BIO 3.5.1 Stimuli, both internal and external are detected and lead to a response

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What is a stimulus?
A stimulus is a change in an organism's environment (both internal and external) that can be detected by receptor cells.
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What is a receptor?
A receptor is a specialised cell that detects a stimulus and initiates an impulse..
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Receptors can exist as ...
Either individual cells (e.g. in the skin), or concentrated in sense organs (e.g. the eye)
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What is a simple behavioural response seen in organisms that can move?
Taxes and Kineses. E.g. Animals/aquatic organisms that can move in a particular direction
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What is a kinesis?
a CHANGE in SPEED of RANDOM MOVEMENTS in response to an ENVIRONMENTAL STIMULUS (to find more favourable conditions)
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What is a taxis?
A direct movement TOWARDS or AWAY from a particular stimulus. There are positive and negative taxes.
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What is a tropism?
A tropism is usually seen in PLANTS. They have NO MUSCLES, but RESPOND by GROWING IN A PARTICULAR DIRECTION.
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In tropisms, how would you describe the MOVEMENT, and HOW is it brought about?
Movement is SLOW. It is brought about by CONTROL OF CELL DIVISION and ELONGATION.
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What is a reflex arc (definition)?
A reflex is a FIXED MOVEMENT of some part of an animal in response to a particular stimulus. (e.g. BLINKING or Knee-JERK
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There are 4 essential components to the reflex arc. What is the 1st one?
Sensory Neurone. It is a NERVE CELL that carries IMPULSES from a RECEPTOR to a CNS
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There are 4 essential components to the reflex arc. What is the 2nd one?
The CNS. Which consists of the BRAIN and SPINAL CORD. It processes incoming information and produces a response, OFTEN BASED on PREVIOUS EXPERIENCE
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There are 4 essential components to the reflex arc. What is the 3rd one?
Motor Neurone. It is a NERVE CELL that carries IMPULSES from the CNS to the EFFECTOR
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There are 4 essential components to the reflex arc. What is the 4th one?
Effectors. An ORGAN that brings about a response; usually a GLAND or MUSCLE
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Dorsal, Ventral. Which side is Front or Back?
Dorsal is front, Ventral is back.
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The reflex is a RAPID, FIXED RESPONSE that cannot be controlled. Why is this?
Because the nerve impulses involved do NOT pass through the CONSCIOUS PARTS OF THE BRAIN
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A reflex that avoids danger....
minimises damage to the body (e.g. touching something hot)
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What is a postural reflex?
A reflex that maintains our position and body control without having to constantly think about fine adjustments.
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The Knee-Jerk reaction is a 3 neurone reflex arc. What is the 1st one?
Sensory Neurone. It links the RECEPTOR to the SPINAL CORD.
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The Knee-Jerk reaction is a 3 neurone reflex arc. What is the 2nd one?
Short Relay Neurone. It CONNECTS INCOMING and OUTGOING neurones
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The Knee-Jerk reaction is a 3 neurone reflex arc. What is the 3rd one?
Motor Neurone. It transmits impulses to the effector
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The Knee-Jerk reaction happens well before the conscious brain has time to stop it, but one is aware that the eg has moved. Why?
Sensory information passes from the spinal cord to the brain.
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What does being MYOGENIC mean?
It means that the MUSCLE CONTRACTION originates from within the heart muscle itself.
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What happens to if the nerves to heart are severed?
The heart will continue to beat at a slow, regular pace. BUT, it cannot be matched to the changing needs of the body.
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What part of the BRAIN is responsible for modifying heart rate (rather than initiating it) ?
The cardiovascular centre in the medulla.
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The cardiovascular centre receives its information from what 2 main sources?
CHEMORECEPTORS CELLS (in the carotid and aortic bodies) and BARORECEPTOR CELLS (in the carotid sinus)
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Describe chemoreceptor cells.
They are sensitive to CO2 levels in the blood. When CO2 levels rise during exercise, impulses PASS MORE FREQUENTLY to the cardiovascular centre.
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Describe baroreceptor cells.
They transmit impulses to the CV centre when BLOOD PRESSURE RISES.
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What kind of system keeps blood pressure within certain limits?
The negative feedback system.
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From the CV centre, there are two ANTAGONISTIC NERVES, leading to where?
The SINO-ATRIAL NODE in the heart.
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What are the 2 antagonistic nerves leading from the CV centre to the SA NODE?
The SYMPATHETIC NERVE and PARASYMPATHETIC NERVE.
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What does the sympathetic nerve do?
It carries impulses that SPEED UP HEART RATE
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What does the parasympathetic nerve do?
It carries impulses that SLOW DOWN HEART RATE
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The SNS and the PSNS (nervous systems) form part of what overall system?
The ANS or Autonomic Nervous System.
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E.g. when we exercise, increased CO2 levels are detected and impulses are transmitted to the medulla. In response, impulses transmit down WHICH nerve to do WHAT?
The sympathetic nerve to increase rate of heartbeat
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Where is the Pacinian corpuscle found, and what is its function?
It is found deep in MAMMALIAN skin, and it detects changes in PRESSURE and VIBRATION - it is a mechanoreceptor
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What does the Pacinian corpuscle consist of?
It consists of a SINGLE SENSORY NEURONE. Surrounded by 20-60 lamellae (layers) of fibrous connective tissue separated by viscous gel
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When a CHANGE of PRESSURE is transmitted through to the sensory nerve, what happens after?
The sensory nerve DEFORMS, which opens STRETCH-MEDIATED SODIUM ION CHANNELS (in the axon membrane) to open.
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When stretch-mediated sodium ion channels open, what happens?
Sodium ions diffuse in
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When sodium ions diffuse in, what happens?
A generator potential is created
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When does an impulse finally pass down the sensory nerve?
When the generator potential reaches a THRESHOLD (a certain) level
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If the stimulus is too small, what happens?
Only a small amount of sodium will diffuse in, the threshold is not reached, and no impulse is generated.
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The Pacinian corpuscle detects CHANGES of pressure, but NOT ______
not PROLONGED PRESSURE.
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The layers of gel act as ____, which _______
Act as a shock absorber, which prevents the receptor being STIMULATED for more than a brief moment
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Adaptation is
When the layers of gel quickly allows the sensory nerve to assume its normal shape
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Important thing to remember about receptors:
They will respond to CHANGES IN ENVIRONMENT, NOT CONSTANT STIMULI.
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What is the retina? Where is it?
A LAYER of LIGHT-SENSITIVE RECEPTOR CELLS, together with CONNECTING NEURONES, on the back of the eye
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What are the light-sensitive receptor cells?
Rods and Cones
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What is the overall function of the retina?
To gather information about the incoming light and relay it to the brain
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How is the information about incoming light relayed to the brain?
via the optic nerve
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Where is the actual image formed?
In the VISUAL CORTEX of the brain
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Rods and Cones differ in 2 things: 1)_______ 2)_____
SENSITIVITY and VISUAL ACUITY
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Define Sensitivity
The amount of light needed for the cells to function
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Define Visual Acuity
The ability of the cell to PERCEIVE DETAIL/ DISTINGUISH OBJECTS that are CLOSE TOGETHER
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There are 2 reasons why RODS are more sensitive and can function in dim light what are they?
1. The pigment in rod cells (rhodopsin) is MORE EASILY BLEACHED/BROKEN DOWN than the pigment in Cone cells (iodopsin) 2. RETINAL CONVERGENCE
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Define RETINAL CONVERGENCE
is when many ROD cells CONVERGE into 1 neurone - so they can ALL CONTRIBUTE TO THE GENERATOR POTENTIAL
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When more rod cells contribute to the generator potential, what is more likely to be reached? What is called when many converge into one?
the threshold. Many converging into one is called SUMMATION.
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Which of the receptor cells has a higher visual acuity than the other?
The Cone cells
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What is the key idea about visual acuity?
Cone cells send more information to the brain than is sent by the rod cells PER UNIT AREA OF RETINA
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Why do the cone cells send more information to the brain per unit area of retina?
Because of the ONE CONE: ONE NEURONE connection. Any impulses generated by an INDIVIDUAL CONE are transmitted to the brain as SEPARATE IMPULSES
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What is the name of the part of the retina that has the greatest concentration of cone cells?
The Fovea
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Only when light falls on the _____, can we see things in ____
Fovea. Detail.
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