The Human Nervous System and Sense Organs
Coordinating Systems in Humans: Nervous vs. Endocrine
Humans possess two primary coordinating systems that allow them to respond to changes in both internal and external environments.
The Endocrine System:
Consists of endocrine glands that secrete hormones directly into the bloodstream.
Hormones travel via blood to a specific target organ or target tissue to elicit a response.
Response Characteristics: Generally takes longer to take effect and the effects endure for a longer period.
Example: If you see a snake and realize it is rubber, your heart rate and sweaty palms (triggered by an adrenaline rush) remain elevated for some time even after you know there is no threat.
The Nervous System:
Utilizes neurons and electrical impulses to transmit information from a receptor to the central nervous system, and then toward an aector (effector) to bring about a response.
Response Characteristics: Much more rapid and the duration of the effect is much shorter.
Example: Placing a hand on a hot plate causes a rapid reflex to pull the hand away to prevent injury.
Divisions of the Nervous System
Central Nervous System (CNS):
Comprised of the brain and the spinal cord.
Acts as the central hub for integrating and processing functions.
Receives information from the peripheral nervous system and decides on the appropriate response.
Peripheral Nervous System (PNS):
Consists of cranial nerves (branching from the brain) and spinal nerves (branching from the spine).
Collects information from all other organs and tissues and transmits it back to the CNS.
Relays impulses from the CNS to the aectors (muscles and glands).
Protection of the Central Nervous System
The nervous system is essential for survival, necessitating three main types of protection:
1. The Menangies:
Three layers of membranes located between the skull and the brain (specifically the cerebrum).
Provide a protective layer for the brain tissue.
2. Bony Structures:
Cranium: Part of the skull that surrounds and protects the brain.
Vertebral Column: A bony structure that surrounds and protects the spinal cord.
3. Cerebrros Spinal Fluid (CSF):
Located in both the cerebrum area and the spinal cord (central canal).
Functions as a shock absorber.
Ensures adequate pressure within the brain and spinal cord.
Prevents the dehydration of neurons and nervous tissue.
Anatomy and Functions of the Brain
Cerebrum:
The largest part of the brain, consisting of two hemispheres (left and right).
Highly folded to provide a large surface area for brain matter.
Functions:
Interprets all sensory information (senses like smell, hearing, touch, taste, and sight).
Controls voluntary actions (initiates the thought process behind movement).
Controls higher cognitive functions: memory, judgment, reasoning, and emotional intelligence.
Lobes of the Cerebrum:
Occipital Lobe (back): Processes visual information/sight.
Temporal Lobe (side): Processes smell, hearing, taste, and language association.
Parietal Lobe (top): Interprets skin sensations.
Frontal Lobe (front): Responsible for higher cognitive skills, judgment, and emotional intelligence.
Note on Brain Maturity: The brain matures from back to front; the frontal lobe (responsible for impulse control) is the last part to become fully mature in adolescents.
Cerebellum:
Located at the back of the brain.
Functions:
Coordinates voluntary movement by controlling skeletal muscles.
Maintains muscle tone, balance, and equilibrium.
Receives information from the ear to help maintain bodily balance.
Corpus Colosum:
A white structure that connects the left and right hemispheres.
Coordinates activities between the two hemispheres, allowing them to communicate.
Hypothalamus:
Located just above the pituitary gland.
Functions:
Controls the thirst center (water balance) and temperature regulation.
Manages blood pressure, sleeping rhythms, appetite, and emotions.
Controls ADH secretion and communicates with the pituitary gland.
Medulla Blangata:
Sometimes called the "brain stem," located diagonally below the cerebellum, just above the forarm and magnum.
Functions:
Controls vital functions: heart rate, breathing rate, and blood pressure.
Controls less vital functions: swallowing and salivation.
Nerve Crossover: Information from the right side of the body crosses to the left hemisphere here, and vice versa.
Critical Note: Any damage to this area results in immediate death.
The Spinal Cord: Structure and Functions
Positioning: Exits the skull through the forarm and magnum and extends down the vertebral column.
Cross-section Components:
Central Canal: Contains cerebrros spinal fluid.
Grey Matter: H-shaped area containing cell bodies and inter neurons; gray color comes from the cytoplasm.
White Matter: Surrounding area containing axons coated in myelin sheaths (fatty substance giving a white appearance).
Neural Circuitry:
Dorsal Root: Entry point for sensory neurons. Contains the Dorsal Root Ganglen, a "bubble" or swelling where the sensory neuron cell body is located.
Ventral Root: Exit point for motor neurons.
Functions:
Pathway for nerve impulses traveling to and from the brain.
Center for reflex actions.
Neurons and Nerves
Analogy: A nerve is like a laptop charger cable. The copper wires inside are the neurons (the small functional unit), and the plastic casing is the nerve.
Types of Neurons:
Motor Neuron:
Structure: Cell body with protrusions (dendrites) leading to a long axon and axon terminals.
Direction of impulse: Dendrites $\rightarrow$ Cell body $\rightarrow$ Axon $\rightarrow$ Axon terminals.
Sensory Neuron:
Structure: Cell body is located in the center (unipolar/bipolar look).
Direction of impulse: Dendrites $\rightarrow$ Cell body $\rightarrow$ Axon.
Reflex Arc vs. Reflex Action:
Reflex Action: A quick, automatic response to a stimulus that does not involve the brain (e.g., blinking, coughing, jerking a foot away from a nail).
Reflex Arc: The specific pathway an impulse follows from the receptor to the aector.
Pathway: Receptor $\rightarrow$ Sensory Neuron $\rightarrow$ Inter Neuron (in spinal cord) $\rightarrow$ Motor Neuron $\rightarrow$ Aector (muscle or gland).
Questions & Discussion: Damage to the Spinal Cord
Scenario: What happens if the Dorsal Root is damaged?
If the dorsal root is cut, the impulse from the receptor cannot reach the inter neuron or the CNS.
The person will not feel the stimulus (e.g., a pinprick) and there will be no reflex reaction. They have not lost muscle control, but they have lost sensation.
Scenario: What happens if the Ventral Root is damaged?
The impulse can reach the CNS via the dorsal root, so the person will feel the pain.
However, the impulse cannot reach the aector (muscle) because the motor neuron path is broken. The person can feel it but cannot respond or move the limb.
Synapses
A sinapse is the gap between two neurons.
Mechanism:
Impulse reaches axon terminals $\rightarrow$ Neurotransmitters are released into the synaptic cliff (cleft) $\rightarrow$ They bind to receptors on the next neuron $\rightarrow$ A new impulse is generated.
Neurotransmitters are reabsorbed by the presinaptic membrane to prevent continuous stimulation.
Significance:
Ensures impulses move in one direction only.
Allows impulses to be transmitted to more than one neuron.
Determines which impulses are transmitted forward.
Peripheral Nervous System (PNS) Divisions
Somatic Nervous System:
Controls voluntary actions (deciding to run, pick up an object).
Autonomic Nervous System (Automatic):
Controls involuntary actions (heartbeat, sneezing, blinking).
Divided into two antagonistic systems:
1. Sympathetic Nervous System:
Prepares the body for "Fight or Flight."
Effects: Pupils dilate, heart rate increases, airways relax, stomach activity is inhibited (blood redirected to muscles), glucose is released for energy, adrenaline is secreted.
2. Parasympathetic Nervous System:
Active during "Rest and Digest."
Effects: Pupils constrict, heart rate calms, respiratory system slows, blood flow increases to the digestive system.
Disorders of the Nervous System
Alzheimer's Disease:
Characterized by the slow degeneration of nervous tissue.
The brain appears shriveled. Thermal imaging shows black areas where no impulses are processed.
Symptoms include memory loss and confusion.
Multiple Sclerosis:
Caused by the degeneration of the myelin sheath that insulates neurons.
Analogy: Like a charging cable with the plastic coating eaten away by a bunny; the connection (impulse) is lost.
Symptoms: Loss of muscle control and coordination. It is a progressive disease that may eventually affect breathing.
Anatomy of the Eye
Binocular/Stereoscopic Vision:
Our eyes are at the front of the face, allowing visual fields to overlap.
Benefits: Creates a 3D image and provides better depth perception (essential for predators).
Sclera:
The white, inelastic outer layer of the eye that maintains shape.
Muscles that move the eye within the socket are attached here.
Cornea:
The transparent, inelastic extension of the sclera at the front.
Allows light to enter; it is the primary site of light refraction ().
Koid (Choroid):
Darkly pigmented middle layer.
Absorbs excessive light rays to prevent internal reflection.
Contains blood vessels that supply oxygen and nutrients.
Retina:
Inner layer containing photo receptors:
Rods: For black and white vision.
Cones: For color vision.
Yellow Spot (Fovea):
Area on the retina providing the clearest (HD) vision.
Blind Spot:
The point where the optic nerve and blood vessels exit the eye.
Contains no photo receptors; light falling here is not seen.
Iris:
The colored part of the eye containing circular and radial muscles.
Responsible for the pupillary mechanism.
Lens:
An elastic, transparent, jelly-like structure behind the iris.
Changes shape to focus light on the yellow spot (Accommodation).
Humors:
Aqueous Humor: Watery fluid in the front chamber; provides nutrients and oxygen to the lens and cornea (which lack blood vessels).
Vitrius Humor: Jelly-like fluid in the large back cavity; maintains the eye's shape and keeps layers pressed together.
The Pupillary Mechanism
A reflex action to protect the retina from overexposure or to allow more light in.
In Bright Light:
Circular muscles of the iris contract.
Radial muscles of the iris relax.
Pupil constricts (becomes smaller).
Less light enters the eye.
In Dim Light:
Radial muscles of the iris contract (like pulling up blinds).
Circular muscles of the iris relax.
Pupil dilates (becomes larger).
More light enters the eye.
Eye Accommodation
The series of changes in the shape of the lens to focus on objects at various distances.
Distant Objects (> 6\,m):
Ciliary muscles relax.
Ciliary body moves further from the lens.
Suspensory ligaments become tight.
Tension on the lens increases.
Lens becomes less convex (flatter).
Light rays are refracted less and focused on the yellow spot.
Near Objects (< 6\,m):
Ciliary muscles contract (hard work; can cause headaches).
Ciliary body moves closer to the lens/bulges.
Suspensory ligaments become slack.
Tension on the lens decreases.
Lens becomes more convex (rounder/bulges).
Light rays are refracted more and focused on the yellow spot.
Visual Defects
Shortsightedness (Myopia):
Can see near objects clearly; distant objects are blurry.
Cause: Eyeball too long or cornea too curved. Light focuses in front of the retina.
Correction: Concave lens.
Long-sightedness (Hypermetropia):
Can see distant objects clearly; near objects are blurry.
Cause: Eyeball too short or lens cannot become convex enough. Light focuses behind the retina.
Correction: Convex lens.
Astigmatism:
Eyeball is rugby ball-shaped instead of soccer ball-shaped.
Light falls short of the retina or is distorted.
Correction: Corrective lenses or laser surgery.
Cataracts:
Proteins coagulate in the lens, making it opaque and dispersing light.
Leads to semi-blindness or full blindness.
Correction: Surgery to replace the lens.