Control and Coordination Notes - Nervous System, Reflex Arc, Brain, Plant Hormones and Endocrine Glands

In organisms, many movements are responses to changes in the environment or to utilize changes in the environment.

• Plants grow out into the sunshine.
• Children get pleasure and fun out of swinging.
• Buffaloes chew cud for better digestion.
• Falling light on eyes or touching a hot object cause responses.

Such movements are carefully controlled and coordinated by specialized tissues. They are also connected to the recognition of various events in the environment.

ANIMALS - NERVOUS SYSTEM

• In animals, control and coordination are provided by nervous and muscular tissues.
• Nervous tissue is made up of a network of nerve cells (neurons). It conducts information (electrical impulses) from one part of the body to another.
• Some nerve cells have specialized tips called receptors. They are usually located in sense organs and detect information from the environment. For example, gustatory receptors detect taste, and olfactory receptors detect smell.
• General perception of taste is jointly created by the tongue (taste) and nose (smell). That’s why if the nose is blocked or we have a cold, there is a difference in the taste of foods.

TRANSMISSION OF NERVE IMPULSE

The transmission of a nerve impulse follows this pathway:

Information from receptors → dendritic tip of nerve cell → sets off a chemical reaction → generation of electrical impulse → impulse to cell body → axon → axonal end releases chemicals → chemicals cross the gap (synapse) → generation of electrical impulse in dendrite of the next neuron → impulses deliver to muscle cells or gland.

WHAT HAPPENS IN REFLEX ACTIONS?

• Reflex actions are sudden, unconscious actions of the body in response to a stimulus in the environment. E.g.,
• Withdrawal of hand when we touch a flame.
• Blinking of eyes when light falls on them.
• These actions do not involve thinking, as they require an immediate response.
• Thinking requires the complex interaction of many nerve impulses from multiple neurons.
• In the brain, the thinking tissue is located in the forward end of the skull. It receives signals from all over the body and processes them before responding.
• If the thinking part of the brain instructs muscles to move, nerves must carry this signal to different parts of the body. This process takes time and can prevent a quick response. For example, when touching a hot object, involving the thinking tissue would delay hand withdrawal.
• The pathway of impulses in a reflex action is called a reflex arc. It includes the receptor, sensory neuron, CNS, motor neuron, and effector (muscle or gland).

• Nerves from all body parts meet in the spinal cord on their way to the brain.
• Reflex arcs evolved as efficient ways of functioning in the absence of true thought processes in animals. Even after complex neuron networks evolved, reflex arcs remain more efficient for quick responses.
• The sequence of events in a reflex arc when bright light is focused on the eyes is:

Receptor → Sensory neuron → Brain → Motor neuron → Eye → Eye muscle contracts.

HUMAN BRAIN

• The brain and spinal cord constitute the central nervous system (CNS). They receive information from all parts of the body and integrate it.
• The brain is protected in a fluid-filled balloon (shock absorber) inside the bony box (cranium).
• The spinal cord is protected in the vertebral column or backbone.
• The brain is the main coordinating center of the body. It involves complex mechanisms and neural connections for processes such as thinking.
• The spinal cord contains nerves that supply information for the brain to process.
• The brain sends messages to muscles to control voluntary actions such as writing, talking, moving, clapping, etc.
• Communication between the CNS and other parts of the body is facilitated by the peripheral nervous system (PNS), which consists of cranial nerves (from the brain) and spinal nerves (from the spinal cord).
• The brain has three regions: forebrain, midbrain, and hindbrain.

• The forebrain is the main thinking part of the brain. It has the following regions:
• Sensory regions: Receive sensory impulses for hearing, smell, sight, etc.
• Association areas: Interpret sensory information by associating it with information from other receptors and previously stored information. A decision is made to respond, and this information is passed to the motor areas, which control the movement of voluntary muscles.
• Centre of hunger: Provides sensations such as feeling full when food is eaten.
• In the body, there are involuntary actions between simple reflex actions and thought-out actions. These are not under conscious control, e.g., salivation, heartbeat. They are controlled by the midbrain and hindbrain.
• The medulla in the hindbrain controls involuntary actions such as blood pressure, salivation, vomiting, etc.
• The cerebellum in the hindbrain controls voluntary actions like walking, riding a bicycle, picking up a pencil, etc. It also maintains the posture and balance of the body.

HOW DOES NERVOUS TISSUE CAUSE ACTION?

• When a nerve impulse reaches the muscle, the muscle fibers (muscle cells) move by changing their shape, causing them to shorten.
• Muscle cells have special proteins that change their shape and arrangement in response to nervous electrical impulses, resulting in a shorter form.
• Voluntary muscles: Muscles attached to the skeleton that can be moved as we decide.
• Involuntary muscles: Muscles found in visceral organs that are not under our control.

COORDINATION IN PLANTS

Plants have no nervous system or muscles, but they can respond to stimuli.

Like animals, plants show two types of movement:

• Dependent on growth: For example, when a seed germinates, the root grows downward, and the stem grows upward.
• Independent of growth: For example, when the leaves of a chhui-mui (the ‘sensitive’ or ‘touch-me-not’ plant of the Mimosa family) are touched, they quickly fold up and droop.

IMMEDIATE RESPONSE TO STIMULUS

• In the sensitive plant, movement occurs at a point different from the point of touch, indicating that information about the touch is communicated. Plants use electrical-chemical means to conduct information from cell to cell.
• In animals, some muscle proteins help change the shape of cells. In contrast, plant cells change shape (swelling or shrinking) by altering the amount of water in them.

MOVEMENT DUE TO GROWTH

• Some plants, such as peas, have tendrils to climb up supports or fences. These tendrils are sensitive to touch.
• When tendrils come into contact with a support, the touching part grows more slowly than the part away from the object, causing the tendril to circle around and cling to the support.
• Plants respond to stimuli slowly by growing in a specific direction. This directional growth makes it appear as if the plant is moving.
• Environmental triggers such as light or gravity change the direction of plant growth. These are called tropic (directional) movements, which can be toward the stimulus (positive) or away from it (negative).

TYPES OF TROPIC MOVEMENTS

Phototropism

• The tropic movement in response to light.
• Shoots respond by bending toward light (positive).
• Roots respond by bending away from light (negative).
• This can be demonstrated by the following activity:
• Fill a conical flask with water and cover its neck with a wire mesh containing 2-3 freshly germinated bean seeds.
• Place the flask in a cardboard box such that its open side faces light coming from a window.
• After 2 or 3 days, observe that the shoots bend toward the light and the roots bend away from it.
• Turn the flask so that the shoots are away from the light and the roots are toward it. Leave it for a few days.
• The old parts of the shoot and root show no noticeable change in direction, but the new growth parts show a change: shoots bend toward the light, and roots bend away from it.

Geotropism

• Geotropism is the movement in response to the pull of the Earth or gravity.
• Roots always grow downward (positive).
• Shoots grow upward and away from the Earth (negative).

Hydrotropism

• The movement of a plant toward or away from water.
• For example, roots bend toward moist soil.

Chemotropism

• The movement of a plant toward or away from chemicals.
• For example, the growth of pollen tubes toward ovules.

Controlled movements in plants can be either slow or fast. For example:

• The sensitive plant quickly moves in response to touch.
• Sunflowers slowly move in response to day or night.
• Growth-related movements of plants are slower.

In animal bodies, growth also occurs in controlled directions, such as the growth of arms and fingers.

For fast responses to stimuli, information must be transferred quickly. Electrical impulses are an excellent means for this, but they have limitations:

• They reach only the cells connected to nervous tissue.
• Once an impulse is generated and transmitted, the cell takes time to reset its mechanism to generate a new impulse, so cells cannot continuously create and transmit electrical impulses.

Therefore, most multicellular organisms use chemical (hormone) communication between cells.

In this process, stimulated cells release a hormone that diffuses around the original cell. Other cells detect the hormone using special molecules on their surfaces, recognize the information, and transmit it.

This is a slower process but can reach all cells and can be done steadily and persistently.

Plant hormones coordinate growth, development, and responses to the environment. They are synthesized at specific locations and diffuse to the area of action.

SOME PLANT HORMONES

• Auxin: Regulates growth in plants. When plants detect light, auxin, synthesized at the shoot tip, helps cells grow longer. When light comes from one side, auxin diffuses toward the shady side of the shoot, causing cells on the shady side to grow longer, making the plant bend toward the light.
• Gibberellins: Help in the growth of the stem.
• Cytokinins: Promote cell division. They are present in greater amounts in areas of rapid cell division, such as fruits and seeds.

The above hormones promote growth.

• Abscisic acid: Inhibits growth and causes wilting of leaves.

HORMONES IN ANIMALS (ENDOCRINE SYSTEM)

• Electrical impulses can instruct only a few tissues to prepare for an activity. Therefore, animals have another method of control and coordination called the Endocrine System.
• It includes endocrine glands and their secretions called hormones (chemical signals). These can reach all cells and provide wide-ranging changes. For example, adrenal glands secrete adrenaline.
• Adrenaline prepares the body to cope with emergency situations. For example, when encountering a scary animal, such as a squirrel, the body prepares to fight or run away, requiring more energy.
• Though fighting and running are different, both require some common preparations.
• Adrenaline is secreted into the blood and carried to different body parts, resulting in the following events:
• Heart beats faster to supply more oxygen to muscles.
• Blood to the digestive system and skin is reduced by contracting muscles around small arteries in these organs, diverting blood to skeletal muscles.
• Breathing rate increases due to the contraction of the diaphragm and rib muscles.
• These responses prepare the body to deal with the situation (fighting or running).
• The timing and amount of hormone released are regulated by feedback mechanisms. For example, if the blood sugar level increases, it is detected by cells of the pancreas, which produce more insulin. As the blood sugar level falls, insulin secretion is reduced.

SOME IMPORTANT HORMONES AND THEIR FUNCTIONS