Biology Term 2 Year 11

Homeostasis

The relatively constant physiological state of the body despite fluctuations in the environment.

Coordination and Control

When different body parts work together to maintain a steady internal state or homeostasis.

The Two Coordinating Systems

The nervous system and the endocrine (hormonal) system are the two systems responsible for maintaining homeostasis.

Receptors

Specialised cells or sense organs that detect stimuli and send signals toward the control centre.

Control Centre (Regulator)

The coordinating centre (spinal cord or brain) that receives signals from receptors and determines whether a response is required.

Effectors

Organs or cells (muscles or glands) that carry out the response instructed by the control centre.

Stimulus Response Model

The sequential process by which the body detects and responds to change: Stimulus then Receptor then CNS then Effector then Response then Feedback.

Stimulus

A change in the internal or external environment that is detected by a receptor and triggers a response.

Feedback

A reaction to a response that either stops or maintains a process.

Negative Feedback

A self-regulating control mechanism where a system’s output acts to reduce or counteract changes, pushing the system back toward a stable set point

Negative Feedback Examples

Thermoregulation, blood sugar regulation and osmoregulation.

Positive Feedback

A feedback mechanism that reinforces and amplifies the initial change detected, continuing until the stimulus is removed. Unresolved positive feedback can be fatal.

Positive Feedback Examples

Childbirth, lactation, ovulation, blood clotting and fever.

Negative vs Positive Feedback

Negative feedback restores balance and is the dominant mechanism in homeostasis. Positive feedback amplifies change and is used for specific processes that need to reach a conclusion.

Sensory Receptors

Specialised cells that allow the body to detect and respond to a range of stimuli in both internal and external environments.

Mechanoreceptors

Sensory receptors that respond to physical pressure or distortion. Found throughout the body including skin, muscles, joints and the inner ear.

Thermoreceptors

Sensory receptors that detect changes in skin temperature and relay this to the hypothalamus. More are found in skin areas more sensitive to temperature changes.

Nociceptors

Sensory neurons activated by noxious or potentially damaging stimuli including extremes of temperature, pressure and toxic chemicals. Experienced as pain. Found externally in the skin, cornea, mouth and nose, and internally in several organs.

Photoreceptors

Sensory cells in the eyes that detect colour, intensity and movement of light.

Chemoreceptors

Sensory receptors that detect the presence of chemicals including smell, taste and the detection of CO₂ and O₂ levels in the blood. Used to adjust breathing and heart rate to regulate blood composition.

Neuron

A nerve cell that transmits information as electrical impulses. Consists of a cell body, dendrites and an axon.

Nerve

A bundle of neuronal fibres.

Cell Body (Soma)

The part of the neuron that contains the nucleus and other organelles.

Dendrites

The branching structure of a neuron that receives incoming messages and carries them toward the cell body.

Axon

The long extension of a neuron that carries nerve impulses away from the cell body.

Axon Terminals

The hair-like ends of the axon.

Synaptic Knobs

Located at the end of an axon and release neurotransmitters across the synapse.

Myelin Sheath

A fatty substance that insulates the axon.

Nodes of Ranvier

Gaps in the myelin sheath where the action potential occurs during saltatory conduction.

Schwann Cells

Cells located within the myelin sheath of the PNS that produce myelin.

Synapse

The cell junction (gap) between a synaptic terminal (knob) and a target cell such as another neuron or effector cell.

Ganglia

Nodes or swellings formed when nerve cell bodies are grouped together.

Central Nervous System (CNS)

Made up of the brain and spinal cord.

Peripheral Nervous System (PNS)

Made up of all cranial and spinal nerves outside of the brain and spinal cord.

CNS vs PNS

The CNS (brain and spinal cord) is the processing and control centre. The PNS (cranial and spinal nerves) connects the CNS to the rest of the body.

Autonomic Nervous System

Regulates involuntary bodily processes including heart rate, digestion and respiration without conscious effort.

Sympathetic Nervous System

A division of the autonomic nervous system that prepares the body for action and stress (fight or flight).

Parasympathetic Nervous System

A division of the autonomic nervous system that calms the body and conserves energy. Heart rate lowers and bladder relaxes

Sympathetic vs Parasympathetic

The sympathetic system activates the body under stress. The parasympathetic system restores the body to a calm resting state.

Afferent (Sensory) Neurons

Neurons that receive stimuli and send signals toward the CNS.

Association Neurons (Interneurons)

Neurons found only in the CNS that connect sensory neurons to motor neurons.

Efferent (Motor) Neurons

Neurons that carry signals from the CNS to effectors such as muscles or glands.

Afferent vs Efferent Neurons

Afferent neurons carry signals toward the CNS. Efferent neurons carry signals away from the CNS.

Reflex Arc

An automatic response to a stimulus.

Signal Transduction (Nervous System)

The process by which a stimulus is converted and transmitted. A receptor cell generates a nerve impulse, which passes through the cytoplasm to the axon, travels along the axon membrane, and then triggers neurotransmitter release at the synapse to stimulate the next neuron.

Nerve Impulse

The movement of an action potential along a neuron as a series of electrical depolarisation events in response to a stimulus. Results from changes in Na⁺ and K⁺ ion concentrations across the neuron membrane.

Resting Potential

The electrical potential across the membrane of a neuron that is not conducting an impulse, maintained at approximately 70mV. Maintained by the Na⁺/K⁺ pump which actively transports Na⁺ out and K⁺ in.

Action Potential

The depolarisation and repolarisation of the electrical potential across the neuron membrane as a nerve impulse passes along a neuron.

Depolarisation

Occurs when voltage-gated Na⁺ channels open and Na⁺ rushes into the neuron, causing the outside of the neuron to become negative.

Repolarisation

Occurs after depolarisation when K⁺ channels open and K⁺ diffuses out, restoring the resting potential.

Resting Potential vs Action Potential

Resting potential is the stable charge across the membrane when the neuron is inactive. Action potential is the temporary reversal of this charge during impulse transmission.

Threshold

The minimum level of stimulus required to trigger an action potential.

Refractory Period

The brief recovery period after an action potential during which ion channels reset. Ensures the nerve impulse travels in one direction only along the axon.

Synaptic Transmission

The process by which a nerve impulse is passed from one neuron to the next across a synapse via neurotransmitters released from synaptic vesicles.

Synaptic Vesicles

Membrane-bound sacs in the axon termianls that store and release neurotransmitters into the synapse.

Neurotransmitters

Chemical messenger substances released from synaptic knobs into the synapse to transmit signals to the next neuron or effector cell.

Excitatory Synapse

A synapse where the neurotransmitter promotes the continuation of the action potential in the next neuron. Adrenaline is an example of an excitatory neurotransmitter.

Inhibitory Synapse

A synapse where the neurotransmitter prevents or reduces the likelihood of an action potential in the next neuron. Acetylcholine is an example of an inhibitory neurotransmitter.

Excitatory vs Inhibitory Synapses

Excitatory synapses continue the nerve impulse. Inhibitory synapses dampen or block it, allowing the nervous system to fine-tune responses.

Drugs and Poisons

Exert their effects by interfering with synaptic mechanisms, primarily by altering the activity of neurotransmitters such as dopamine and serotonin.

Amphetamines

Noradrenalin mimics that increase arousal. Used clinically as antidepressants to elevate mood, increase alertness and reduce fatigue. Can lead to hallucinations due to lack of sleep.

Stimulants

Substances that increase activity of the nervous system.

Hallucinogens

Substances that alter perception and cause hallucinations.

Depressants

Substances that slow down nervous system activity.

Opiates

Substances that act on the nervous system to relieve pain.

Hormones

Chemical messengers produced by glands that regulate the activity of specific cells and organs. They are specific, slow acting and long lasting.

Hormones vs Nerve Impulses

Hormones are slow acting and long lasting chemical signals that travel via the bloodstream. Nerve impulses are fast and short-lived electrical signals that travel along neurons.

Endocrine System

The system of glands that secrete hormones directly into the bloodstream. Glands work together despite being physically isolated from one another.

Stimuli for Hormonal Release

Three triggers exist: the presence of a specific metabolite in the blood, the presence of another hormone in the blood, or stimulation by the autonomic nervous system.

Hypothalamus

The brain region that controls homeostasis. Collects information from other parts of the brain, monitors hormone levels and other chemicals in the blood, and communicates changes to the pituitary gland via nervous conduction.

Pituitary Gland

A gland attached to the hypothalamus that produces hormones instructing other cells, glands and organs to regulate metabolism, growth, sexual maturation, reproduction, blood pressure and other vital functions. Has a posterior and anterior lobe.

Hypothalamus vs Pituitary Gland

The hypothalamus detects changes and passes this to the pituitary gland via nervous conduction. The pituitary gland then responds with negative feedback by releasing hormones that instruct other glands and organs.

Thyroid Gland

An endocrine gland that regulates heart rate, blood pressure, body temperature, growth and metabolism. Uses iodine to produce its hormones.

The Two Most Important Hormones of the Thyroid Gland

Thyroxine (T4) and Triiodothyronine (T3).

Pancreas

An endocrine organ involved in regulating blood sugar levels.

Adrenal Complex

Endocrine glands involved in the stress response and regulation of metabolism.

Autocrine Signalling

A form of hormonal signalling where the hormone binds to receptors on the same cell that produced it.

Paracrine Signalling

A form of hormonal signalling where the hormone binds to receptors on neighbouring cells.

Endocrine Signalling

A form of hormonal signalling where the hormone travels long distances through the bloodstream to reach target cells.

Autocrine vs Paracrine vs Endocrine Signalling

Autocrine acts on the same cell. Paracrine acts on neighbouring cells. Endocrine acts on distant cells via the bloodstream.

Pheromones

Hormones that act at a distance between organisms.

Other Chemical Signals

Neurotransmitters, messages from the immune system, growth factors and toxins can all affect change in cells.

Hormone Receptors

Protein structures on the cellular membrane or within the cytoplasm whose shape is complementary to a specific hormone. There are many different types that each respond to specific signals.

Receptor Regulation

A cell constantly monitors what signals are coming to its receptors and adjusts their number accordingly. More receptors means stronger sensitivity.

Up-Regulation

The increase in the number of receptors on a cell in response to a weak signal, increasing the cell's sensitivity to a hormone. Example: the uterus up-regulates oxytocin receptors during pregnancy.

Down-Regulation

The recycling or removal of receptors from the cell membrane in response to a strong or prolonged signal, reducing the cell's sensitivity to a hormone. Example: insulin receptors are down-regulated when the body is exposed to high insulin levels.

Up-Regulation vs Down-Regulation

Up-regulation increases receptor numbers to amplify sensitivity to a weak signal. Down-regulation reduces receptor numbers to dampen sensitivity to an excessive signal.

Signal Transduction (Hormonal)

A three stage process by which a hormonal signal produces a cellular response: signal perception (hormone binds to receptor), intracellular signal transduction (signal is relayed within the cell), and cellular response (the cell carries out the appropriate change).

What is the purpose of myelin?

Myelin acts as a insulator to increase the speed that the impulses travel because it forces the impulse to jump from one uninsulated region to another.

How do action potentials move in myelinated vs non myelinated neurones?

In myelinated neurones the action potential jumps between uninsulated regions (nodes of ranvier), in non myelinated action potentials move as a continuous wave of depolarization.

Monosynaptic vs polysynaptic reflex arc

A monosynaptic reflex arc involves only one CNS synapse, whereas polysynaptic reflex arcs involve two or more.

Sequence of events in a neurone when it receives a stimulus sufficient to reach threshold

When a neuron receives a stimulus that reaches the threshold (approx. \(-55\text{mV}\)), voltage-gated sodium (\(Na^{+}\)) channels open, causing a rapid influx of sodium ions that depolarizes the membrane to about \(+40\text{mV}\). Next, these voltage-gated sodium channels inactivate, while voltage-gated potassium (\(K^{+}\)) channels open, allowing potassium ions to rush out of the cell. This outflow of positive ions causes repolarization, which returns the interior of the neuron to a negative charge. Finally, because potassium channels close slowly, the membrane briefly hyperpolarizes—becoming more negative than the resting potential—before the sodium-potassium pump restores the resting state.