CNS: The Brain & Spinal Cord
Sensory Organs & the Afferent Nerves
Humans have 5 basic senses - biological methods of sensing changes in the world around us. Changes in air density caused by sound waves... changes in temperature... changes in pressure at the surface of our skin... change in chemical gradients internally... changes in our balance... we are capable of sensing myriad external and internal stimuli at any moment.
Eyes are the sensory organs designed to receive visible light, a small slice of the electromagnetic spectrum. The eyes receive incoming light through the protective cornea, focus it via a flexible lens, projecting an inverse image at the back of the eye, the retina. The sensitive rod and cone-shaped nerve endings at the back of the retina are stimulated by light (this is a lot more complicated than can be summed up in a short paragraph!). These nerves communicate to the brain via the optic nerve. The eyes see in stereo, enabling most humans to have excellent depth perception. Visual information is processed at the back of the brain in the occipital lobe.
Humans are able to sense longitudinal sound waves through the auditory pathway of the ear. Sounds enter the pinna of the ear and proceed down the auditory canal towards the tympanic membrane (eardrum). This drum vibrates, suspended by a delicate architecture of tendons (easily damaged!). Vibrations are further amplified by the 3 bones of the middle ear, the incus, malleus, and stapes. The sound waves stimulate extremely tiny hairs, in the inner ear's organ of corti, which are designed to vibrate at certain frequency ranges. These hairs are known to degenerate over time - young people can sense higher sound frequencies than older people. The vibration of these hairs converts the sound signal into an electrochemical impulse via the auditory nerve . These ultra-fast signals are then communicated to the brain.
Sense of Hearing Links:
What are neurons?
Neurons are the basic unit of the human nervous system. Neurons, also called nerve cells, are responsible carrying electrochemical impulses throughout the nervous system. These impulses are transmitted along oftentimes complex pathways. Chemicals called neurotransmitters make it possible for messages to be relayed along this system.
[ To watch neurotransmission in action visit this NIH interactive. ]
Each neuron consists of a cell body (containing the nucleus) that is intimately covered with dendrites, the "beginning" of the cell. An axon leads outward from the nerve cell body toward the "end" of the nerve cell. Nerve impulses travel from the dendrites, through the cell body and down the axon toward the axon terminal. Axons can then connect nerves to the dendrites of other nerves, or to body tissues such as muscles. The point at which nerve cells interact with muscle tissue is referred to as a neuromuscular junction.
The brain contains millions of nerve cells. Research into brain function is still in its nascent phases, but scientists have been able to generally categorize the functions that various regions of the brain play. Some regions of the brain consist of neurons that are responsible for speech and language. Other cells are directly related to muscle movements, memory, sight, or a host of other things. When neurons make connections with other neurons, pathways are formed within the nervous system. This is essentially the mechanism for most learning humans experience.
The axon serves a vital function for nerve cells. The axon is where a complex action potential occurs as a result of an ion gradient along its periphery. It is now thought that a substance called myelin plays a role in lowering the electrical resistance along the axon, allowing for more efficient nerve impulse transmissions. The learned actions of a champion athlete or concert pianist have been attributed to the repeated myelination of neurons.
Neurons are thought to be difficult to replace in the central nervous system (CNS), although new research is showing that even adults can regenerate some nerve cells in the CNS. Drugs such as cocaine can greatly interfere with neurotransmission, releasing natural body chemicals (such as dopamine) in unnatural ways. Drugs negatively affect how neurons communicate with each other. Utilizing several specific pathways to the brain, drugs create an altered sensory experience for the user. This experience is not "real" but is an internally created "mirage", deceiving the brain through chemical means. Stay away from these substances!
[ Meet addicts and their families in these NIH interview videos ]