8.1.2 Systems to Maintain Internal Environment Flashcards
(16 cards)
Systems
…and Types of Receptors included
The nervous and endocrine systems work together and individually to maintain homeostasis by creating a pathway to relay messages via nerves (nervous system) or hormones (endocrine system) from the receptors to the control centre and then to the effectors to initiate a response.
In both systems, receptors are responsible for detecting stimuli using their sensory cells.
- External receptors - grouped together in sense organs
- Interoceptors - within the body to detect internal stimuli
- Thermoreceptors - detects changes in temperatures
- Chemoreceptors - detects changes in pH and concentrations of chemicals
- Osmoreceptors - detects changes in osmotic pressure
Parts of the Nervous System
The nervous system has two main parts:
Central Nervous System (CNS) - the brain and spinal cord
- Hypothalamus is the control centre for regulation and directs effectors to carry out responses via neural or chemical messages.
- Spinal cord is the conduction pathway for nerve impulses and coordinate reflex actions
Peripheral Nervous System (PNS) - all other nerves throughout the body that carry messages in the form of electrochemical impulses to and from the CNS, causing voluntary or involuntary actions
- Neurons (nerve cells) can be sensory, motor or interneurons
Structure of a Neuron
Nervous System
Soma is the cell body containing the nucleus and other organelles, forming the grey matter of the CNS
Axon is the long extension of the cytoplasm that carries messages away from the cell body to form the white matter of the CNS
- Myeline sheath is an insulating layer that forms around the axons, allowing electrical impulses to transmit quickly
- Terminal branches change electrical impulses into chemical messages (neurotransmitters)
- Axon terminal is at the end of the axon to facilitate communication with other structures
Dendrites are the fine branches of the cytoplasm that receive messages from other axons in the form of impulses
Types of Neurons
Nervous System
Sensory Neurons - carry impulses from the sensory cells in the PNS to the CNS (cell body to the side, long dendron and short axon)
- Associated with specialised receptor cells
- Dendrites found outside the spinal cord in the skin, muscle or receptor gland. Axons end in the spinal cord where they connect with other dendrites.
Motor Neurons - transfers messages from CNS to effectors (short dendrites and long axon)
- Dendrites and cell body in the spinal cord, while axons are in the effector
Interneurons - located within the CNS and link sensory and motor neurons (short dendrites)
Transmitting Messages Along the Nervous System
Nervous System
Neutrons transmit the message as an electrochemical signal. Though they themselves are not touching, the axons of one and the dendrite of another will be very close. The gap between them is a synapse (synaptic gap).
A nerve impulse reaches the axon terminal as electrical potential.
- Electrical potential is the amount of work that must be done to separate oppositely charged particles or the work that can be produced by the spontaneous motion of charged particles.
The action potential triggers to release of neurotransmitters, which are released into the synapse to bind the receptors to another dendrite.
- The change in concentration of chemicals (e.g. Na+, K+, Cl-) by movement across the cell membrane causes an electrical impulse. These ions are present on both sides of the membrane in different concentrations, and the selectively permeable membrane allows them to diffuse.
This initiates an action potential in the next neuron, continuing the message.
At Rest (Not Transmitting Messages)
Nervous System
If no electrochemical message is transmitting, the neuron is at rest, and the ions on either side attempt to balance out. This is not possible as Na+ can only move through channels that are closed when the neuron is at rest, but the K+ channels are open. The higher negative charge on the inside causes it to be polarised (resting potential: -70mV) and attract the positively charged sodium ions. The pressure on sodium ions to enter the neuron builds up due to the concentration gradient.
- The cell membrane is a non-polar region, making it difficult for the ions to diffuse and leaving only ion channels as passageways for travel.
- Since ions can’t diffuse through the membrane, the electric potential difference (voltage) is constant, called the resting membrane potential.
Upon a stimulus being detected, neurotransmitters can come and attach to the receptors, a signal causing the channels “membrane potential” to change at times. This can result in depolarisation (excites the neuron to make resting potential less negative) or hyperpolarisation (inhibits the neuron to make the resting potential more negative). The intensity of the signal or number of neurotransmitters are proportional to the change in membrane potential.
- The refractory period is after the neuron become hyperpolarised.
As ions pass through the membrane, the voltage changes and causes nearby voltage-gated ion channels to open up. This restarts the diffusion process and allows more ions to pass through.
It is possible for all the activity to eventually depolarise the membrane beyond its threshold (55mV). This would involve all the voltage-gated sodium channels in the membrane along the axon opening up and allowing sodium ions to rush in. This would then trigger the opening of voltage-gated potassium channels and let the potassium ions rush out.
- The action potential is the rapid depolarisation and repolarisation, and is only activated if the stimulus reaches a threshold of -55mV
The terminal end of the axon will recognise this activity and trigger the release of more neurotransmitters. These travel across the synaptic space to communicate with another neuron, allowing each action potential to cause another action potential along the neuron and eventually in other neurons.
Action Potential Graphs
Nervous System
Diagrams can be used to show how membrane potential changes over time.
- The curve is initially flat, which is the resting potential around -70mV.
- If depolarisation of sufficient magnitude occurs to surpass the threshold of excitation, the sodium and potassium channels will open, causing a sharp spike to occur which is the action potential produced to around 55mV.
- This is due to the ions diffusing across the membrane along the entire axon at rapid rates (half a millisecond)
- Repolarisation occurs when the sodium channels close again and the efflux of potassium ions causes the potential to drop.
- A variety of membrane proteins will allow ion concentrations to reset using active transport shuttling sodium and potassium ions to reset their locations. This restores the concentration gradient that caused the resting potential.
Hormones
Endocrine System
Chemical messenger molecules that are produced and secreted into the bloodstream to initiate cellular reactions in specific target cells, tissues or organs in the body.
Endocrine glands are responsible for releasing hormones in response to a specific stimulus.
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Pituitary Gland - (“the master gland” releases hormones on direction from the hypothalamus to regulate the activity of other glands.
- Includes the anterior and posterious that are controlled by the hypothalamus and nerve impulses respectively
- Thyroid Gland - produces the hormone thyroxine and is controlled by thyroid-stimulating hormone (TSH) released by the anterior pituitary.
- Adrenal Gland - regulates levels of aldosterone (reabsorption of sodium and decreases reabsorption of potassium)
- Pancreas - regulate insulin and glucagon
Stimuli for Hormone Release
Endocrine System
Stimuli can be divided into three categories:
- Humoral - the changing concentration of specific chemicals in the chemical can result in the glands secreting hormones.
- Neural - the stimulation of the glands via neurons can result in the secretion of hormones.
- Hormonal - hormones can be secreted by the pituitary gland to control the number of hormones secreted by other endocrine glands.
Role of Brain
Endocrine System
The brain is the main control centre of the body. The hypothalamus is a small area of the brain that is located centrally and close to the pituitary gland.
- The hypothalamus regulates many of the activities in the body by directing effectors to carry out a response either by sending message through neural pathways or chemical messages (hormones). It is also the main link between the nervous and endocrine system.
- The medulla oblongata (or medulla) is a long, stem-like structure which makes up part of the brainstem. It is responsible for regulating several basic functions of the autonomic nervous system.
Endotherms
An endotherm is an organism capable of maintaining their core body temperature within a very narrow range despite variations in the external temperature/environment due to a variety of adaptations.
Behavioral Adaptations in Endotherms
- Changes in position/alightment of the body
- Burrowing
- Nocturnal activity
- Migration
Structural Adaptations in Endotherms
- Insulation
- SA:V ratio (i.e. body shape and size of structures on the body)
Physiological Adaptations of Endotherms
- Changes to metabolic rate
- Hibernation/’Torpor’
- Evaporative cooling
- Vasodilation/Vasoconstriction (i.e. regulation of blood flow to the surface and extremeties)
- Counter-current blood exchange
- Muscle reactions
Maintaining Water Balance in Plants
Where there is limited water, plants must achieve a balance between how much it can lose through cooling, transpiration, exchange of gases and dehydration.
The main form of water loss in plants is transpiration, the evaporation of water and dissolved ions from the roots to the stomata of leaves. It is also a form of evaporative cooling to regulate temperatures.
- Some plants open their stomata during cooler periods of the day, so evaporation is reduced and water loss minimised.
Xerophytes
Maintaining Water Balance in Plants
Xerophytes are plants that have evolved over time live in arid conditions and have adaptations to equip them to achieve survival. They have been evolved to hold large amounts of water for a long period time as arid environments do not provide enough water to sustain life.
- Stems and leaf stalks (petioles) have sparsely distributed stomata but still contain photosynthetic tissue, allowing them to photosynthesis while reducing water loss.
- Reduced/rolled leaves prevent water loss through evaporation. (e.g. flax lily plant curl their leaves when the temperature is high to hide stomata)
- Thick, waxy or leathery cuticle ensures that all epidermal cells are waterproof. (e.g. eucalyptus and banksia plants)
- Leaves may have white hairs to reflect sunlight and reduce water temperature on the surface or to trap a thin layer of humid air. (e.g. eucalyptus reflects sunlight; paper daisy traps a layer of humid air)
- Stomata opening and closing (e.g. eucalyptus, stag horn fern)
- Shedding of leaves allows surface area exposed to sunlight to be reduced. (e.g. eucalyptus)
- A vertical leaf orientation can also reduce surface area. (e.g. eucalyptus)
- CAM physiology
- Lower growth to ground
