Neural Control and Coordination: Class 11 NCERT Biology Guide

Understanding the Neuron: Structure and Function

A neuron is the fundamental structural and functional unit of the neural system. It is a microscopic cell specialised to transmit nerve impulses. A typical neuron consists of three main parts:

• Cell Body (Soma): Contains the nucleus, mitochondria, ribosomes, and Nissl's granules responsible for protein synthesis.
• Dendrites: Short, branched fibers that receive impulses from other neurons and carry them towards the cell body.
• Axon: A long fiber that conducts impulses away from the cell body to other neurons or muscles.

The axon ends in synaptic knobs containing neurotransmitters that help transmit signals across synapses. Neurons are classified based on their structure:

Axons may be myelinated (covered with Schwann cells forming myelin sheath) or non-myelinated. Myelinated axons conduct impulses faster due to nodes of Ranvier.

Brain Structure: The Central Control Unit

The brain is the central organ of the nervous system, protected by the skull and meninges. It is divided into three main parts:

• Forebrain: Includes the cerebrum and diencephalon. The cerebrum controls voluntary actions, intelligence, and memory.
• Midbrain: Connects the forebrain and hindbrain and controls reflexes related to vision and hearing.
• Hindbrain: Comprises the cerebellum, pons, and medulla oblongata. It regulates balance, posture, and vital functions like breathing.

The brain has two types of tissues:

• Grey Matter: Contains neuronal cell bodies and is found on the brain's surface.
• White Matter: Contains myelinated axons and lies beneath the grey matter.

Understanding brain structure helps explain how different parts coordinate to control body functions.

Central Nervous System vs Peripheral Nervous System

The nervous system is divided into two major parts:

The CNS acts as the control centre, while the PNS connects the CNS to limbs and organs, facilitating communication throughout the body.

Nerve Impulse: Resting Potential and Action Potential

Neurons transmit signals via electrical impulses. Two key states are:

• Resting Potential: The neuron membrane is polarised with a charge of about $-70$ mV inside relative to outside. This is maintained by the sodium-potassium pump, which moves $3$ Na$^+$ ions out and $2$ K$^+$ ions in.

• Action Potential: When stimulated, voltage-gated Na$^+$ channels open, allowing Na$^+$ ions inside, reversing the polarity (depolarisation). The membrane potential becomes positive temporarily, then returns to resting state (repolarisation).

Worked Example:

If the resting potential is $-70$ mV, and during depolarisation the membrane potential reaches $+40$ mV, the total change in potential is:

$$+40 - (-70) = 110 \text{ mV}$$

This rapid change propagates along the neuron as a nerve impulse.

Transmission of Nerve Impulses Across Synapses

Neurons communicate at junctions called synapses. Transmission occurs chemically:

1. When an impulse reaches the synaptic knob, it triggers the release of neurotransmitters stored in synaptic vesicles. 2. These chemicals cross the synaptic cleft and bind to receptors on the next neuron or muscle cell. 3. This binding generates a new impulse in the receiving cell.

This process ensures one-way signal flow and allows modulation of signals. Common neurotransmitters include acetylcholine and dopamine.

Reflex Actions: Quick Responses for Survival

Reflex actions are rapid, involuntary responses to stimuli that protect the body. They involve a simple neural pathway called the reflex arc:

• Receptor: Detects stimulus.
• Sensory Neuron: Transmits impulse to spinal cord.
• Interneuron: Processes information in the spinal cord.
• Motor Neuron: Sends impulse to effector.
• Effector: Muscle or gland that responds.

Reflexes bypass the brain for faster response. For example, touching a hot object causes immediate withdrawal without conscious thought.