Understanding the Human Eye and Nervous System
Adnexia:
Lacrimal glands produce a slightly salty watery fluid that is responsible for keeping the eyeball moist. They contain lysozyme, an enzyme responsible for destroying certain bacterial agents.
Eyelids
Eyelids prevent the entry of foreign agents to the eye, such as dust and sweat. They are responsible for distributing tears all over the eyeball by blinking.
Muscles of the Eye
Muscles in the eye are responsible for moving the eyeballs up and down, left and right, and rotating them.
Eyeball:
- Sclera: The outer layer that protects the rest of the globe.
- Cornea: The transparent front part of the eye.
- Choroid: The black membrane that contains the iris diaphragm, which determines eye color. This membrane has a central hole called the pupil.
- Iris: Regulates the amount of light passing through the pupil. It contracts in bright light and expands in dim light.
- Lens: A biconvex lens that focuses light.
- Retina: The innermost layer where photoreceptors are located. The axons of these neurons form the optic nerve, which reaches the brain.
Rods: Involved in vision under low light intensity. Cones: Responsible for vision under high light intensity and for perceiving different colors. There are different types of cones sensitive to the primary colors.
Mechanism of Vision:
The light enters the pupil through the cornea, passes through the lens (which refracts it), and the retina forms an image that is equal to the real one but reversed. This is where the image is converted into nerve signals that travel through the optic nerve to the brain, where the image is processed.
Brain
The brain is the most voluminous organ and is divided into two hemispheres. Both hemispheres are connected by the corpus callosum. The outermost part is called the cortex, where sensations are processed, motor responses are made, and memory and consciousness reside. The cortex has numerous wrinkles called convolutions.
Cortex Functions
- Sensory Cortex: Processes auditory, olfactory, and visual information.
- Motor Cortex: Responsible for voluntary movements.
- Association Areas: Involved in decision-making, language, and memory.
Spinal Cord
The spinal cord is located in the spine and coordinates reflexes. It allows the brain to communicate with external receptors via sensory and motor nerves. The nerves that run through the spinal cord cross at the medulla, so the right brain controls the left side of the body and vice versa.
Peripheral Nervous System
The peripheral nervous system consists of 31 pairs of spinal nerves that leave the spinal cord and 12 pairs of nerves from the brain. It is divided into:
- Somatic Nervous System: Responsible for conscious movement and voluntary muscle control.
- Autonomic Nervous System: Composed of sympathetic (alert) and parasympathetic (rest) nerves. Their actions on the organs are antagonistic; for example, the parasympathetic system slows the heart rate, while the sympathetic system accelerates it.
Reflexes and Voluntary Actions:
Reflexes are involuntary responses of the body to stimuli. They are responsible for reflex actions, such as the knee jerk and closing your eyes when an object approaches. Reflexes act quickly, automatically, and involuntarily, controlled by the spinal cord. The elements of a reflex arc include:
- Stimulus receptor
- Sensory neuron that carries information to the spinal cord
- Interneuron or association neuron
- Motor neuron that carries information to the effector organ (muscle movement)
Voluntary Actions:
For a sense to be perceived, it must be processed by the brain. Voluntary actions require the intervention of various brain structures, including the sensory cortex and motor areas.
Endocrine System:
The endocrine system consists of a set of effector organs called endocrine glands that secrete hormones into the blood. Hormones are chemical messengers with several characteristics:
- Varied chemical nature
- Synthesized by endocrine glands
- Released into the bloodstream
- Act on target cells
Major Glands and Hormones
- Pituitary: Controls the activity of other glands and produces growth hormone.
- Thyroid: Produces thyroxine, which controls the body’s metabolism.
- Pancreas: Secretes insulin and glucagon.
- Gonads (Testes and Ovaries): Produce estrogens (in women) and testosterone (in men).