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.