# 10.3: Biological Energy Conversion

The human body can be considered an energy conversion device. Humans take in chemical energy in the form of food and convert it to kinetic energy, heat, and other forms of energy. Some components of the human body are also energy conversion devices. Muscles convert chemical energy to kinetic energy. Photoreceptors in the retina of the eye convert optical energy of photons to electrical energy of neurons. The ear converts sound waves to energy stored in the pressure of the fluid of the inner ear, kinetic energy of moving hairs that line the cochlea of the inner ear, and electrical energy of neurons. The human body also stores energy. Muscles can store energy as they stretch and contract. Human fat cells store energy in chemical form. When you walk, your center of mass moves up and down storing energy in pendulum-like motion [155]. Additionally, bone, skin, and collagen exhibit piezoelectricity [156].

Neurons are nerve cells that convert chemical energy to electrical energy. The human brain has around $$10^{11}$$ neurons [157, p. 135]. They are composed of a cell body, an axon, dendrites, and synapses [158]. The axon is the fibrous part that transmits information to other neurons. The dendrites are the fibrous part that receives information from other neurons. A synapse is a gap between neurons. Ions, such as Na$$^+$$, K$$^+$$, or Cl$$^-$$, build up on the membrane or in the gap between two neurons, and the charge separation of the ions causes an electrical potential [157]. Ions sometimes cross the gap between neurons. Neurons may be classified as sensory afferents, interneurons, or motoneurons [157]. Sensory afferents transmit a signal from sensory receptors to the nervous system. Interneurons transmit the signal throughout the nervous system, and motoneurons transmit the signal from the nervous system to muscles [157]. Electrical signals transmitted along the nervous system involve pulses with a duration of a few milliseconds [157]. The information is encoded in the frequency rate of the pulses [157].