Sensory Systems and the Nervous and Endocrine Systems Study Guide
Receptors and Stimuli
Definitions:
Stimulus: A change in the environment, which can be internal or external.
Receptor: A specialized cell that detects a stimulus.
Types of Stimuli:
External Stimuli: Factors outside the body, such as light, sound, and temperature.
Internal Stimuli: Factors inside the body, such as hunger, thirst, or dehydration.
The Five Major Receptors:
Photoreceptors: Detect light; found in the eyes.
Mechanoreceptors: Detect sound and touch/pressure; found in the ears and skin.
Chemoreceptors: Detect chemicals involved in smell and taste; found in the nose and tongue.
Thermoreceptors: Detect temperature changes; found in the skin.
Nociceptors: Detect pain; found in all organs.
Mechanism of Action: Receptors detect changes (stimuli) and send this information to the brain to create an appropriate response. For example, when the body detects dehydration, it signals thirst, leading to the response of drinking water or sweating.
The Sense Organs: Sight and Hearing
The Distance Senses: Sight and hearing allow for the detection of stimuli from a distance.
Eyes (Sense of Sight):
Stimulus: Light waves.
Anatomy and Function:
Cornea: Bends incoming light.
Iris: Controls the amount of light entering the eye.
Lens: Focuses incoming light onto the back of the eye.
Retina: Contains photoreceptor cells that convert light into nerve signals.
Optic Nerve: Carries nerve impulses from the retina to the brain.
Ears (Sense of Hearing):
Stimulus: Sound waves (vibrations).
Pathway of Sound:
Sound enters through the ear canal.
Vibrations hit the eardrum.
Vibrations pass through the middle ear bones.
Vibrations reach the cochlea, which contains fluid.
Movement of the cochlear fluid is converted into electrical signals (nerve impulses) by nerve cells.
Nerve impulses travel through the auditory nerve to the brain.
The Sense Organs: Taste and Smell
The Chemical Senses: Both taste and smell rely on chemoreceptors to detect molecules.
Tongue (Sense of Taste):
Stimulus: Chemicals in food.
Anatomy: The tongue is covered in bumps called papillae. These papillae contain taste buds, which house the taste receptor cells.
Taste Categories: Sweet, salty, sour, bitter, and umami (savory).
The Taste Pathway: Chemical molecules in food dissolve in saliva $\rightarrow$ bind to taste receptors in taste buds $\rightarrow$ nerve signals $\rightarrow$ brain interprets flavor.
Supertasters: Individuals with many more papillae than average, resulting in more intense flavor perception (e.g., finding black coffee extremely unpleasant).
Nose (Sense of Smell):
Stimulus: Chemicals in the air.
Anatomy:
Nostrils: Entry point for air.
Mucus: Produced by nasal sinuses to trap bacteria and particles.
Smell Receptors: Located above the nasal cavity; they stimulate the olfactory bulb.
Olfactory Bulb: Interprets odors and sends messages to the brain.
The Link Between Smell and Taste: Up to of what we perceive as flavor is actually smell. When the nose is blocked (e.g., during a cold), smell signals cannot reach the brain, making food taste bland.
Animal Senses:
Dogs: Use tongues for taste and temperature control (panting).
Elephants: Use trunks to detect water and danger from kilometers away.
Bats: Use large ears for echolocation to locate prey.
The Sense Organs: Touch
Distribution: Unlike other senses confined to specific organs, touch receptors are distributed across the entire body surface (the skin).
Skin Layers:
Epidermis: The outer layer that provides a protective barrier.
Dermis: The inner layer containing nerve endings (receptors), blood vessels, and sweat glands.
Subcutaneous Fat Layer: Below the dermis; provides insulation.
Sensitivity: The fingertips have approximately .
Stimuli Detected: Heat, cold, pressure, pain, and light touch.
The Touch Pathway: Stimulus (e.g., heat) $\rightarrow$ nerve endings in dermis detect it $\rightarrow$ signals sent to brain $\rightarrow$ brain generates response (e.g., withdrawing hand).
Neurons: Nerve Cell Structure
Definition: Neurons are specialized cells that carry electrical messages (nerve impulses) throughout the body. There are approximately billion neurons in the human body.
Parts of a Neuron:
Cell Body (Soma): Contains the nucleus and genetic material; controls cell activity.
Dendrites: Branch-like endings that receive signals from other neurons.
Axon: A long fiber that carries impulses AWAY from the cell body.
Myelin Sheath: A fatty (lipid) layer wrapping around the axon. It acts like insulation on a wire, preventing signal leakage and increasing electrical speed.
Node of Ranvier: Gaps in the myelin sheath where signals 'jump,' greatly increasing speed.
Synaptic Terminal (Axon Terminal): Small bulbs at the end of the axon that release neurotransmitters.
Signal Transmission (The Synapse):
An electrical impulse travels along the axon.
The signal reaches the synaptic terminal.
Neurotransmitters (chemical messengers) are released into the synapse (the tiny gap between neurons).
Neurotransmitters cross the gap and bind to receptors on the dendrites of the next neuron.
Types of Neurons
Sensory Neurons (Afferent):
Function: Carry messages FROM receptors (eyes, ears, skin) TO the Central Nervous System (CNS).
Structure: Long dendrites, short axon.
Interneurons (Connector):
Function: Link sensory and motor neurons. Responsible for processing and decision-making.
Location: Found only in the brain and spinal cord (CNS). They are the most common type of neuron.
Motor Neurons (Efferent):
Function: Carry messages FROM the CNS TO effectors (muscles or glands) to produce a response.
Structure: Long axon reaching to effectors.
The Stimulus-Response Model
The Pathway:
Stimulus: Environmental change.
Receptor: Detects stimulus, converts to electrical signal.
Sensory Neuron: Carries signal to CNS.
Control Centre (CNS): Processes information and decides response.
Motor Neuron: Carries response signal to effector.
Effector: Muscle or gland that carries out the action.
Response: The final action (e.g., pupil constriction, muscle contraction).
Reflex Actions
Definition: An involuntary, nearly instantaneous, and automatic movement in response to a stimulus.
The Reflex Arc:
The signal travels to the spinal cord instead of waiting for brain processing.
The interneuron in the spinal cord sends signals simultaneously to the motor neuron (for immediate movement) and the brain (for awareness after the fact).
Purpose: Survival and protection from danger before the brain has time to "think."
Examples: Knee-jerk (patellar reflex), blinking, sneezing, and pulling away from heat.
Key Distinction: Reflexes are faster than voluntary actions because they bypass the brain for the initial response.
The Central and Peripheral Nervous Systems
Central Nervous System (CNS):
Consists of the Brain and Spinal Cord.
The control center protected by the skull and vertebrae.
Peripheral Nervous System (PNS):
Consists of all nerves outside the CNS.
Carries messages to and from the CNS.
Divisions of the PNS:
Somatic Nervous System: Controls voluntary skeletal muscle movements (e.g., walking, writing).
Autonomic Nervous System: Controls involuntary functions (e.g., heartbeat, digestion). It maintains homeostasis automatically.
Divisions of the Autonomic Nervous System:
Sympathetic: "Fight or Flight" response. Increases heart rate, dilates pupils, increases breathing, inhibits digestion.
Parasympathetic: "Rest and Digest" response. Slows heart rate, constricts pupils, promotes digestion.
Anatomy of the Brain
Main Parts:
Cerebrum: Largest part; responsible for memory, personality, and voluntary movement. Divided into four lobes.
Cerebellum: Back of the brain; controls fine movement, balance, and coordination.
Brain Stem (including Medulla Oblongata): Connects to the spinal cord; controls vital automatic functions like breathing and heart rate.
The Four Lobes of the Cerebrum:
Frontal Lobe: Emotions, personality, reasoning, problem-solving.
Parietal Lobe: Perception of taste, pain, pressure, and temperature.
Temporal Lobe: Hearing, sound recognition, and memory formation.
Occipital Lobe: Vision processing and object identification.
Spinal Cord: Acts as the highway for messages between the brain and the rest of the body.
Nervous System Disorders
Spinal Cord Damage: Signals below the injury cannot reach the brain. High injury results in quadriplegia (arms and legs); lower injury results in paraplegia (legs only).
Slipped Disc: A weak intervertebral disc bulges and presses on a spinal nerve, causing pain or numbness.
Multiple Sclerosis (MS): An autoimmune disease where the immune system destroys the myelin sheath. This leads to lost or slowed signals, causing muscle weakness and vision problems.
Motor Neurone Disease (MND/ALS): Motor neurons weaken and lose function, leading to progressive muscle wasting and eventually affecting breathing.
Alzheimer's Disease: Progressive damage to brain neurons. Plaques develop around neurons, leading to brain shrinkage, memory loss, and confusion.
The Endocrine System
Definition: A collection of glands that secrete hormones directly into the bloodstream.
Hormone: A chemical messenger carried by the blood to target cells.
Mechanism: Hormones use a "lock and key" model; only target cells with specific receptors that match the hormone will respond.
Key Glands and Hormones:
Pituitary Gland: Known as the "Master Gland"; produces ADH (kidneys), Growth Hormone (bones), and TSH (thyroid).
Hypothalamus: Controls hunger, thirst, and temperature.
Thyroid: Produces Thyroxine to regulate metabolism.
Pancreas: Produces Insulin (lowers blood glucose) and Glucagon (raises blood glucose).
Adrenal Glands: Produce Adrenalin for the fight-flight-freeze response.
Gonads (Ovaries/Testes): Produce Oestrogen, Progesterone, and Testosterone.
Homeostasis and Negative Feedback
Homeostasis: The process of maintaining a stable internal state (e.g., temperature at $\sim37\,^{\circ}C$, blood glucose, water levels).
Negative Feedback Loop: A mechanism where the body's response opposes the original stimulus to restore balance.
Pathway: Stimulus $\rightarrow$ Receptor $\rightarrow$ Control Centre $\rightarrow$ Effector $\rightarrow$ Response (which removes the stimulus).
Blood Glucose Regulation:
High Glucose: Pancreas (receptor) $\rightarrow$ Insulin released $\rightarrow$ Liver/muscles (effectors) store glucose as glycogen $\rightarrow$ Glucose levels fall.
Low Glucose: Pancreas (receptor) $\rightarrow$ Glucagon released $\rightarrow$ Liver (effector) breaks down glycogen into glucose $\rightarrow$ Glucose levels rise.
Water Regulation (ADH):
Low Water: Hypothalamus detects low water $\rightarrow$ Pituitary releases ADH (Antidiuretic Hormone) $\rightarrow$ Kidneys reabsorb more water $\rightarrow$ Small volume of concentrated (dark) urine.
High Water: Hypothalamus signals stop ADH $\rightarrow$ Kidneys reabsorb less water $\rightarrow$ Large volume of dilute (clear) urine.
CO2 and O2 Regulation:
Monitored by the Medulla in the brain stem. High $CO_2$ levels (e.g., during exercise) trigger a faster heart and breathing rate to remove $CO_2$ and deliver $O_2$.
Questions & Discussion
Think-Pair-Share: "If the cochlea is damaged, can the brain still 'hear' the sound?"
Answer: No, because the cochlea is responsible for converting physical vibrations into electrical nerve impulses. Without this conversion, no signal reaches the brain.
Checkpoint Question: "Why are reflexes faster than normal responses?"
Answer: They are processed in the spinal cord and happen before conscious thought, bypassing the delay of brain processing.
Checkpoint Question: "Explain why a supertaster might find black coffee more unpleasant than a non-taster."
Answer: Supertasters have significantly more papillae/taste buds, making them perceive bitter chemicals in coffee much more intensely.
Checkpoint Question: "Compare the fight-flight-freeze response of a real threat versus a panic attack."
Answer: Both share identical physical symptoms (adrenaline surge, racing heart). The difference is context: a real threat is a survival response to danger, whereas a panic attack is a "false alarm" in the absence of a threat.
Retrieval Question: "Which type of neuron detects a hot stove?"
Answer: Sensory neurons detect the stimulus and send the message to the CNS.