Neural Control and CoordinationClass 11 Biology NCERT Solutions

The human brain is the central information processing organ and is protected by the skull. It is covered by three layers of cranial meninges: dura mater (outer), arachnoid (middle), and pia mater (inner). The brain is divided into three major parts:

The midbrain, pons, and medulla oblongata together form the brain stem, which connects the brain to the spinal cord.

(a) Polarisation of the membrane of a nerve fibre: In the resting state, a neuron's membrane is said to be polarised. This is due to a difference in the distribution of ions across the axonal membrane. The axoplasm (fluid inside the axon) has a high concentration of potassium ions ($K^+$) and negatively charged proteins, and a low concentration of sodium ions ($Na^+$). Conversely, the extracellular fluid has a high concentration of $Na^+$ and a low concentration of $K^+$. This concentration gradient is maintained by the sodium-potassium pump, which actively transports 3 $Na^+$ ions out of the cell for every 2 $K^+$ ions it pumps in. Additionally, the resting membrane is more permeable to $K^+$ ions and nearly impermeable to $Na^+$ ions and negatively charged proteins. As a result, the outer surface of the axonal membrane acquires a positive charge, while the inner surface becomes negatively charged. This electrical potential difference across the resting membrane is called the resting potential.

(b) Depolarisation of the membrane of a nerve fibre: When a stimulus is applied to a polarised membrane, it causes a rapid change in membrane potential. The stimulus triggers the opening of voltage-gated $Na^+$ channels, making the membrane freely permeable to $Na^+$ ions at that site. This leads to a rapid influx of $Na^+$ ions into the axoplasm, driven by the concentration gradient. The influx of positive ions reverses the polarity of the membrane at that site: the inner surface becomes positively charged, and the outer surface becomes negatively charged. This reversal of polarity is called depolarisation. The electrical potential difference across the membrane during this phase is known as the action potential, which is effectively the nerve impulse.

(c) Transmission of a nerve impulse across a chemical synapse: A chemical synapse is a junction between a pre-synaptic neuron and a post-synaptic neuron, separated by a fluid-filled space called the synaptic cleft. The transmission of an impulse occurs as follows:

Since drawing is not possible in this format, a description of the key labelled parts for each diagram is provided below, based on the textbook figures.

(a) Neuron (based on Figure 18.1): A labelled diagram of a neuron would show the following parts:

(b) Brain (based on Figure 18.4 - Sagittal Section): A labelled diagram of the human brain would show the three main regions with their key components:

(a) Neural coordination: Neural coordination is the process through which the neural system, an organised network of neurons, provides quick, point-to-point connections to coordinate and integrate the functions of various organs and organ systems. This rapid communication allows the body to respond swiftly to stimuli and maintain homeostasis. For example, during physical exercise, the neural system coordinates the functions of muscles, lungs, heart, and blood vessels to meet the increased demand for energy and oxygen. It works in conjunction with the endocrine system to ensure that all bodily activities function in a synchronised manner.

(b) Forebrain: The forebrain is the largest and most complex part of the human brain. It consists of the cerebrum, thalamus, and hypothalamus. The cerebrum is divided into two hemispheres connected by the corpus callosum and is the centre for higher functions like thought, memory, communication, and voluntary actions. The thalamus acts as a major relay and coordinating centre for sensory and motor signals. The hypothalamus controls vital functions such as body temperature, hunger, thirst, and also regulates the pituitary gland. The inner parts of the forebrain, along with structures like the amygdala and hippocampus, form the limbic system, which is involved in emotional responses, motivation, and sexual behaviour.

(c) Midbrain: The midbrain is a relatively small region located between the forebrain's thalamus/hypothalamus and the hindbrain's pons. A canal called the cerebral aqueduct passes through it. The dorsal part of the midbrain contains four round swellings known as the corpora quadrigemina. The midbrain acts as a relay centre for auditory and visual information and plays a crucial role in controlling eye movement and other motor functions.

(d) Hindbrain: The hindbrain is located at the lower back part of the brain and comprises the pons, cerebellum, and medulla oblongata. The pons consists of fibre tracts that interconnect different regions of the brain. The cerebellum has a highly convoluted surface and is responsible for coordinating voluntary movements, posture, and balance. The medulla oblongata connects the brain to the spinal cord and contains vital centres that control autonomic functions such as respiration, cardiovascular reflexes (like heart rate), and gastric secretions.

(e) Synapse: A synapse is a specialized junction where a nerve impulse is transmitted from one neuron (the pre-synaptic neuron) to another (the post-synaptic neuron) or to an effector cell like a muscle or gland. There are two types: electrical and chemical. In an electrical synapse, the neurons are in very close proximity, allowing direct flow of electrical current, resulting in very fast transmission. In a chemical synapse, the neurons are separated by a fluid-filled gap called the synaptic cleft. Transmission occurs via the release of chemical messengers called neurotransmitters from the pre-synaptic terminal, which then bind to receptors on the post-synaptic membrane to generate a new potential.

The mechanism of synaptic transmission, particularly at a chemical synapse, involves the transfer of a nerve impulse from a pre-synaptic neuron to a post-synaptic neuron using neurotransmitters. The process occurs in the following steps:

Sodium ions ($Na^+$) play a crucial role in the generation of an action potential in a neuron. The process can be explained as follows:

Thus, the rapid influx of $Na^+$ is the direct cause of the depolarisation phase, which is the defining characteristic of an action potential.

(a) The cerebrum is the most developed part of the human brain. It forms the major part of the brain and its outer layer, the cerebral cortex, is responsible for complex functions like memory, communication, intelligence, and consciousness.

(b) The hypothalamus acts as a master clock for the body. It contains centres that control and regulate the circadian (24-hour) rhythms, which govern our sleep-wake cycles and other daily physiological and behavioural patterns.