Human Body Systems: Nerves, Senses, and Coordination
Body Coordination: Nervous System & Senses
Interaction and Coordination
The nervous system interacts with both internal and external environments, enabling the body to detect changes and provide appropriate responses.
The Basic Pathway of a Response
- Stimulus: An external or internal change.
- Receptors: Detect the stimulus and convert it into an electrical impulse.
- Central Nervous System (CNS): Processes the impulse.
- Responses:
- Muscle Effector: Leads to a motor response (movement).
- Gland Effector: Leads to a secretory response (e.g., hormone release).
Coordination Systems
- Nervous System:
- Receives and transmits information rapidly via electrical impulses through sensory neurons.
- Responses are short-lived.
- Endocrine System:
- Consists of glands that secrete hormones.
- Hormones are carried in the blood to target cells.
- Responses are slow but long-lasting.
Sensory Receptors
Sensory receptors capture information from both external and internal environments. When a receptor detects a stimulus, it converts it into an electrical impulse.
There are two main types:
- Interoreceptors: Detect internal body conditions and sensations (e.g., thirst, the need to urinate, etc.).
- Exteroreceptors: Detect external stimuli. These can be:
- Chemoreceptors: (Nose and tongue) Detect chemical substances.
- Mechanoreceptors: (Skin and ears) Detect physical stimuli (e.g., pressure, sound).
- Photoreceptors: (Retina) Detect light.
- Thermoreceptors: (Skin) Detect temperature changes.
- Nociceptors: (Throughout the body) Detect stimuli that cause pain.
The Sense Organs
The Sense of Sight
Light stimuli are detected by the eyes. Each eye consists of the eyeball and accessory structures. The eyeball is a sphere made up of three layers:
- Sclera: The tough, white outer layer.
- Choroid: The middle layer, rich in blood vessels.
- Retina: The innermost layer, containing photoreceptors.
Parts of the Eye
Key parts of the eye include:
- Extraocular Muscles
- Sclera
- Choroid
- Optic Nerve
- Retina
- Vitreous Humor
- Aqueous Humor
- Cornea
- Iris
- Crystalline Lens
Accessory Structures of the Eye
In addition to the eyeball, accessory structures enable movement and protection:
- Extraocular Muscles: Enable eye movement.
- Eyebrows: Protect from sweat and debris.
- Eyelids: Cover the eyes and spread tears.
- Lacrimal Glands: Secrete tears to clean and lubricate the eye surface.
- Lacrimal Sac: Collects and drains excess tears.
How the Eyes Work
Light reflected off objects passes through the cornea and the lens. The lens becomes more rounded when focusing on nearby objects; this process is called accommodation. The image formed on the retina is inverted and smaller. Photoreceptors in the retina convert the image into nerve impulses, which are then carried by the optic nerve to the cerebral cortex for interpretation.
The Sense of Touch
The skin has two main layers:
- Epidermis: The outer layer, made up of several layers, with the outermost cells being dead.
- Dermis: The inner layer, which contains various touch receptors.
Types of Touch Receptors in the Dermis
- Tactile Corpuscle (Meissner’s Corpuscle)
- Ruffini Corpuscles
- Krause Corpuscle
- Vater-Pacini Corpuscle
- Free Nerve Endings
Typical Disorders and Healthy Habits
| Organ | Disorder | Healthy Habits |
|---|---|---|
| Mouth | Caries (Cavities) | Reduce sugar in your diet |
| Skin | Psoriasis | Reduce your stress levels |
| Nasal Cavities | Rhinitis | Avoid exposure to allergens |
| Eyes | Myopia (Nearsightedness) | Use sunglasses to protect from UV |
| Ear | Otitis (Ear Infection) | Remove excess earwax carefully |
Components of the Nervous System
The nervous system detects and interprets external and internal stimuli. Neural tissue is made up mainly of two types of cells: neurons and glial cells.
Neurons
Neurons generate and transmit nerve impulses. They have three main parts:
- Cell Body (Soma): Contains the nucleus and most of the cytoplasm.
- Axon: A long fiber that transmits nerve impulses away from the cell body.
- Dendrites: Branch-like structures that receive nerve impulses from receptors or other neurons.
Classification of Neurons by Function
- Sensory Neurons: Send information from receptors to the Central Nervous System (CNS).
- Motor Neurons: Send responses from the CNS to effectors (muscles or glands).
- Relay Neurons (Interneurons): Connect sensory and motor neurons within the CNS.
Glial Cells (Neuroglia)
Glial cells support and protect neurons. The main types include:
- Astrocytes: Star-shaped cells that transport nutrients and maintain the blood-brain barrier.
- Microglia: Remove cellular waste and pathogens.
- Oligodendrocytes: Produce the myelin sheath, a fatty covering found on some axons, which insulates and speeds up nerve impulse transmission.
Transmission of Nerve Impulses
Nerve impulses are transmitted as electrical signals. The impulse always travels along the neuron in the same direction.
Synapse
Neurons are not physically connected; there is a microscopic gap between them called the synaptic cleft. Chemical messengers called neurotransmitters are released from the presynaptic neuron into this cleft. These neurotransmitters travel across the synaptic cleft and are detected by receptors on the postsynaptic neuron, typically located in the dendrites and/or cell body, thus transmitting the signal.