What this chapter is about
The human body performs countless activities simultaneously — walking, breathing, digesting food, maintaining balance, and responding to sudden dangers. All these actions require precise coordination, which is achieved through the nervous system. This chapter introduces students to how nerve cells generate and transmit electrical signals, how the brain and spinal cord process information, and how our sense organs detect changes in the environment.
A Class 11 student encounters this topic to understand the biological basis of behaviour, reflexes, and sensory perception. The nervous system works alongside the endocrine system (chemical coordination) to maintain homeostasis and enable rapid responses. Understanding neural mechanisms is essential for later studies in physiology, medicine, and neuroscience.
After studying this chapter, students should be able to describe the structure and function of neurons, explain how nerve impulses are generated and transmitted, outline the organisation of the central and peripheral nervous systems, and describe how the eye and ear function as sensory receptors.
Key ideas
- Neurons are the structural and functional units of the nervous system. Each neuron has a cell body (containing the nucleus), dendrites (receiving signals), and an axon (transmitting signals away from the cell body).
- The resting membrane potential of a neuron is about −70 mV. This is maintained by the sodium-potassium pump, which moves 3 Na⁺ ions out and 2 K⁺ ions into the cell, using ATP.
- An action potential is an all-or-none electrical signal. When a stimulus reaches threshold, voltage-gated Na⁺ channels open, causing depolarisation; then K⁺ channels open, leading to repolarisation.
- Synapses are junctions between neurons. At chemical synapses, neurotransmitters (such as acetylcholine) are released from synaptic vesicles, cross the synaptic cleft, and bind to receptors on the postsynaptic membrane.
- The central nervous system (CNS) consists of the brain and spinal cord. The brain has three main regions: forebrain (cerebrum, thalamus, hypothalamus), midbrain, and hindbrain (cerebellum, pons, medulla oblongata).
- The peripheral nervous system (PNS) includes cranial and spinal nerves. The autonomic nervous system, part of PNS, has sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.
- Reflex arcs provide rapid, automatic responses. A simple reflex arc involves a receptor, sensory neuron, integration centre in the spinal cord, motor neuron, and effector organ.
- The eye and ear are specialised sense organs. The retina contains photoreceptors (rods and cones), while the cochlea in the inner ear contains hair cells that detect sound vibrations.
Formulas and facts to remember
- Resting potential: approximately −70 mV; inside of neuron is negative relative to outside.
- Action potential sequence: resting state → depolarisation (Na⁺ influx) → repolarisation (K⁺ efflux) → hyperpolarisation → return to resting state.
- Saltatory conduction: in myelinated neurons, impulses jump between nodes of Ranvier, increasing conduction speed up to 100 m/s.
- Neurotransmitters: acetylcholine (at neuromuscular junctions), dopamine, serotonin, norepinephrine, GABA.
- 12 pairs of cranial nerves arise from the brain; 31 pairs of spinal nerves arise from the spinal cord.
- Photoreceptors: rods (dim light, no colour) and cones (bright light, colour vision — red, green, blue types).
- Accommodation: the ability of the eye to change lens curvature to focus on near or distant objects.
- Organ of Corti: located in the cochlea; contains hair cells that convert sound vibrations into nerve impulses.
Worked examples
### Example 1: Understanding the action potential
Problem: A neuron at rest has a membrane potential of −70 mV. After stimulation, the potential rises to +30 mV, then returns to −70 mV. What ion movements cause these changes?
Solution: 1. At rest, the membrane is polarised at −70 mV due to more K⁺ inside and more Na⁺ outside. 2. When stimulated above threshold, voltage-gated Na⁺ channels open rapidly. Na⁺ ions rush into the cell (down concentration gradient), making the inside positive — this is depolarisation to +30 mV. 3. Na⁺ channels close and K⁺ channels open. K⁺ ions move out of the cell, restoring the negative charge inside — this is repolarisation back toward −70 mV. 4. The sodium-potassium pump restores the original ion distribution for future impulses.
### Example 2: Tracing a reflex arc
Problem: When Ravi accidentally touches a hot pan, he withdraws his hand before consciously feeling pain. Trace the pathway of this withdrawal reflex.
Solution: 1. Receptor: Heat receptors (thermoreceptors) in the skin of Ravi's finger detect the high temperature. 2. Sensory neuron: Carries the impulse from receptors to the spinal cord via the dorsal root. 3. Integration centre: In the grey matter of the spinal cord, the sensory neuron synapses with an interneuron, which connects to a motor neuron. 4. Motor neuron: Carries the impulse from the spinal cord via the ventral root to the effector. 5. Effector: Muscles of the arm contract, pulling the hand away from the pan.
This reflex is rapid because it does not require brain processing; the spinal cord handles it directly.
### Example 3: Image formation in the eye
Problem: Explain how the eye adjusts to focus on a book held 25 cm away after looking at a distant tree.
Solution: 1. When viewing the distant tree, the ciliary muscles are relaxed, the suspensory ligaments are taut, and the lens is thin (less curved). Light from far objects is focused on the retina. 2. To focus on the nearby book, the ciliary muscles contract, reducing tension on the suspensory ligaments. 3. The elastic lens becomes thicker (more curved), increasing its refractive power. 4. This process is called accommodation. The increased curvature bends light rays from the near object more sharply, focusing the image precisely on the retina.
Common mistakes
- Thinking dendrites transmit impulses away from the cell body → dendrites receive signals; axons transmit impulses away.
- Confusing depolarisation with repolarisation → depolarisation is Na⁺ entry making inside positive; repolarisation is K⁺ exit restoring negative inside.
- Believing reflexes are controlled by the brain → simple reflexes are coordinated by the spinal cord; the brain is informed afterward.
- Mixing up rods and cones → rods work in dim light and give black-and-white vision; cones need bright light and detect colours.
- Assuming the sympathetic and parasympathetic systems always oppose each other exactly → they often have complementary but not simply opposite effects on different organs.
Quick revision
- Neurons have dendrites (receive), cell body (integrate), and axon (transmit).
- Resting potential is −70 mV; action potential involves Na⁺ influx (depolarisation) then K⁺ efflux (repolarisation).
- Synapses use neurotransmitters to pass signals between neurons.
- CNS = brain + spinal cord; PNS = cranial nerves (12 pairs) + spinal nerves (31 pairs).
- Reflex arcs enable rapid, involuntary responses without brain involvement.
- Eye: lens accommodation for focusing; retina has rods and cones. Ear: cochlea converts sound to nerve impulses.