Human Nervous System and Vision: Functions and Structures
Nervous System
A complex network of nerves and cells that transmit signals between different parts of the body, responsible for controlling and coordinating bodily functions.
Neuron: A nerve cell that carries electrical signals in the nervous system.
Neurotransmitter: Chemical substances that transmit signals across a synapse from one neuron to another or to a target cell (e.g., dopamine, serotonin).
Synapse: The junction between two neurons, or between a neuron and a target cell (e.g., muscle, gland), where signal transmission occurs.
Action Potential: An electrical impulse that travels along the axon of a neuron, allowing communication between neurons and other cells.
Types of Nervous Systems
Central Nervous System (CNS): Composed of the brain and spinal cord; processes sensory information and coordinates response.
Peripheral Nervous System (PNS): Includes all the nerves outside the CNS; it connects the brain and spinal cord to the rest of the body.
Autonomic Nervous System (ANS): A division of the PNS that controls involuntary bodily functions like heart rate, digestion, and breathing.
Somatic Nervous System (SNS): A division of the PNS that controls voluntary movements of skeletal muscles.
Neural Structures and Components
Axon: The long, slender projection of a neuron that conducts electrical impulses away from the cell body to other neurons or target cells.
Dendrite: Branched extensions of a neuron that receive signals from other neurons and transmit them toward the cell body.
Myelin Sheath: Fatty layer that surrounds the axon of many neurons, speeding up the transmission of electrical signals.
Node of Ranvier: Gaps in the myelin sheath where action potentials are accelerated as they jump from node to node (saltatory conduction).
Cell Body (Soma): The part of the neuron that contains the nucleus and organelles, responsible for metabolic functions.
Sensory Neurons: Neurons that carry sensory information from receptors to the central nervous system (e.g., touch, temperature, pain).
Motor Neurons: Neurons that carry signals from the CNS to muscles or glands to produce a response or action.
Interneurons: Neurons that connect sensory and motor neurons within the CNS, enabling processing and reflexes.
Reflex Arc: The simplest form of nervous system response, in which an external stimulus is quickly processed to produce an involuntary motor response.
Sympathetic Nervous System: The “fight or flight” system; prepares the body for stressful or emergency situations (increases heart rate, dilates pupils, etc.).
Parasympathetic Nervous System: The “rest and digest” system; calms the body down after stress, promoting relaxation and recovery (slows heart rate, promotes digestion).
Enteric Nervous System: A division of the ANS that directly controls the gastrointestinal system.
Cerebrum: The largest part of the brain, responsible for higher functions like thought, memory, and voluntary movement.
Cerebellum: Located at the back of the brain; coordinates voluntary movements, balance, and motor skills.
Brainstem: The lower part of the brain, connecting to the spinal cord; regulates basic functions like heart rate, breathing, and reflexes.
Thalamus: The relay station for sensory information, directing signals to appropriate areas of the cerebral cortex.
Hypothalamus: Regulates homeostasis (e.g., hunger, thirst, body temperature) and controls the release of hormones from the pituitary gland.
Medulla Oblongata: Part of the brainstem responsible for autonomic functions like breathing, heart rate, and blood pressure.
Corpus Callosum: A bundle of nerve fibers that connects the left and right hemispheres of the brain.
Astrocytes: Star-shaped glial cells that support neurons, maintain the blood-brain barrier, and regulate blood flow.
Oligodendrocytes: Glial cells that produce myelin in the CNS, enhancing the speed of electrical signal transmission.
Schwann Cells: Glial cells that produce myelin in the PNS.
Microglia: The immune cells of the nervous system; they remove waste and protect against infection.
Ependymal Cells: Line the ventricles of the brain and produce cerebrospinal fluid (CSF).
Resting Potential: The stable, negative charge inside a neuron when it is not transmitting an impulse.
Threshold: The level of stimulus required to trigger an action potential in a neuron.
Depolarization: The process during an action potential when the neuron’s membrane potential becomes more positive due to the influx of sodium ions.
Repolarization: The process by which a neuron restores its resting potential after depolarization, usually by expelling potassium ions.
Hyperpolarization: When the membrane potential becomes more negative than the resting potential, often due to excessive potassium ion outflow.
Refractory Period: The period during which a neuron cannot generate another action potential immediately after firing.
Multiple Sclerosis (MS): A disease in which the immune system attacks the myelin sheath, impairing neural transmission.
Parkinson’s Disease: A neurodegenerative disorder that affects movement, caused by the loss of dopamine-producing neurons in the brain.
Alzheimer’s Disease: A progressive neurodegenerative disorder that affects memory and cognitive function, often associated with the buildup of amyloid plaques in the brain.
Epilepsy: A neurological disorder characterized by recurrent, unprovoked seizures.
Stroke: A disruption of blood flow to the brain, leading to a loss of function in the affected brain area.
How Light Enters the Eye and is Processed
Light Entry
- Light from the environment enters the eye through the cornea, the clear, curved outer layer of the eye.
- The cornea helps focus light, bending (refracting) it towards the pupil.
Pupil and Iris
- The pupil is the dark circular opening in the center of the iris, the colored part of the eye.
- The iris controls the size of the pupil, regulating how much light enters the eye. It adjusts the pupil size based on lighting conditions—constricting in bright light and dilating in dim light.
Focusing Light on the Retina
- After passing through the pupil, light travels through the lens.
- The lens changes its shape (a process called accommodation) to focus light onto the retina, which is the light-sensitive layer at the back of the eye.
- The lens focuses the light to form a sharp image.
Retina and Photoreceptors
- The retina contains specialized cells called photoreceptors that detect light. These photoreceptors come in two types:
- Rods: Sensitive to low light and responsible for black-and-white vision and peripheral vision.
- Cones: Responsible for color vision and function best in bright light. There are three types of cones that detect different wavelengths of light, corresponding to red, green, and blue.
Signal Transduction
- When light hits the photoreceptors, it triggers a chemical reaction that converts the light into electrical signals.
- These electrical signals are then transmitted to the brain via the optic nerve.
Optic Nerve and Visual Pathways
- The electrical signals from the retina travel through the optic nerve to the brain.
- The optic nerves from both eyes meet at a point called the optic chiasm, where some of the nerve fibers cross over to the opposite side. This ensures that visual information from the left visual field is processed by the right side of the brain, and vice versa.
Processing in the Brain
- The signals travel to the visual cortex in the occipital lobe of the brain, located at the back of the head.
- The visual cortex processes these signals, interpreting aspects of vision like color, motion, depth, and shape.
- The brain then creates a coherent image based on this processed information, allowing you to perceive the world visually.
Depth Perception and Stereoscopic Vision
- Vision involves binocular vision (using both eyes), which provides depth perception. The brain compares the slightly different images from each eye (due to their positions on your face) to judge distance and depth.
Additional Key Structures
- Fovea: The central part of the retina, where visual acuity is highest. The fovea is packed with cones and provides sharp, detailed vision, especially for tasks like reading.
- Macula: The area around the fovea responsible for central vision.