Animals detect changes in the outside world and act according to the situation. Let us learn about the functions of sense organs and the nervous system.
Basic Knowledge
Correspondence between sense organs and stimuli:
① Eyes (eye): light → retina → optic nerve
② Ears (ear): sound → eardrum → auditory ossicles → cochlea → auditory nerve (also balance)
③ Nose (nose): odor molecules → olfactory cells
④ Tongue (tongue): taste molecules → taste receptors (taste buds)
⑤ Skin (skin): pressure, temperature, pain, and so on
The nervous system is divided into two parts:
・Central nervous system (CNS): brain and spinal cord; the center for judgment and commands
・Peripheral nervous system (PNS): sensory nerves (carry information to the central nervous system) and motor nerves (carry commands to the ends of the body)
Pathway of a conscious response: stimulus → sense organ → sensory nerve → spinal cord → brain → spinal cord → motor nerve → muscle → response
Unconscious reflex (for example, pulling your hand away after touching something hot): stimulus → sense organ → sensory nerve → spinal cord → motor nerve → muscle → response (fast because it does not pass through the brain)
Sense organs (organs such as the eyes, ears, nose, tongue, and skin that receive stimuli from the outside world)
Retina (the part at the back of the eyeball that detects light; visual cells are arranged there)
Central and peripheral nervous systems (the brain and spinal cord / nerves throughout the body)
Sensory and motor nerves (nerves from sense organs to the central nervous system / nerves from the central nervous system to muscles)
Reflex (an unconscious response that does not involve the brain; the spinal cord gives the command)
Neuron (a nerve cell that transmits information as electrical signals)
Further Study (Background and Significance)
The structure of the eye consists of the cornea, lens, retina, and optic nerve. The lens changes its shape through the ciliary muscle to focus on nearby and distant objects (accommodation). Nearsightedness occurs when an image is formed in front of the retina because of an abnormal lens or eyeball shape; farsightedness is the opposite.
In the ear, sound vibrations travel through the eardrum → auditory ossicles (malleus, incus, and stapes) → cochlea, and vibrations in the lymph fluid inside are converted into signals for the auditory nerve. The ear also contains the semicircular canals (sense rotation) and the vestibule (sense tilt), which are responsible for balance.
The significance of reflexes is to protect the body quickly from danger. The response time when touching something hot is less than half that of a conscious response. If the brain's judgment had to be awaited, we would be burned, so the spinal cord gives the command directly. Representative examples include the patellar tendon reflex, pupillary reflex, and sneezing.
Nerve cells (neurons) transmit information through electrical signals. A single neuron can be more than 1m long in some cases (from a toe to the spine), and the human brain contains about 86 billion neurons. Together they form complex networks that produce thought, memory, and emotions.
- Sense organs: eyes (light), ears (sound), nose (smell), tongue (taste), skin (touch, temperature, pain)
- Central nervous system=brain+spinal cord
- Peripheral nervous system=sensory nerves+motor nerves
- Conscious response: sense→spinal cord→brain→spinal cord→muscle
- Reflex: sense→spinal cord→muscle (does not pass through the brain)
- Significance of reflexes=danger avoidance (fast)
- Ear: eardrum→auditory ossicles→cochlea
Points of Caution (Easily Confused)
① The difference between the pathways of a conscious response (passes through the brain) and a reflex (does not pass through the brain) is frequently tested. ② Do not reverse sensory nerves (sense organs → central nervous system) and motor nerves (central nervous system → muscles). ③ The semicircular canals (rotation) and vestibule (tilt) are both inside the ear. They are responsible for balance, not hearing. ④ A reflex (an unconscious response) and a response (any response to a stimulus) are different terms. Every reflex is a response, but not every response is a reflex.
Practice
- Name the two parts that make up the central nervous system.
- Explain the pathway of the response in which you pull your hand away after touching something hot, without using the “brain.”
- Name the part of the eye that detects light.