Nervous System Chapter Notes | Biology Class 7 ICSE PDF Download

Coordination

All living organisms rely on various interconnected processes such as digestion, respiration, and movement to meet their bodily needs. These processes work together, not independently, to maintain internal and external balance, a phenomenon known as coordination. Coordination ensures that the body's activities align to fulfill its requirements effectively.

There are two primary types of coordination: (i) nervous coordination and (ii) chemical coordination.

(i) Nervous Coordination: This type is managed by the nervous system, which includes the brain, spinal cord, nerves, and sense organs. It facilitates rapid responses to stimuli through coordinated actions. Examples include:

• Eating Food: When hunger strikes, multiple actions are coordinated. The eyes observe food on the table, the brain processes this information, and signals the arms to move. The hands pick up the plate, fingers select the food, and guide it to the mouth. The food then travels through the digestive system, eventually reaching the bloodstream to address the hunger.
• Playing Tennis: Fast-paced activities like tennis demand precise coordination. The brain makes quick judgments, the eyes track the ball, and muscles execute movements to respond effectively.
• Reacting to Pain: A sprained ankle sends pain signals, prompting the brain to restrict movement of the affected area to aid recovery.

(ii) Chemical Coordination: This involves chemical messengers, such as hormones, that regulate bodily functions. For instance, when faced with danger, like a threat on the street, the body releases specific chemicals into the bloodstream. These hormones provide the energy and strength needed to run away, ensuring a swift response to the situation.

Structural and Functional Unit of Nervous System - The Neuron

• The nervous system is made up of special cells called neurons or nerve cells.
• A neuron has two main parts:
  • Cyton: The main cell body containing a nucleus, which gives out fine processes called dendrites.
  • Axon: A long extension that carries messages from the cyton to terminal branches.
• Dendrites receive messages from organs and pass them through the cyton to the axon.
• Terminal branches of the axon connect to dendrites of another neuron at a point called a synapse.
• The synapse is where messages are transmitted from one neuron to the next.

Types of Neurons

• Sensory neurons: Carry impulses from sense organs to the spinal cord or brain.
• Motor neurons: Carry messages from the brain or spinal cord to muscles or glands.
• Association neurons: Located in the spinal cord, they transmit impulses between neurons.

Nerve

• Sensory nerve: Contains only sensory neurons, e.g., optic nerve of the eye.
• Motor nerve: Contains only motor neurons, e.g., nerves of the eyeball muscles.
• Mixed nerve: Contains both sensory and motor neurons, e.g., nerve to the tongue.

Human Nervous System

The human nervous system comprises two main components:

1. Central Nervous System (CNS): This includes the brain, located within the skull, and the spinal cord, housed within the vertebral column.
2. Peripheral Nervous System (PNS): This consists of nerves that extend from the central nervous system to all parts of the body. The PNS is further divided into two subdivisions: the somatic nervous system and the autonomic nervous system.

The Brain

• The brain is the most complex and important organ, weighing about 1.5 kg in adults.
• It is protected by the skull and three membranes called meninges, separated by cerebrospinal fluid.
• The brain has three main parts:
  • Cerebrum: The largest part, divided into two cerebral hemispheres with folded surfaces to accommodate many neurons (about 9 billion). It controls intelligence, consciousness, memory, and voluntary activities.
  • Cerebellum: Located under the cerebrum, it manages body balance and muscle coordination. It is affected by alcohol, causing poor muscle coordination.
  • Medulla oblongata: The lowest part, connecting to the spinal cord, it controls involuntary actions like heartbeat, breathing, swallowing, and digestion. Injury to it can cause immediate death.

The Spinal Cord

Extends from the medulla oblongata and runs through most of the backbone.

Main functions:

• Controls reflexes below the neck.
• Carries messages from skin and muscles to the brain.
• Transmits commands from the brain to muscles of the trunk and limbs.

Parts of Central Nervous System and Their Main Functions
Brain:

• Cerebrum: Controls intelligence, consciousness, memory, and will power.
• Cerebellum: Manages muscle coordination and body balance.
• Medulla oblongata: Regulates breathing, digestion, heartbeat, and other involuntary actions.

Spinal Cord:

• Controls reflexes below the neck.
• Conducts messages from skin and muscles to the brain.
• Transmits commands from the brain to muscles of the trunk and limbs.

Peripheral Nervous System

The peripheral nervous system (PNS) comprises nerves that link the central nervous system (CNS) to all parts of the body. It is categorized into two main subdivisions: the somatic nervous system and the autonomic nervous system.

Somatic Nervous System: 

• This system includes two types of nerves: cranial nerves (originating from the brain) and spinal nerves (originating from the spinal cord). 
• The somatic nervous system is responsible for transmitting sensory information (such as smell, taste, and sound) from the body to the CNS and relaying commands from the CNS to skeletal muscles, which control voluntary movements.

Autonomic Nervous System: 

• This system consists of paired chains of ganglia (clusters of nerve cell bodies) and nerves located along both sides of the spinal cord. It operates largely unconsciously, regulating involuntary functions of internal organs, such as heart rate, blood pressure, breathing, and pupil dilation. 
• The autonomic nervous system is further divided into two opposing subsystems: the sympathetic nervous system, which triggers immediate "fight or flight" responses during stressful or abnormal conditions, and the parasympathetic nervous system, which restores normal conditions. For example, if the sympathetic nervous system increases heart rate, the parasympathetic nervous system works to slow it down.