Biology Revision – Module 1 Irritability Table of Contents Vocabulary ...................................................................................................................................... 2 Taxis Behaviour .............................................................................................................................. 3 Stimulus & Response ...................................................................................................................... 3 Neurons ........................................................................................................................................... 4 Parts of a Neuron ............................................................................................................................ 4 The Nervous System ....................................................................................................................... 4 Central Nervous System ............................................................................................................. 4 Peripheral Nervous System ......................................................................................................... 4 Receptors..................................................................................................................................... 4 Control System............................................................................................................................ 5 Effectors ...................................................................................................................................... 5 Neuron Pathway .............................................................................................................................. 5 Reflex Action .................................................................................................................................. 5 Steps Involving Reflex Action: ................................................................................................... 6 Normal Neuron Pathway V.S. Reflex Arc ...................................................................................... 6 Synaptic Transmission .................................................................................................................... 7 Nerve Impulse/Action Potential ...................................................................................................... 8 Vocabulary Irritability – living things responding to changes in their environment. Response – is any change in the state of an organism as a result of a stimulus. - May be internal or external. - Response increases survival. Tropism – is a plant’s growth towards or away from unidirectional stimuli. - Positive tropism – plants grow towards the stimulus. - Negative tropism – plants grow towards the stimulus. Taxis – is a behavioural response that produces movement towards or away from a stimulus. - The movement of an organism in response to a stimulus is known as a kinesis or a taxis. Kinesis – is a behavioural response that produces random movements when a stimulus is present. There are no clear movements towards or away from the stimulus. Reflex – an involuntary response to a stimulus. - A reflex action only happens when responding to a stimulus. PNS – Peripheral Neuron System CNS – Control Neuron System Synaptic transmission – is the biological process by which a neuron communicates with a target cell across a synapse. Nerve Impulse – is a wave of depolarization traveling along the axon of the motor nerve such that the resting membrane potential of about −70 millivolt is reversed, becoming briefly positive. Polarization – is when a neuron is not stimulated and have no impulse to carry or transmit. - Being polarized means that the electrical charge on the outside of the membrane is positive while the electrical charge on the inside of the membrane is negative. Depolarization – occurs when a stimulus reaches a resting neuron. - During the depolarization phase, the gated sodium ion channels on the neuron’s membrane suddenly open and allow sodium ions (Na+) present outside the membrane to rush into the cell. Repolarization – is caused by the closing of sodium ion channels and the opening of potassium ion channels. Sodium Potassium Pump – a protein that uses ATP and energy that takes out sodium out of the axons so it can return to the resting potential phase. Taxis Behaviour Living organisms are exposed to a multitude of environmental changes over their lifetime. Changes in the environment serve as external stimuli that result in behavioural changes in the organisms. For example, a pill bug will move away from bright light. The light serves as a stimulus for the response, the movement away from the light. The movement of an organism in response to a stimulus is known as a kinesis or a taxis. A kinesis is a behavioural response that produces random movements when a stimulus is present. There are no clear movements towards or away from the stimulus. A taxis, on the other hand, is a behavioural response that produces movements either towards or away from a stimulus. •A phototaxis is the response to light. •A hydrotaxis is an organism’s movement in response to moisture. •A chemotaxis is an organism’s response to a specific chemical. •A geotaxis is an organism’s movement in response to gravity. Taxis behaviours are classified according to the stimulus producing the response. The presence of stimuli such as light, gravity, or chemicals can result in taxis behaviours. If the organism moves toward the stimulus, then the movement is referred to as a positive taxis. Avoidance or movement away from a stimulus is a negative taxis. Stimulus & Response Irritability is whenever living things respond to a stimulus or the changes in the environment, if in danger, a response increases the survival of a living thing. A stimulus can be either internal or external. An Internal Stimulus is a stimulus that comes from the inside of an organism. Example, you have an infection, so you run a fever. External stimuli are factors outside of the body that are taken in by receptors and will lead to a response. Examples of external stimuli include temperature changes, vision, hearing, tasting, smelling, and even sunlight for plants. Tropism is the turning of all or part of an organism (specifically in plants) in a particular direction in response to an external stimulus. There are two types of tropism: positive tropism is when a plant grows toward the stimulus, example a plant grows towards the sunlight, this is positive tropism. Negative tropism is when a plant grows away from the stimulus. Plants can respond to light, gravity, water and touch. Plants use chemical and hormones to coordinate growth, development and responses to the environment in order to maximize survival, photosynthesis and reproduction. Neurons Sensory Neurons are stimulated by our senses, and it senses the stimulus. Sensory neurons transmit impulses from the body to the CNS. Motor Neurons is responsible for the response and controls the muscles. Relay Neuron (brain & spinal cord) connects neurons together, it transfers the sensory feeling from the sensory neuron to the motor neuron. Reflex Arc is the pathway that contains all these neurons. Parts of a Neuron Cell Body (soma) – is the cell body is the core section of the neuron. The cell body contains genetic information, maintains the neuron's structure and provides energy to drive activities. Axon – is where electrical impulses from the neuron travel away to be received by other neurons. Dendrites - is where a neuron receives input from other cells. Dendrites branch as they move towards their tips, just like tree branches do, and they even have leaf-like structures on them called spines. The Nervous System The nervous system allows humans to react to their surroundings and coordinate their behaviour. It has millions of neurons and uses electrical impulse to communicate quickly. Central Nervous System The CNS is made up the brain and the spinal cord. Muscles contracting are the response of effectors coordinated by the CNS. Peripheral Nervous System The PNS connects the CNS to the rest of the body because it contains the nerves found outside the CNS. Nerves that send and receive messages to internal organs are part of the PNS. Receptors Cells called receptors detect stimuli. Receptors receive stimuli from the environment while responses are brought about by the effectors. Control System The spinal coordinator centre, such as the brain, spinal cord, or pancreases, receives and process information from receptors all over the body. Effectors Effectors can restore optimum levels such as core body temperature and blood glucose levels. Effectors include muscles and glands, so responses can include muscle contraction. Stimulus Receptor Coordinator Effector Response Neuron Pathway A neural pathway is a series of connected neurons that send signals from one part of the brain to another. The nerve pathway involved in a reflex action, including at its simplest a sensory nerve and a motor nerve with a synapse between. The central nervous system (CNS) contains numerous nerve fibres that group together to form pathways between its various parts. These neural pathways represent the communicating highways of the CNS. They can be located solely within the brain, providing connections between several of its structures, or they can link the brain and the spinal cord together. When a regular response occurs involving the neuron pathway, processing the information in our brain come before a reflex action. Reflex Action PNS When the body detects a stimulus, it can have two types of responses: Voluntary Response Involuntary Response An involuntary response to a stimulus is called a reflex action. All reflex actions follow an overall sequence through the nervous system which is called the reflex arc. Crucially this does not involve the conscious part of the brain, which makes it much quicker. This in turn reduces damage to the body. The pathway of a reflex action does not initially go to the brain to increase the speed of reactions The pathway of a reflex action only travels through relay neurones in the spinal cord and not the brain Sensory Division Motor Division (Effren) Autonomic Response Sympathetic (Affren) Somatic Response Parasympathetic Autonomic – involuntary trigger Somatic – voluntary trigger Sympathetic (fight or flight) – controls functions when in threat. Parasympathetic (rest & digest) – controls functions when in rest. Steps Involving Reflex Action: Body detects stimulus. Receptors respond to the stimulus. Receptors sends electrical impulses to the sensory neurons. The electrical signal travels behind the spinal cord to the relay neuron. The electrical signal travels Infront of the spinal cord to the motor neuron. Then the motor neuron sends the signals to the effectors. The effector produces the response, allowing muscles to contract, thus causing the reflex action. * Note that the brain is not involved until after the reflex takes place. Normal Neuron Pathway V.S. Reflex Arc In vertebrates, most sensory neurons do not pass directly into the brain, but the synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. Reflexes are faster than our normal reactions because reflex actions use a different neural pathway. Unlike normal movement, most reflexes bypass the brain, receiving signals only from the spinal cord. Synaptic Transmission Synaptic transmission is the biological process by which a neuron communicates with a target cell across a synapse. Chemical synaptic transmission involves the release of a neurotransmitter from the pre-synaptic neuron, and neurotransmitter binding to specific post-synaptic receptors. Most communication between neurons occurs at a specialized structure called a synapse. A synapse is an area where two neurons come close enough to one another that they can pass chemical signals from one cell to another. The neurons are not actually connected but are separated by a microscopely small space called the synaptic cleft. Nerve Impulse/Action Potential A Nerve Impulse is a series of depolarization along the length of the axon. The axon is powered up by ATP molecules. 1. Resting Potential: Sodium on outside Chlorine inside Voltage = -70mv 2. Depolarization: The channels opens and sodium rushes in. Inside becomes positive. Voltage = - 55 mv 3. Repolarization: The channels remain open, and potassium moves outside. Sodium Inside Inside becomes negative again Using ATP molecules, the sodium potassium pump rapidly moves sodium out of the cell. 4. Hyperpolarization: Inside of the axon is more negative than normal for a brief period, Too much potassium moves out causing the inside to become overly negative. Voltage = - 90mv 5. Refractory Period: The axon unable to pick up another stimulus/impulse during this period. The refractory period is one millisecond long. The axon returns to resting potential with the help of the sodium potassium pump and when receptors detect a stimulus, the entire process occur once more.
