Neural Control and Coordination
Welcome to HSLC Guru. This page provides complete English-medium notes, summary, and question-answer practice for ASSEB Class 11 Biology Chapter 21 — Neural Control and Coordination. Students preparing for the Assam State School Education Board (ASSEB) Class 11 final examination will find detailed explanations of the human nervous system, neuron structure, generation and conduction of nerve impulses, brain regions, reflex action, and sensory organs. The material follows the prescribed ASSEB syllabus and is suitable for board exam revision.
Summary
Multicellular animals require rapid coordination among different organs and tissues to respond to changes in their internal and external environment. This is achieved through two principal control systems — the nervous system and the endocrine system. The nervous system uses electrical and chemical signals (nerve impulses) and provides quick, point-to-point control, while the endocrine system uses hormones for slower but longer-lasting effects. In humans, the nervous system is broadly divided into two parts: the Central Nervous System (CNS), comprising the brain and the spinal cord, and the Peripheral Nervous System (PNS), comprising all the nerves arising from the CNS. The PNS is further classified into the somatic nervous system, which carries impulses from the CNS to the skeletal muscles for voluntary action, and the autonomic nervous system, which controls involuntary functions of internal organs. The autonomic system itself has two antagonistic divisions — the sympathetic nervous system, which prepares the body for emergency “fight or flight” responses, and the parasympathetic nervous system, which conserves energy and promotes “rest and digest” activities.
The structural and functional unit of the nervous system is the neuron or nerve cell. A typical neuron consists of a cell body (cyton or soma) containing the nucleus and Nissl’s granules, short branched processes called dendrites that receive impulses, and a long process called the axon that conducts the impulse away from the cell body. In many vertebrate neurons the axon is wrapped in a fatty insulating layer called the myelin sheath, formed by Schwann cells in the peripheral nervous system. The gaps between successive Schwann cells where the axon is exposed are called nodes of Ranvier; impulses jump from node to node in a fast process called saltatory conduction. Neurons may be classified by structure into multipolar, bipolar, and unipolar types, and by function into sensory (afferent), motor (efferent), and association (interneuron) neurons.
A nerve impulse is the electrical signal that travels along a neuron. In a resting neuron the axonal membrane is more permeable to potassium ions and almost impermeable to sodium ions; the sodium-potassium pump maintains a higher concentration of Na+ outside and K+ inside, so the inside of the membrane becomes negatively charged with respect to the outside. This electrical potential difference (about -70 mV) is called the resting membrane potential, and the membrane is said to be polarised. When a stimulus is applied above the threshold, sodium channels open and Na+ rushes inwards. The membrane briefly becomes positive inside (about +30 mV) — this reversal is called depolarisation, and the resulting wave is the action potential or nerve impulse. Soon afterwards K+ channels open and K+ flows out, restoring the negative interior; this recovery is called repolarisation. The pump then re-establishes the original ionic distribution.
Nerve impulses are transmitted from one neuron to the next across a junction called a synapse. There are two kinds of synapses — electrical synapses, where the membranes of pre- and post-synaptic neurons are very close and current flows directly, and chemical synapses, where the two neurons are separated by a gap (synaptic cleft) and signalling occurs through neurotransmitters such as acetylcholine. The human brain, the master coordinator, is protected by the cranium and three meninges and is divided into three parts. The forebrain includes the cerebrum (centre of voluntary action, intelligence, memory and speech), thalamus (relay centre) and hypothalamus (controls body temperature, hunger, thirst, and links to the limbic system that governs emotions). The midbrain connects the fore- and hindbrain and processes visual and auditory reflexes. The hindbrain includes the cerebellum (coordinates muscular movement and balance), pons (carries signals between the cerebrum and cerebellum) and medulla oblongata (controls heartbeat, breathing, and blood pressure). Sudden, automatic responses to stimuli are called reflex actions, mediated by a reflex arc passing through the spinal cord. Sensory perception is achieved through specialised receptor organs — the eye, with cornea, lens, iris and retina (containing rods and cones), and the ear, with outer, middle, and inner regions, where the cochlea perceives sound and the semicircular canals maintain balance.
Very Short Answer Questions (1 Mark)
Q1. What is a neuron?
Answer: A neuron is the structural and functional unit of the nervous system, specialised to receive, conduct, and transmit nerve impulses.
Q2. Name the two main divisions of the human nervous system.
Answer: The Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
Q3. What are nodes of Ranvier?
Answer: The unmyelinated gaps between consecutive Schwann cells along a myelinated axon are called nodes of Ranvier.
Q4. What is the resting membrane potential of a neuron?
Answer: About -70 mV; the inside of the axonal membrane is negatively charged relative to the outside.
Q5. Name a common neurotransmitter at chemical synapses.
Answer: Acetylcholine.
Q6. Which part of the brain controls balance and coordination of muscular movement?
Answer: The cerebellum.
Q7. What is the function of the medulla oblongata?
Answer: It controls involuntary activities such as heartbeat, breathing, blood pressure, and reflexes like coughing and vomiting.
Q8. Define a reflex action.
Answer: A reflex action is a sudden, involuntary, and automatic response of the body to a stimulus, controlled mainly by the spinal cord.
Q9. Name the photoreceptor cells of the retina.
Answer: Rods (for dim light vision) and cones (for colour and bright light vision).
Q10. Which division of the autonomic nervous system prepares the body for emergencies?
Answer: The sympathetic nervous system (“fight or flight” response).
Short Answer Questions (2-3 Marks)
Q1. Distinguish between the somatic and autonomic nervous systems.
Answer: The somatic nervous system carries motor impulses from the CNS to the skeletal (voluntary) muscles, allowing conscious control of movement. The autonomic nervous system carries impulses to involuntary structures such as smooth muscle, cardiac muscle, and glands; it is not under conscious control. The somatic system has only one motor neuron between the CNS and the muscle, while the autonomic system has two — preganglionic and postganglionic.
Q2. Differentiate between sympathetic and parasympathetic nervous systems with two examples.
Answer: The sympathetic system prepares the body for stress and emergency (“fight or flight”), while the parasympathetic system promotes “rest and digest” activities. For example, sympathetic stimulation increases heart rate and dilates pupils, whereas parasympathetic stimulation decreases heart rate and constricts pupils. The two systems are antagonistic and together maintain physiological balance.
Q3. Draw and label a typical neuron.
Answer: A typical multipolar neuron consists of:
- Cell body (cyton) — contains nucleus and Nissl’s granules.
- Dendrites — short branched processes that receive impulses.
- Axon — long single process conducting impulses away from the cell body.
- Myelin sheath — fatty insulating cover formed by Schwann cells.
- Nodes of Ranvier — unmyelinated gaps between Schwann cells.
- Axon terminals — branched ends ending in synaptic knobs.
Q4. What is meant by polarisation, depolarisation, and repolarisation of a neuron?
Answer: Polarisation is the resting state in which the inside of the axon membrane is electrically negative (-70 mV) compared to the outside, due to the unequal distribution of Na+ and K+ ions. Depolarisation is the rapid inflow of Na+ ions on stimulation, making the inside positive (about +30 mV). Repolarisation is the outflow of K+ ions that restores the original negative resting potential.
Q5. Differentiate between electrical and chemical synapses.
Answer: In an electrical synapse, the membranes of the pre- and post-synaptic neurons are in very close contact and the impulse passes directly as electric current; transmission is very fast. In a chemical synapse, the two neurons are separated by a synaptic cleft, and a neurotransmitter (e.g., acetylcholine) is released from the pre-synaptic vesicles, diffuses across the cleft, and binds to receptors on the post-synaptic membrane to generate a new impulse. Chemical synapses are slower but more common in the human nervous system.
Q6. What is a reflex arc? Give one example of a spinal reflex.
Answer: A reflex arc is the pathway through which a reflex impulse travels — receptor → sensory (afferent) neuron → spinal cord (interneuron) → motor (efferent) neuron → effector (muscle or gland). Example: withdrawal of the hand on touching a hot object (knee-jerk reflex is another).
Long Answer Questions (5-7 Marks)
Q1. Describe the structure of the human nervous system with a flow chart.
Answer: The human nervous system is the most highly developed coordinating system in the body and may be divided as follows:
- Central Nervous System (CNS): consists of the brain and spinal cord. It receives information, processes it, and issues instructions.
- Peripheral Nervous System (PNS): consists of nerves arising from the brain (12 pairs of cranial nerves) and spinal cord (31 pairs of spinal nerves).
- Somatic Nervous System (SNS): carries impulses to skeletal muscles; controls voluntary actions.
- Autonomic Nervous System (ANS): controls involuntary activities of internal organs.
- Sympathetic system — “fight or flight” responses.
- Parasympathetic system — “rest and digest” activities.
Together, these divisions ensure rapid sensing of stimuli, integration of information in the CNS, and appropriate responses by effectors throughout the body.
Q2. Explain the generation and conduction of a nerve impulse along an axon.
Answer: Generation of a nerve impulse occurs at the axonal membrane:
- Resting potential: In the resting condition the membrane is more permeable to K+ than to Na+. The Na+/K+ pump moves 3 Na+ out for every 2 K+ in, so the inside of the axon becomes about -70 mV with respect to the outside. The membrane is said to be polarised.
- Stimulation and depolarisation: A stimulus above threshold opens voltage-gated Na+ channels. Na+ rushes into the axon, and the inside becomes positive (about +30 mV). This rapid reversal is the action potential.
- Repolarisation: Na+ channels close and K+ channels open. K+ flows out of the axon, returning the inside to its negative resting state.
- Restoration: The Na+/K+ pump restores the original ionic balance.
- Conduction: The action potential moves along the axon as adjacent regions are depolarised. In myelinated axons the impulse jumps from one node of Ranvier to the next (saltatory conduction), making conduction much faster than in non-myelinated fibres.
Q3. Describe how a nerve impulse is transmitted across a chemical synapse.
Answer: Transmission of a nerve impulse across a chemical synapse takes place in the following steps:
- The action potential reaches the synaptic knob of the pre-synaptic neuron.
- Voltage-gated Ca2+ channels open and Ca2+ enters the knob.
- Ca2+ causes synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitter (e.g., acetylcholine) into the synaptic cleft by exocytosis.
- The neurotransmitter diffuses across the cleft and binds to specific receptors on the post-synaptic membrane.
- This opens ion channels, causing depolarisation of the post-synaptic membrane and generating a new action potential in the next neuron.
- The neurotransmitter is then rapidly broken down (e.g., by acetylcholinesterase) so that the synapse is ready for the next impulse.
Thus, transmission across a chemical synapse is unidirectional and chemical in nature.
Q4. Describe the structure and functions of the human brain.
Answer: The human brain weighs about 1.3-1.4 kg and lies within the cranial cavity, protected by three meninges (dura mater, arachnoid mater, and pia mater) with cerebrospinal fluid in between. It is divided into three main parts:
- Forebrain (Prosencephalon):
- Cerebrum — largest part; divided into two hemispheres connected by the corpus callosum. It controls voluntary actions, intelligence, memory, learning, speech, and consciousness.
- Thalamus — major coordinating centre for sensory and motor signalling.
- Hypothalamus — regulates body temperature, hunger, thirst, sleep, and emotional behaviour. Contains centres of the limbic system, which governs emotions and motivation.
- Midbrain (Mesencephalon): Located between thalamus and pons; contains corpora quadrigemina that control visual and auditory reflexes.
- Hindbrain (Rhombencephalon):
- Cerebellum — coordinates body movement and posture; maintains balance.
- Pons — relays signals between cerebrum and cerebellum and contains pneumotaxic centres that regulate breathing.
- Medulla oblongata — connects to the spinal cord; controls heartbeat, breathing, blood pressure, salivation, and reflexes such as vomiting.
Together these regions form the master control centre of the body.
Q5. Briefly describe the structure of the human eye and explain how it perceives images.
Answer: The human eye is a roughly spherical organ housed in the orbit of the skull. Its wall has three layers — the outer fibrous layer (sclera and transparent cornea), the middle vascular layer (choroid, ciliary body, and iris with the central pupil), and the inner nervous layer (retina). The retina contains photoreceptors — rods (sensitive to dim light, contain rhodopsin) and cones (sensitive to bright light and colour). The lens, held by suspensory ligaments, divides the eye into the aqueous chamber (in front) and vitreous chamber (behind).
Light entering the eye passes through the cornea, aqueous humour, pupil, lens, and vitreous humour, and is focused on the retina. Photochemical changes in rhodopsin and other photopigments generate impulses that are conducted by the optic nerve to the visual cortex of the cerebrum, where the image is interpreted. The ear, similarly, has outer, middle, and inner regions; the cochlea of the inner ear converts sound vibrations into nerve impulses sent to the brain through the auditory nerve, while the semicircular canals maintain dynamic balance.
Multiple Choice Questions (MCQ)
Q1. The structural and functional unit of the nervous system is:
(a) Nephron (b) Neuron (c) Axon (d) Dendron
Answer: (b) Neuron
Q2. Resting membrane potential of a neuron is approximately:
(a) +30 mV (b) -30 mV (c) -70 mV (d) +70 mV
Answer: (c) -70 mV
Q3. The myelin sheath in peripheral nerves is formed by:
(a) Astrocytes (b) Schwann cells (c) Oligodendrocytes (d) Microglia
Answer: (b) Schwann cells
Q4. Saltatory conduction occurs at:
(a) Cell body (b) Synapse (c) Nodes of Ranvier (d) Dendrites
Answer: (c) Nodes of Ranvier
Q5. Which of the following is a neurotransmitter?
(a) Insulin (b) Acetylcholine (c) Haemoglobin (d) Pepsin
Answer: (b) Acetylcholine
Q6. The largest part of the human brain is the:
(a) Cerebellum (b) Medulla (c) Cerebrum (d) Pons
Answer: (c) Cerebrum
Q7. Which part controls heartbeat and breathing?
(a) Cerebrum (b) Medulla oblongata (c) Cerebellum (d) Hypothalamus
Answer: (b) Medulla oblongata
Q8. The “fight or flight” response is governed by the:
(a) Parasympathetic system (b) Somatic system (c) Sympathetic system (d) Sensory system
Answer: (c) Sympathetic system
Q9. Photoreceptor cells responsible for colour vision are:
(a) Rods (b) Cones (c) Bipolar cells (d) Ganglion cells
Answer: (b) Cones
Q10. The reflex arc is the functional unit of:
(a) Reflex action (b) Voluntary action (c) Endocrine system (d) Hearing
Answer: (a) Reflex action
Fill in the Blanks
Q1. The fatty insulating cover around the axon is called the __________.
Answer: myelin sheath
Q2. The reversal of membrane potential during a nerve impulse is called __________.
Answer: depolarisation
Q3. The junction between two neurons is known as a __________.
Answer: synapse
Q4. The __________ controls body temperature, hunger, and thirst.
Answer: hypothalamus
Q5. The cochlea of the inner ear is responsible for __________.
Answer: hearing
True or False
Q1. The cerebellum is part of the forebrain.
Answer: False (it is part of the hindbrain).
Q2. Acetylcholine is a neurotransmitter released at chemical synapses.
Answer: True.
Q3. Saltatory conduction is faster than continuous conduction.
Answer: True.
Q4. The sympathetic and parasympathetic systems have similar effects on the body.
Answer: False (they are antagonistic).
Q5. Rods of the retina contain the pigment rhodopsin and help in dim light vision.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Neuron | Structural and functional unit of the nervous system. |
| Dendrite | Short branched process of a neuron that receives impulses. |
| Axon | Long process of a neuron that conducts impulses away from the cell body. |
| Myelin sheath | Fatty insulating layer around axons formed by Schwann cells. |
| Node of Ranvier | Unmyelinated gap between Schwann cells along an axon. |
| Resting potential | Electrical potential (~ -70 mV) across a resting neuronal membrane. |
| Action potential | Rapid reversal of membrane potential during a nerve impulse. |
| Depolarisation | Inflow of Na+ that makes the inside of the membrane positive. |
| Repolarisation | Outflow of K+ that restores the resting potential. |
| Synapse | Functional junction between two neurons or between a neuron and an effector. |
| Neurotransmitter | Chemical messenger (e.g., acetylcholine) released at a chemical synapse. |
| CNS | Central Nervous System; brain and spinal cord. |
| PNS | Peripheral Nervous System; cranial and spinal nerves. |
| Somatic NS | Division of PNS controlling voluntary skeletal muscles. |
| Autonomic NS | Division of PNS controlling involuntary internal organs. |
| Sympathetic NS | Autonomic division responsible for “fight or flight” responses. |
| Parasympathetic NS | Autonomic division responsible for “rest and digest” activities. |
| Cerebrum | Largest part of the brain; centre of intelligence and voluntary action. |
| Cerebellum | Hindbrain region controlling balance and muscle coordination. |
| Medulla oblongata | Lowest part of the brain; controls heartbeat and breathing. |
| Thalamus | Relay station for sensory and motor signals to the cerebrum. |
| Hypothalamus | Region controlling temperature, hunger, thirst, and emotions. |
| Limbic system | Group of structures regulating emotions and memory. |
| Reflex action | Sudden involuntary response to a stimulus. |
| Reflex arc | Pathway of a reflex through receptor, neurons, and effector. |
| Retina | Inner light-sensitive layer of the eye containing rods and cones. |
| Cochlea | Spiral organ of the inner ear that detects sound vibrations. |