Human Sensory Perception and Nervous System Fundamentals
Understanding Sensory Perception
Perceptions are created when information gathered by receptors is sent to the brain through the nerves.
The receptor’s function is to gather information from both the external and internal environments.
The Sense of Touch: Skin
The skin, our largest organ, contains numerous receptors for touch, temperature, and pain. It is composed of two primary layers:
- Epidermis: The outer layer of the skin.
- Dermis: The inner layer of the skin.
The Sense of Taste
Taste buds are specialized receivers that capture different tastes.
The Sense of Smell
Olfactory Receptors
In the nasal passages, the olfactory mucosa contains chemoreceptors capable of detecting chemical stimuli, allowing us to perceive smells.
The Sense of Hearing: The Ear
The ear can be distinguished into three main regions:
- Outer Ear: The outer ear captures sound waves, and the ear canal transports these waves to the eardrum.
- Middle Ear: The eardrum vibrates, transmitting waves into the tympanic cavity. From there, a chain of three tiny ossicles (malleus, incus, stapes) transmits these vibrations to the cochlea.
- Inner Ear: The cochlea, a snail-shaped structure, contains a liquid through which waves travel to stimulate receivers. These receivers convert the vibrations into nerve impulses.
How Hearing Works
Hearing occurs when sound waves hit the eardrum and cause it to vibrate. These vibrations move the three middle ear bones. The vibrations then reach a membrane called the oval window, which communicates with the cochlea. The cochlea captures these vibrations and converts them into nerve signals that are transmitted to the brain.
The Sense of Vision: The Eye
The eye is composed of several layers, each with a specific function:
- Sclera: The outer layer, which includes the transparent and curved cornea. The cornea allows the passage of light into the eye.
- Choroid: The middle layer, rich in blood capillaries that supply nutrients. At its front, below the ciliary body, is the iris. The iris is the colored part of the eye and controls the size of the pupil. Behind the iris is the lens, which helps to focus light for clear vision.
- Retina: The innermost layer of the eye, containing two types of photoreceptors:
- Rods: Responsible for vision in low light and detecting shades of gray.
- Cones: Responsible for color vision and detail perception in bright light.
How Vision Works
Light passes through the cornea, then through the pupil, and finally through the lens to the retina. The amount of light entering the lens is regulated by the iris. Focusing is achieved by the cornea and lens. The image focused by the lens is projected onto the retina upside down. The photoreceptors in the retina capture and transform the light signal into a nerve signal, which reaches the brain via the optic nerve. In the brain, the image is inverted again and finally perceived correctly.
The Nervous System
The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body.
Neurons: The Building Blocks
Neurons are the fundamental cells that form the nervous system. They are composed of:
- Cell Body (Soma): Contains the nucleus.
- Dendrites: Branch-like extensions that receive signals from other neurons.
- Axon: A long projection that transmits signals away from the cell body to other neurons or effector cells.
Synapse
The synapse is the junction between two neurons. Nerve signals pass from one cell to another across the synaptic cleft by means of chemical neurotransmitter molecules. This typically occurs between the axon terminal of one neuron and the dendrite of another cell.
Myelin
Myelin is a fatty substance that forms an insulating sheath around axons. It allows for the rapid and efficient transmission of nerve impulses between different parts of the body due to its insulating properties. Myelin is produced by Schwann cells, which are present in the peripheral nervous system, and oligodendrocytes in the central nervous system.
Divisions of the Nervous System
The nervous system is broadly divided into two main parts:
- Central Nervous System (CNS): Consists of the brain and spinal cord.
- Peripheral Nervous System (PNS): Composed of all the nerves that connect the Central Nervous System to the body’s tissues and organs.
The Spinal Cord
The spinal cord’s functions include immediate control and involuntary movements, such as reflexes, and regulating the sympathetic and parasympathetic nervous systems. It serves as the primary communication pathway between the brain and the rest of the body.
The Reflex Arc
Some actions are involuntary and do not involve the brain; only the spinal cord is involved. These actions are called reflex arcs. Reflex arcs are essential defense mechanisms that require rapid reaction.
For example, when you prick a finger, you automatically withdraw it. You don’t consciously think and process the information before reacting and pulling it away.
Subdivisions of the Peripheral Nervous System (PNS)
The PNS is further divided into:
- Somatic Nervous System (SNS): Composed of nerves that coordinate the body’s voluntary movements, such as moving your arms, legs, etc.
- Autonomic Nervous System (ANS): Consists of nerves connecting the CNS with various internal organs. This control is involuntary and is managed through two types of nerves that determine two divisions: the sympathetic and parasympathetic divisions.
Autonomic Nervous System Divisions
- Sympathetic Division: Prepares the body for ‘fight or flight’ responses. It generally increases heart rate, dilates pupils, and inhibits digestion.
- Parasympathetic Division: Promotes ‘rest and digest’ functions. It generally slows heart rate, constricts pupils, stimulates digestion, and promotes bowel movements and gastric juice production.
Basic Biological Components
Nucleotides are made up of three elements: a sugar, a phosphate group, and a nitrogenous base.