxygen doesn’t do that.

Baroreceptors are also in key places to monitor blood pressure.  The aorta is a key place for baroreceptors monitoring blood pressure. The renal arteryThe artery that supplies blood to the kidney. accepts close to 25% of the blood from the aorta and is also important for this function.  You also have different types of baroreceptors in your skin.  However, these are talked about in another mini-lecture because these types of baroreceptors are not associated with free nerve endings.