Plant Movements: Locomotion and Curvature

Plant Movement

• There are two basic types of plant movement: locomotion and curvature

Movements of Locomotion

• Involves movement from one location to another
• Aquatic plants demonstrate this type of movement

Movements of Curvature

• Plants exhibit slight bending and fixed movement
• Rooted, terrestrial plants are examples

Types of Plant Movements

• Plant movements are categorized as movements of locomotion or curvature
• Movements of Curvature are divided into Growth and Variation movements
• Growth movements are classified as Autonomic or Paratonic
• Autonomic growth movements can be nasty or nutational
• Paratonic growth movements are tropic
• Variation movements are classified as Autonomic or Paratonic
• Paratonic variation movements are called nastic

Paratonic Locomotion Movements: Phototactic

• Ciliated algae and zoospores possess a light-sensitive eye spot
• Positive phototactic movement occurs in low light, and the organism moves towards the light
• Negative phototactic movement occurs in high light, and is repelled away, similar to chloroplast and mesophyll cells

Paratonic Locomotion Movements: Chemotactic

• Bryophytes and pteridophytes show swimming movements
• Antherozoids move toward archegonial tips, which secrete sugar and malic acid

Paratonic Locomotion Movements: Thermotactic

• Ciliated algae move from colder to warmer places
• Example: Chlamydomonas moves towards the warmer side in cold water, and vice versa

Autonomic Growth Movement: Nutational

• Shoot tips of certain species grow in a zig-zag manner due to an equal pattern on the opposite side
• Circumnutation of tendrils is an example

Paratonic Growth Movement: Hydrotropism

• Wire gauze is used to grow seedlings covered with moist dust
• Radicles grow downward and then move toward the moist sawdust (positive hydrotropism)

Paratonic Growth Movement: Thigmotropism (Haptotropism)

• Tendrils come into contact with solid objects that have a rough surface
• If tendrils do not have touch stimulus they fail to develop
• After touching, tendrils start twinning around
• The side near the support has less growth, while the opposite side has more growth

Paratonic Growth Movement: Chemotropism

• Fungal hyphae and pollen tubes exhibit positive tropic movement under the influence of chemical structure
• Chemicals are generally sugars and other nutrient substances
• Positive chemotropism is exhibited in the presence of chemicals

Clinostat

• It is used to study gravity's effect in detail
• Presentation Time: Minimum period of exposure to the stimulus which produces curvature, stimulus received, but no response
• Relaxation Time: Clinostat is rotated very slowly with no response until the stimulus ceases
• Reaction Time: Visible effect of stimulus appearing after a delay

Autonomic Variation Movement

• Example: Desmodium gyrans (dancing leaves)
• Larger terminal leaflets move up and down throughout the day
• Smaller lateral leaflets exhibit rhythmic movements within minutes

Paratonic Variation Movement: Nyctinastic

• It is the circadian rhythmic nastic movement of higher plants in response to darkness (plant "sleeping")
• Photonastic and Thermonastic movements are seen in tulips
• Office time plants are an example

Paratonic Variation Movement: Seismonastic

• Demonstrated by the "touch-me-not" plant (Mimosa pudica)

Paratonic Variation Movement: Thigmonastic

• Drosera tentacles show variant move

Neurophysiology: Structural Classification of Neurons

• Neurons are classified based on the number of processes extending from the cell body
• The three types of neurons are Multipolar, Bipolar and Unipolar

Multipolar Neurons

• Usually have several dendrites and one axon
• Most neurons of this type are in the brain and spinal cord and all motor neurons

Bipolar Neurons

• Have one main dendrite and one axon
• Located in the retina of the eye, the inner ear, and the olfactory area (to smell) of the brain

Unipolar Neurons

• Are fused dendrites and one axon forming a continuous process
• Called pseudounipolar neurons because they begin in the embryo as bipolar neurons

Neuroglia

• Neuroglia, also known as glia, makes up about half the volume of the central nervous system (CNS)
• Neuroglia actively participates in the activities of nervous tissue
• Neuroglia is smaller and 5 to 25 times more numerous.
• Glia do not generate or propagate action potentials, and they can multiply and divide in the mature nervous system
• The six types of neuroglia: astrocytes, oligodendrocytes, microglia, and ependymal cells are found only in the CNS
• Schwann cells and satellite cells are present in the PNS

Astrocytes

• Star-shaped cells that are the largest and most numerous of the neuroglia
• Protoplasmic astrocytes have many short branching processes and are found in gray matter
• Fibrous astrocytes have many long unbranched processes and are located mainly in white matter
• Processes of astrocytes contact blood capillaries, neurons, and the pia mater

Functions of Astrocytes

• Astrocytes contain microfilaments for considerable strength to support neurons
• Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from potentially harmful substances in blood. They do this by secreting chemicals that maintain the properties of the endothelial cells of the capillaries
• In the embryo, astrocytes secrete chemicals to regulate the growth, migration, and interconnection among neurons in the brain

Oligodendrocytes

• They resemble astrocytes but are smaller and contain fewer processes
• They for and maintain the myelin sheath around axons in the CNS
• The myelin sheath a multilayered lipid and protein covers insulates certain axons and increases nerve impulse conduction
• Axons are said to be myelinated

Microglial Cells or Microglia

• Small cells with slender processes that can give off spiny projections
• Function as phagocytes removing cellular debris that formed during development and to phagocytize microbes

Ependymal Cells

• Cuboidal to columnar cells arranged in a single layer with microvilli and cilia
• They line the ventricles of the brain and central canal of the spinal cord filled with cerebrospinal fluid
• Ependymal cells produce, monitor, and assist in the circulation of cerebrospinal fluid and form the blood-cerebrospinal fluid barrier

Neuroglia of the PNS

• Completely surround axons and cell bodies and are schwann and satellite cells

Schwann Cells

• Encircle PNS axons and form myelin sheath around axons with a single schwann cell for each axon.
• A single Schwann cell can also enclose as many as 20 or more unmyelinated axons
• Schwann cells participate in axon regeneration

Satellite Cells

• Flat cells that surround the cell bodies of PNS ganglia
• Regulate the exchanges of materials between neuronal cell bodies and interstitial fluid

Action Potentials

• The action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state, it has a depolarizing and a repolarizing phase.
• During the depolarizing phase, the negative membrane potential becomes less negative, reaches zero, and becomes positive. During the repolarizing phase, the membrane potential is restored to the resting state of –70 mV.
• The plasma membrane and axon terminals mainly contain voltage-gated channels that enable these phases
• The voltage-gated Na+ channels open cause Na+ to rush into the cell, which causes the depolarizing phase. Then voltage-gated K+ channels open, allowing K+ to flow out, which produces the repolarizing phase.
• The threshold value is -55 mV in neurons, the amplitude of an action potential is always the same and does not depend on stimulus intensity
• The after-hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends

Depolarizing Phase

• A depolarizing graded potential or some other stimulus causes the membrane of the axon to depolarize to threshold, voltage-gated Na+ channels open rapidly
• An inrush of Na+ causes the depolarizing phase of the action potential changing the membrane potential from -55 mV to +30 mV
• At the peak of the action potential, the inside of the membrane is 30 mV more positive than the outside

Voltage-gated Na+ channels

• Contain an activation gate and an inactivation gate
• In the resting state of the inactivation gate is open, but the activation gate is closed preventing Na+ from moving into the cell
• At threshold the activation gates opens, as more channels open, the more the membrane depolarizes
• During the few ten-thousandths of a second that the voltage-gated Na+ channel is open, about 20,000 Na+ flow
• Sodium-potassium pumps easily bail out the 20,000 or so Na+ that enter the cell during a single action potential and maintain the low concentration of Na+ inside the cell

Repolarizing Phase

• Shortly after the activation gates of the voltage-gated Na+ channels open, the inactivation gates close, putting it an inactivated state.
• A threshold-level depolarization opens voltage-gated K+ channels more slowly.
• The slower opening of voltage-gated K+ channels and the closing of previously open voltage-gated Na+ channels produce repolarizing phase
• Voltage gated K channels are still open, outflow of K+ may be large enough to cause after-hyperpolarizing phase where membrane potential becomes -90 mV
• Repolarization also allows inactivated Na+ channels to revert to the resting state

Refractory Period

• The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus
• During the absolute refractory period, even a very strong stimulus cannot initiate a second action potential because inactivated Na+ channels cannot reopen and must first return to the resting state.
• Graded potentials do not exhibit a refractory period.

Ion Channel Gate

• When ion channels are open, they allow specific ions to move across the plasma membrane, down their electrochemical gradient (concentration and electrical difference)
• Four types of ion channels: leak channels, ligand-gated channels, mechanically gated channels, and voltage-gated channels
• A ligand-gated channel opens and closes upon a binding of a ligand (chemical) stimulus and are located in the dendrites of some sensory neurons, such as pain receptors, and in dendrites and cell bodies of interneurons and motor neurons.

Voltage-gated channel

• Opens in response to a change in membrane potential (voltage).
• Voltage-gated channels participate in the generation and conduction of action potentials in the axons of all types of neurons.

Continuous and Saltatory Conduction

• There are two types of propagation: continuous conduction and saltatory conduction
• Continuous conduction, involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane and note that the action potential propagates only a relatively short distance in a few milliseconds and occurs in unmyelinated axons
• Saltatory conduction: Action potentials propagate more rapidly along myelinated axons because of the uneven distribution of voltage-gated channels so current carried by Na+ and K+ flows across the membrane with more frequency on these nodes
• The action potential at the first node generates ionic currents that depolarize open voltage gated Na+ channels and creates an action potential at the second node

Resting Membrane Potential

• The resting membrane potential exists because of a small buildup of negative ions in the cytosol along the inside of the membrane, and an equal buildup of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane
• Such a separation of positive and negative electrical charges is a form of potential energy, which is measured in volts or millivolts
• In neurons, the resting membrane potential ranges from –40 to –90 mV
• A cell that exhibits a membrane potential is said to be polarized and Most body cells are polarized
• The resting membrane potential can arise from unequal distribution of ECF and cytosol because of more K leak channels than Na leak channels allowing potassium ions to diffuse out and into the cell
• Anions inside the cell cannot leave and follow the potassium out, so most anions in- side the cell are not free to leave
• Electrodes measure electrical charges to detect the electrical difference (voltage) across the plasma membrane (Figure 12.12b)