Neural Control and CoordinationNEET Zoology · Class 11 · NCERT Chapter 18

Introduction

The nervous system is your body's fast communication network. It senses the environment, processes the information, and sends out commands to muscles and glands within milliseconds. It works alongside the endocrine system, but where endocrine signals are slow and long-lasting, nervous signals are fast and short-lived.

Expect 2 to 3 NEET questions every year from this chapter. The most reliable scoring areas are: parts of a neuron, resting and action potential values, ions involved at each phase, saltatory conduction, neurotransmitter release, parts of the brain with their functions, and the components of the reflex arc.

Organisation of the Nervous System

The human nervous system has two main divisions:

• Central Nervous System (CNS): includes the brain and the spinal cord. The CNS processes information and gives commands.
• Peripheral Nervous System (PNS): all the nerves outside the CNS. The PNS has two parts:
  • Somatic neural system: carries signals from the CNS to the voluntary skeletal muscles.
  • Autonomic neural system: controls involuntary actions of smooth muscle, cardiac muscle and glands. It is further divided into sympathetic (fight or flight) and parasympathetic (rest and digest).

Neuron: Structure

A neuron is the functional unit of the nervous system. It has three main regions:

• Cell body (cyton or soma): contains the nucleus, mitochondria, Nissl granules (rough endoplasmic reticulum) and other organelles.
• Dendrites: short, branched processes that receive signals from other neurons and carry them toward the cell body.
• Axon: a single long process that carries the impulse away from the cell body to the next neuron, muscle or gland.

The axon may be wrapped in a fatty myelin sheath. In peripheral nerves the sheath is formed by Schwann cells. The small gaps between Schwann cells where the axon is exposed are called nodes of Ranvier. At the end of the axon, the synaptic terminals contain vesicles of neurotransmitters.

Types of Neurons

Neurons are classified by the number of processes they have:

• Multipolar: one axon and many dendrites. Most common type. Found in the cerebral cortex and as motor neurons.
• Bipolar: one axon and one dendrite. Found in the retina of the eye and the olfactory bulb (smell).
• Unipolar: only one process from the cell body (which divides into two). Usually found in the embryo.

Generation of a Nerve Impulse

Resting Membrane Potential

When a neuron is not firing, there is a voltage difference across its membrane. The inside of the cell is about negative 70 mV compared to the outside. This is the resting membrane potential.

How is the resting potential maintained?

• The Na+/K+ pump moves 3 Na+ out and 2 K+ in for every ATP spent. This makes the inside slightly more negative.
• The membrane is more permeable to K+ at rest (leak channels), so K+ tends to flow out, leaving the inside negative.
• Negatively charged proteins inside the cell cannot leave, adding to the negativity.

Action Potential

If a stimulus is strong enough (above the threshold), it triggers an action potential. This is a sudden, brief reversal of the membrane voltage and follows the all-or-none law. Either the action potential happens fully or it does not happen at all.

1. Depolarisation: voltage-gated Na+ channels open. Na+ rushes IN. Inside becomes positive (peaks around +30 mV).
2. Repolarisation: Na+ channels close, K+ channels open. K+ flows OUT. Inside becomes negative again.
3. Hyperpolarisation: K+ channels stay open a little too long. Voltage dips below -70 mV briefly.
4. Return to resting: Na+/K+ pump restores ion balance over the next several milliseconds.

Saltatory Conduction

In a myelinated axon, the myelin sheath insulates most of the membrane. Only the small nodes of Ranvier are exposed to the extracellular fluid and have ion channels. The action potential jumps from one node to the next instead of moving smoothly down the whole axon. This jumping is called saltatory conduction.

Why saltatory conduction matters: it makes the impulse much faster (up to 120 metres per second in some myelinated axons versus about 1 metre per second in unmyelinated axons), and it uses less energy because the Na+/K+ pump only has to work at the nodes.

The Synapse

A synapse is the junction between two neurons (or between a neuron and a muscle or gland). Synapses come in two types:

Electrical Synapse

• Membranes of pre- and post-synaptic neurons are in direct contact through gap junctions.
• Ion flow passes the signal directly. Transmission is very fast.
• Less common. Found in some heart muscle and embryonic tissue.

Chemical Synapse

Most synapses in the human nervous system are chemical. Steps in transmission:

1. The action potential reaches the pre-synaptic terminal.
2. Voltage-gated Ca2+ channels open. Ca2+ enters the terminal.
3. Ca2+ triggers vesicles of neurotransmitter (such as acetylcholine) to fuse with the membrane and release their contents into the synaptic cleft.
4. Neurotransmitter molecules bind to receptors on the post-synaptic membrane.
5. Receptors open ion channels on the post-synaptic side. Depending on the receptor, the result is either an excitatory or an inhibitory effect.
6. The neurotransmitter is rapidly broken down or taken back up to end the signal.

Common neurotransmitters: acetylcholine (at the neuromuscular junction and many synapses), dopamine, serotonin, GABA, glutamate, norepinephrine.

Central Nervous System (CNS)

The CNS includes the brain and spinal cord, both protected by three layers of membranes called meninges (dura mater, arachnoid, pia mater) and bathed in cerebrospinal fluid (CSF).

The Brain

The adult human brain weighs about 1.3 to 1.5 kg. It has three regions: forebrain, midbrain and hindbrain.

Cerebrum and Its Lobes

The cerebrum is the largest part of the brain. It is divided into two halves called cerebral hemispheres, connected by a sheet of fibres called the corpus callosum. The outer layer is grey matter (cerebral cortex). The inner layer is white matter.

The cortex of each hemisphere is divided into four lobes:

• Frontal lobe: voluntary movement, decision making, planning, personality.
• Parietal lobe: touch sensation, temperature, pain, spatial awareness.
• Temporal lobe: hearing, smell, memory, language understanding.
• Occipital lobe: vision.

The cerebrum is responsible for higher mental functions: intelligence, memory, consciousness, thinking, voluntary movements.

Thalamus and Hypothalamus

• Thalamus: the major sensory and motor relay station for the cerebrum. Almost all sensory signals (except smell) pass through the thalamus before reaching the cerebral cortex.
• Hypothalamus: small but vital region below the thalamus. Contains centres for body temperature, hunger, thirst, sleep, sexual behaviour. Also links the nervous system to the endocrine system by controlling the pituitary gland.

The limbic system (including hippocampus and amygdala) sits inside the forebrain and controls emotion, motivation and memory.

Midbrain

Small region between the forebrain and the hindbrain. Contains the corpora quadrigemina (four small lobes on the dorsal side) which serve as centres for visual and auditory reflexes.

Hindbrain (Pons, Cerebellum, Medulla)

• Pons: connects the medulla to the rest of the brain. Helps regulate the breathing rhythm started by the medulla.
• Cerebellum: second largest part of the brain. Coordinates and fine-tunes voluntary muscle movement and balance. Has a wrinkled (foliated) surface.
• Medulla oblongata: the lowest part of the brain stem. Continuous with the spinal cord. Controls vital involuntary functions: breathing, heart rate, blood pressure, vomiting reflex, gastric secretion. Damage here can be fatal.

Together, pons, midbrain and medulla form the brain stem.

Spinal Cord

The spinal cord is a long tube of nervous tissue running inside the vertebral column. It carries signals between the brain and the rest of the body, and integrates simple reflexes on its own.

• Inner grey matter (butterfly or H-shaped in cross section, contains neuron cell bodies).
• Outer white matter (mostly myelinated axons forming tracts up and down the cord).
• 31 pairs of spinal nerves arise from the spinal cord.

Peripheral Nervous System (PNS)

The PNS connects the CNS to the rest of the body. It has two main types of fibres:

• Afferent (sensory) fibres: carry signals from sense organs to the CNS.
• Efferent (motor) fibres: carry signals from the CNS to muscles or glands.

Nerves attached to the PNS are: 12 pairs of cranial nerves (from the brain) and 31 pairs of spinal nerves (from the spinal cord).

Autonomic Nervous System

The autonomic system controls involuntary body functions. It has two opposing divisions:

Reflex Action and Reflex Arc

A reflex is a quick, involuntary response to a stimulus. The response is generated without conscious thought, often in the spinal cord, which is why it is so fast.

The reflex arc has five parts in order:

1. Receptor: the sense organ or cell that detects the stimulus.
2. Afferent (sensory) neuron: carries the signal toward the CNS.
3. Centre: usually a part of the spinal cord (grey matter). Sometimes the brain stem.
4. Efferent (motor) neuron: carries the signal away from the CNS to the effector.
5. Effector: a muscle or gland that responds.

Monosynaptic reflex: sensory neuron directly synapses with motor neuron. No interneuron. Example: knee jerk (patellar) reflex.

Polysynaptic reflex: sensory and motor neurons are connected through one or more interneurons. Example: withdrawal reflex when you touch a hot object.

Conditioned reflex:a learned reflex that develops after experience. Example: Pavlov's dogs salivating to a bell. Conditioned reflexes involve the cerebrum.

Worked NEET Problems

Summary Cheat Sheet

• CNS: brain + spinal cord. PNS: all nerves outside CNS.
• PNS divisions: somatic (voluntary) and autonomic (involuntary). Autonomic splits into sympathetic and parasympathetic.
• Neuron parts: cell body (cyton) + dendrites (receive) + axon (sends). Axon may be myelinated.
• Myelin sheath: Schwann cells in PNS, oligodendrocytes in CNS. Gaps = nodes of Ranvier.
• Resting potential: negative 70 mV inside. Maintained by Na+/K+ pump (3 Na+ out, 2 K+ in per ATP).
• Action potential: depolarisation (Na+ IN, peak +30 mV) → repolarisation (K+ OUT) → hyperpolarisation → back to resting. All-or-none.
• Saltatory conduction: action potential jumps node to node in myelinated axons. Up to 120 m/s vs 1 m/s in unmyelinated.
• Chemical synapse: AP arrives → Ca2+ enters → neurotransmitter released into cleft → binds receptors → ion channels open on post-synaptic side.
• Common neurotransmitter: acetylcholine (especially neuromuscular junction).
• Brain regions: Forebrain (cerebrum, thalamus, hypothalamus). Midbrain (corpora quadrigemina). Hindbrain (pons, cerebellum, medulla).
• Cerebral lobes: Frontal (movement, planning), Parietal (touch, spatial), Temporal (hearing, memory), Occipital (vision).
• Thalamus: sensory relay. Hypothalamus: temperature, hunger, thirst, links to pituitary.
• Cerebellum: balance, coordination of voluntary movement.
• Medulla: breathing, heart rate, blood pressure. Damage = fatal.
• Sympathetic: fight or flight. Parasympathetic: rest and digest.
• Reflex arc: Receptor → Sensory neuron → CNS (spinal cord) → Motor neuron → Effector.
• Monosynaptic: knee jerk. Polysynaptic: withdrawal from hot object.
• Cranial nerves: 12 pairs. Spinal nerves: 31 pairs.

Next: use the interactive learning widgets to explore the labelled neuron, walk through an action potential, and tour every brain region, or work through the 24+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.