4.5.1 – Homeostasis and Response
o HOMEOSTASIS is the process of keeping internal conditions constant in response to internal and external changes; optimal conditions
for enzyme action and all cell functions.
o As humans are organisms that live in a changing environment, the body’s internal conditions must be regulated to ensure the enzymes
and cells function well. These internal conditions include:
• Internal (in the blood) body temperature (optimum temperature: 37 degrees Celsius); so that enzymes don’t denature at high
temperatures (change shape, reducing their ability to catalyse, or speed up, metabolic reactions – chemical reactions in organisms)
or deactivate at low temperatures.
• Water levels
• Blood glucose concentration
• Urea concentration in urine and carbon dioxide levels
o These automatic control systems may include chemical or nervous responses. All control systems include:
• Cells called receptors which detect stimuli (changes in the environment)
• Coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from the receptor cells
• Effectors, muscles or glands, which bring about responses which restore optimum levels
o In a negative-feedback mechanism: when an internal condition changes and is no longer optimal, this stimulus is detected and the
body works to counteract (reverse) the change, returning the conditions back to the optimum level; requires all of the nervous system’s
components to work together.
o Positive-feedback mechanisms may also happen, making a small change even bigger: eg. The release of oxytocin (hormone) which
increases the number of contractions during childbirth.
4.5.2 – The Human Nervous System
4.5.2.1 – Nervous System Structure and Function
o The nervous system protects organisms from harm by responding to changes in the environment by coordinating communication
between different parts of organisms. It consists of:
• The brain – one of the components of the CNS (central nervous system); the CNS coordinates the response of effectors, muscles or
glands which act in response to a change in the internal or external environment.
• The spinal cord – other component of the CNS; also important in coordinating the response of effectors to changes in the
environment.
• The neurones (nerve cells) – carry electrical impulses (signals) between receptors, the CNS and effectors.
o Synapses are gaps between neurones and can be found at each junction of a reflex arc. Nerve impulses need to travel across synapses.
• An electrical impulse will reach the end of the neurone before the synapse, triggering the release of chemicals called
neurotransmitters.
• The neurotransmitters diffuse (move down a concentration gradient) across the synapse, which then bind to receptors on the next
neurone.
• The presence of the neurotransmitter causes the production of an electrical impulse in the next neurone.
Reflexes
o Reflex actions are automatic and rapid in response to dangerous situations; they don’t involve the conscious part of the brain - the
nervous system responds to stimuli via a reflex arc.
o Reflex actions include dropping hot objects, sneezing or blinking.
o The different components of a reflex arc include:
• The stimulus – any change in the environment which the body needs to respond to, detected by a receptor.
• A receptor – found all over the body (such as in the skin); detects the change in the environment, initiating a signalling process in
the body picked up by a neurone.
• Neurones of 3 types:
• Sensory neurones – carries the signal in the form of an electrical impulse to the CNS.
• Relay neurones (in the CNS) – relays the electrical impulse from the sensory neurone to the appropriate motor neurone.
• Motor neurone – carries the electrical impulse from the CNS to an effector.
• An effector – muscle or gland which brings about an action in response to the change in the internal or external environment.
• A response – any action which helps the organism avoid the harmful situation.
Investigating human reaction times practical
o If the reaction time is reduced it means it takes less time to react; reduction in reaction time means a person is reacting more quickly.
o Caffeine and exercise are factors that may affect human reaction time. Their effects can be tested by dropping and catching a ruler.
• Hold a ruler vertically from one end. The test subject should have their thumb and finger either side of the ruler at the 0cm mark.
• Drop the ruler; the subject should try to catch it between their thumb and forefinger as fast as possible.
• Note down the distance from the 0cm mark that the ruler travels before the subject catches it.
• Use a table to convert this distance into a reaction time.
• Repeat the experiment on a subject who has just had a coffee or done exercise and compare the reaction times.
o The independent variable is whether or not the subject has just had caffeine or exercised.
o The dependent variable is the subject’s reaction time.
, Reflex Arc
o In a reflex arc (where the CNS would be the spinal cord; occurs in the spinal cord’s grey matter):
• The stimulus is detected by a receptor.
• An electrical impulse is sent from the receptor and carried by a sensory neurone to the CNS.
• The electrical impulse is converted into chemical neurotransmitters which diffuse across the synapse between the sensory and
relay neurones.
• The neurotransmitters then diffuse across the synapse between the relay and motor neurones, and the chemical
neurotransmitters bind to receptors triggering an electrical impulse in the motor neurone.
• The electrical impulse is sent from the motor neurone in the CNS to the effector, causing a stimulation resulting in an impulse
within the effector in response to the stimulus.
• Another sensory neurone leading from the relay neurone in the CNS carries an electrical impulse to the brain, sending a
‘message’.
4.5.2.2 - The Brain
o The brain is a very complex organ that controls all conscious and unconscious thoughts and activities in order to keep an organism alive.
o Treatment of brain damage & disease is difficult because the brain is delicate, complex, and not well understood.
o The brain is made up of millions of interconnecting neurones; different regions of the brain are responsible for coordinating different
functions.
o The brain is made up of three main parts:
• The medulla – the part responsible for unconscious activities such as breathing or heartbeat.
• The cerebral cortex – the part responsible for conscious thought such as memory, language and intelligence; highly folded outer
layer.
• The cerebellum – the part responsible for muscle coordination; important for movement, posture, balance and speech.
Investigating the Brain
o Neuroscientists use many methods to help understand the brain:
▪ Brain damage – by studying patients with brain damage, where part of their brain doesn’t function, neuroscientists have been able to
link particular regions of the brain to particular functions.
Non- invasive methods include:
▪ MRI scanners – stands for Magnetic Resonance Imaging; allowed us to learn which areas of the brain are active during different
activities such as moving, speaking and listening.
• Strong magnetic fields and radio waves are used to show details of brain structure and function.
• Patients are asked to perform various tasks and, by looking at the scan, scientists can see which parts of the brain are
active when the task is carried out.
▪ Electrical stimulation – allowed us to treat certain disorders of the brain; as the nervous system communicates with electrical impulses,
electrical stimulation is used to help treat conditions such as Parkinson’s disease (causes tremors).
• Different parts of the brain are stimulated with a weak electrical current.
• If the motor area is stimulated, the patient makes an involuntary movement.
• If the visual area is stimulated, they may see a flash of colour.
• EEGs (Electroencephalograms) can be created and studied, to observe the electrical activity in the brain.
o Risks include:
• Brain surgery may be needed to remove a tumour or excess fluid, such as blood.
• All surgery carries a level of risk, but due to the complexity and delicacy of the brain, investigating and treating brain
disorders can be very difficult.
• If surgery is undergone, more damage or side-effects may be created, which could affect the patients' quality of life.
• Serious considerations about the risks involved against the benefits need to be undertaken first.
Brain and Nervous System Treatment
There’re many procedures for brain and nervous system damage:
o Brain surgery –
• Can be used to remove as much of a brain tumour as possible (the growth of cells uncontrollably within the brain)
• May cause brain damage affecting speech, memory, balance etc; can cause infection or the possibility of a stroke.
o Brain implants –
• Offers hope for early stage Parkinson’s disease (where the brain becomes progressively damaged); can help the brain function.
• May cause brain damage itself.
o Radiotherapy/chemotherapy –
• Can help remove tumours; radiotherapy involves using rays to destroy cancer cells whereas chemotherapy involves using drugs.
• Both can damage perfectly normal cells – chemotherapy may target fast dividing cells like hair cells.
o Monoclonal antibodies –
• Identical copies of antibodies which can bind to cancer cells and help the immune system destroy them; can help with the treatment
of tumours/cancer.
• May cause skin changes like red and sore skin/itchy rashes.
o Stem cell therapies –
• Unspecialised cells which can differentiate or specialise into many other types; helps in repairing damaged nervous systems and
producing cancer-killing molecules.
• Stem cells may get rejected by the body and be regarded as foreign.
o HOMEOSTASIS is the process of keeping internal conditions constant in response to internal and external changes; optimal conditions
for enzyme action and all cell functions.
o As humans are organisms that live in a changing environment, the body’s internal conditions must be regulated to ensure the enzymes
and cells function well. These internal conditions include:
• Internal (in the blood) body temperature (optimum temperature: 37 degrees Celsius); so that enzymes don’t denature at high
temperatures (change shape, reducing their ability to catalyse, or speed up, metabolic reactions – chemical reactions in organisms)
or deactivate at low temperatures.
• Water levels
• Blood glucose concentration
• Urea concentration in urine and carbon dioxide levels
o These automatic control systems may include chemical or nervous responses. All control systems include:
• Cells called receptors which detect stimuli (changes in the environment)
• Coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from the receptor cells
• Effectors, muscles or glands, which bring about responses which restore optimum levels
o In a negative-feedback mechanism: when an internal condition changes and is no longer optimal, this stimulus is detected and the
body works to counteract (reverse) the change, returning the conditions back to the optimum level; requires all of the nervous system’s
components to work together.
o Positive-feedback mechanisms may also happen, making a small change even bigger: eg. The release of oxytocin (hormone) which
increases the number of contractions during childbirth.
4.5.2 – The Human Nervous System
4.5.2.1 – Nervous System Structure and Function
o The nervous system protects organisms from harm by responding to changes in the environment by coordinating communication
between different parts of organisms. It consists of:
• The brain – one of the components of the CNS (central nervous system); the CNS coordinates the response of effectors, muscles or
glands which act in response to a change in the internal or external environment.
• The spinal cord – other component of the CNS; also important in coordinating the response of effectors to changes in the
environment.
• The neurones (nerve cells) – carry electrical impulses (signals) between receptors, the CNS and effectors.
o Synapses are gaps between neurones and can be found at each junction of a reflex arc. Nerve impulses need to travel across synapses.
• An electrical impulse will reach the end of the neurone before the synapse, triggering the release of chemicals called
neurotransmitters.
• The neurotransmitters diffuse (move down a concentration gradient) across the synapse, which then bind to receptors on the next
neurone.
• The presence of the neurotransmitter causes the production of an electrical impulse in the next neurone.
Reflexes
o Reflex actions are automatic and rapid in response to dangerous situations; they don’t involve the conscious part of the brain - the
nervous system responds to stimuli via a reflex arc.
o Reflex actions include dropping hot objects, sneezing or blinking.
o The different components of a reflex arc include:
• The stimulus – any change in the environment which the body needs to respond to, detected by a receptor.
• A receptor – found all over the body (such as in the skin); detects the change in the environment, initiating a signalling process in
the body picked up by a neurone.
• Neurones of 3 types:
• Sensory neurones – carries the signal in the form of an electrical impulse to the CNS.
• Relay neurones (in the CNS) – relays the electrical impulse from the sensory neurone to the appropriate motor neurone.
• Motor neurone – carries the electrical impulse from the CNS to an effector.
• An effector – muscle or gland which brings about an action in response to the change in the internal or external environment.
• A response – any action which helps the organism avoid the harmful situation.
Investigating human reaction times practical
o If the reaction time is reduced it means it takes less time to react; reduction in reaction time means a person is reacting more quickly.
o Caffeine and exercise are factors that may affect human reaction time. Their effects can be tested by dropping and catching a ruler.
• Hold a ruler vertically from one end. The test subject should have their thumb and finger either side of the ruler at the 0cm mark.
• Drop the ruler; the subject should try to catch it between their thumb and forefinger as fast as possible.
• Note down the distance from the 0cm mark that the ruler travels before the subject catches it.
• Use a table to convert this distance into a reaction time.
• Repeat the experiment on a subject who has just had a coffee or done exercise and compare the reaction times.
o The independent variable is whether or not the subject has just had caffeine or exercised.
o The dependent variable is the subject’s reaction time.
, Reflex Arc
o In a reflex arc (where the CNS would be the spinal cord; occurs in the spinal cord’s grey matter):
• The stimulus is detected by a receptor.
• An electrical impulse is sent from the receptor and carried by a sensory neurone to the CNS.
• The electrical impulse is converted into chemical neurotransmitters which diffuse across the synapse between the sensory and
relay neurones.
• The neurotransmitters then diffuse across the synapse between the relay and motor neurones, and the chemical
neurotransmitters bind to receptors triggering an electrical impulse in the motor neurone.
• The electrical impulse is sent from the motor neurone in the CNS to the effector, causing a stimulation resulting in an impulse
within the effector in response to the stimulus.
• Another sensory neurone leading from the relay neurone in the CNS carries an electrical impulse to the brain, sending a
‘message’.
4.5.2.2 - The Brain
o The brain is a very complex organ that controls all conscious and unconscious thoughts and activities in order to keep an organism alive.
o Treatment of brain damage & disease is difficult because the brain is delicate, complex, and not well understood.
o The brain is made up of millions of interconnecting neurones; different regions of the brain are responsible for coordinating different
functions.
o The brain is made up of three main parts:
• The medulla – the part responsible for unconscious activities such as breathing or heartbeat.
• The cerebral cortex – the part responsible for conscious thought such as memory, language and intelligence; highly folded outer
layer.
• The cerebellum – the part responsible for muscle coordination; important for movement, posture, balance and speech.
Investigating the Brain
o Neuroscientists use many methods to help understand the brain:
▪ Brain damage – by studying patients with brain damage, where part of their brain doesn’t function, neuroscientists have been able to
link particular regions of the brain to particular functions.
Non- invasive methods include:
▪ MRI scanners – stands for Magnetic Resonance Imaging; allowed us to learn which areas of the brain are active during different
activities such as moving, speaking and listening.
• Strong magnetic fields and radio waves are used to show details of brain structure and function.
• Patients are asked to perform various tasks and, by looking at the scan, scientists can see which parts of the brain are
active when the task is carried out.
▪ Electrical stimulation – allowed us to treat certain disorders of the brain; as the nervous system communicates with electrical impulses,
electrical stimulation is used to help treat conditions such as Parkinson’s disease (causes tremors).
• Different parts of the brain are stimulated with a weak electrical current.
• If the motor area is stimulated, the patient makes an involuntary movement.
• If the visual area is stimulated, they may see a flash of colour.
• EEGs (Electroencephalograms) can be created and studied, to observe the electrical activity in the brain.
o Risks include:
• Brain surgery may be needed to remove a tumour or excess fluid, such as blood.
• All surgery carries a level of risk, but due to the complexity and delicacy of the brain, investigating and treating brain
disorders can be very difficult.
• If surgery is undergone, more damage or side-effects may be created, which could affect the patients' quality of life.
• Serious considerations about the risks involved against the benefits need to be undertaken first.
Brain and Nervous System Treatment
There’re many procedures for brain and nervous system damage:
o Brain surgery –
• Can be used to remove as much of a brain tumour as possible (the growth of cells uncontrollably within the brain)
• May cause brain damage affecting speech, memory, balance etc; can cause infection or the possibility of a stroke.
o Brain implants –
• Offers hope for early stage Parkinson’s disease (where the brain becomes progressively damaged); can help the brain function.
• May cause brain damage itself.
o Radiotherapy/chemotherapy –
• Can help remove tumours; radiotherapy involves using rays to destroy cancer cells whereas chemotherapy involves using drugs.
• Both can damage perfectly normal cells – chemotherapy may target fast dividing cells like hair cells.
o Monoclonal antibodies –
• Identical copies of antibodies which can bind to cancer cells and help the immune system destroy them; can help with the treatment
of tumours/cancer.
• May cause skin changes like red and sore skin/itchy rashes.
o Stem cell therapies –
• Unspecialised cells which can differentiate or specialise into many other types; helps in repairing damaged nervous systems and
producing cancer-killing molecules.
• Stem cells may get rejected by the body and be regarded as foreign.
