RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
A hand pulled back from a hot pan moves before the brain has consciously registered pain, and this chapter explains how that speed is possible. It builds the neuron as an excitable cell that generates and conducts electrical signals, follows those signals across the synaptic gap between one neuron and the next, and maps the organisation of the human nervous system from spinal reflex arcs up through the structures of the brain that generate thought, emotion, movement and balance.
The resting membrane potential of a neuron, with the inside negative relative to the outside, is chiefly maintained by:
Answer
The sodium-potassium pump and differential membrane permeability is correct — the pump actively expels three Na+ for every two K+ it imports, and the resting membrane is more permeable to K+ than Na+, together keeping the cell interior electrically negative relative to the outside at rest.
In a typical reflex action, such as withdrawing a hand from a hot object, the immediate response is generated primarily by processing in the:
Answer
Spinal cord is correct — a reflex arc routes the sensory signal into the spinal cord, where it is relayed directly to a motor neuron without first travelling up to the brain for conscious processing, allowing an extremely fast protective response.
The part of the hindbrain chiefly responsible for coordinating balance, posture and fine motor movement is the:
Answer
Cerebellum is correct — it continuously compares intended movement with actual movement using sensory feedback, fine-tuning muscle activity for balance, posture and smooth, coordinated motor actions, unlike the medulla, which regulates involuntary functions like breathing and heart rate.
At a typical chemical synapse, the neurotransmitter released from the presynaptic terminal crosses the synaptic cleft and binds to receptors on the:
Answer
Postsynaptic membrane, generating a new impulse is correct — neurotransmitter diffuses across the cleft and binds specific receptors on the postsynaptic membrane, opening ion channels there and generating a fresh electrical signal in the receiving neuron.
Assertion (A): Impulses travel faster along myelinated axons than along unmyelinated axons of similar diameter. Reason (R): In myelinated axons, depolarisation occurs only at the nodes of Ranvier, allowing the impulse to jump from node to node rather than travelling continuously along the entire membrane.
Answer
Both A and R are true and R is the correct explanation of A — the myelin sheath insulates most of the axon so ion channels are effectively concentrated at the nodes of Ranvier, and this saltatory conduction, jumping node to node, is considerably faster than the continuous, point-by-point depolarisation required along an unmyelinated axon.
Describe the sequence of ionic events that generate an action potential in a neuron once a stimulus reaches threshold.
Answer
When a stimulus depolarises the neuron's membrane to threshold, voltage-gated sodium channels open rapidly, and because Na+ is far more concentrated outside the cell, it rushes in down its electrochemical gradient, making the inside of the membrane transiently positive relative to the outside; this is the depolarisation phase of the action potential. Almost immediately afterward, the sodium channels close and voltage-gated potassium channels open, allowing K+, which is more concentrated inside the cell, to flow out; this outward flow restores the negative interior, a phase called repolarisation, and often briefly overshoots the original resting potential before the sodium-potassium pump and ion channels restore the normal resting state.
Describe the components and pathway of a simple reflex arc, using the example of withdrawing a hand from a sharp object.
Answer
A reflex arc is the specific neural pathway involved in a fast, involuntary response and consists of five components acting in sequence. A receptor, here pain receptors in the skin of the fingertip, first detects the sharp stimulus and generates a nerve impulse. This impulse travels along an afferent (sensory) neuron toward the spinal cord, entering through the dorsal root. Within the spinal cord, the impulse is typically relayed, often through one or more interneurons, directly to an efferent (motor) neuron without first travelling up to the brain, allowing an unusually rapid response. This efferent neuron carries the impulse out through the ventral root of the spinal cord to the effector, in this case the skeletal muscles of the arm, which contract to pull the hand away from the sharp object. Because the decision to respond is made at the level of the spinal cord rather than the brain, the whole sequence, from stimulus to withdrawal, happens within a fraction of a second, well before the person consciously registers pain.
Describe the structure of a neuron and explain how an action potential is generated and propagated along its axon.
Answer
A neuron, the structural and functional unit of the nervous system, consists of three main parts. The cell body, or cyton, contains the nucleus and granular structures called Nissl bodies, and is the metabolic centre of the cell. Extending from the cell body are dendrites, short, branched processes specialised to receive incoming signals from other neurons, and a single, often much longer axon, which conducts impulses away from the cell body toward another neuron or an effector organ. Many axons are wrapped in a fatty myelin sheath formed by Schwann cells, which is interrupted at regular intervals by gaps called nodes of Ranvier. At rest, active transport by the sodium-potassium pump, combined with the membrane's greater permeability to potassium, keeps the inside of the axon negatively charged relative to the outside, a state called the resting potential. When a stimulus depolarises the membrane at a given point past a critical threshold, voltage-gated sodium channels at that point open, and Na+ rushes into the axon down its concentration gradient, reversing the polarity so the inside briefly becomes positive; this is the action potential. This local reversal causes neighbouring sodium channels to open in turn, so the depolarisation spreads as a self-propagating wave along the membrane, while behind the advancing impulse, sodium channels close and voltage-gated potassium channels open, allowing K+ to flow out and repolarise the membrane back toward its resting state, briefly leaving that stretch of membrane refractory to a new impulse, which ensures the wave travels in only one direction. On myelinated axons, because ion channels are concentrated almost exclusively at the nodes of Ranvier, the impulse effectively jumps from node to node in a process called saltatory conduction, which is considerably faster than the continuous conduction seen along unmyelinated fibres.
Describe the major regions of the human brain and outline the principal functions associated with each.
Answer
The human brain is organised into three major divisions. The forebrain is the largest and most anterior division, dominated by the cerebrum, which is split into left and right cerebral hemispheres connected internally and responsible for higher functions including conscious thought, memory, reasoning, language, sensory perception and the initiation of voluntary movement; different regions of the cerebral cortex are specialised for different sensory and motor functions. Also within the forebrain, the thalamus acts as a major relay and integrating centre for sensory information travelling toward the cerebral cortex, while the hypothalamus, lying just below the thalamus, regulates body temperature, hunger, thirst and various emotional responses, and also links the nervous system to the endocrine system by controlling the pituitary gland. The midbrain forms a short connecting region between the forebrain and hindbrain and contains centres involved in visual and auditory reflexes, such as reflexively turning the head and eyes toward a sudden sound or moving light. The hindbrain consists of three parts: the cerebellum, positioned behind the brainstem, which continuously compares intended movements against actual body position using sensory feedback to fine-tune balance, posture and coordinated, precise motor activity; the pons, which contains fibre tracts connecting different parts of the brain and contributes to regulating breathing; and the medulla oblongata, the most posterior part, which continues into the spinal cord and controls essential involuntary functions necessary for survival, including heart rate, breathing rate, blood pressure and reflexes such as swallowing and vomiting. Together these regions allow the brain to manage everything from conscious decision-making to the automatic regulation of vital body functions.
A patient who suffered a severe blow to the back of the head begins to show difficulty maintaining balance and struggles to perform smooth, coordinated hand movements, though her memory, speech and reasoning remain unaffected. Doctors also note that her breathing and heart rate, controlled by a separate brain region entirely, remain normal throughout. (a) Identify the brain region most likely damaged by the blow, based on her specific symptoms. (b) Explain why her memory, speech and reasoning are unaffected despite the injury. (c) Name the brain region responsible for regulating her heart rate and breathing, and explain why damage there would be far more immediately dangerous. (d) Suggest one type of rehabilitation exercise that might help retrain her affected functions.
Answer
(a) Her symptoms of impaired balance and loss of smooth, coordinated movement point to damage in the cerebellum, the hindbrain structure specifically responsible for fine-tuning motor coordination and posture using continuous sensory feedback. (b) Memory, speech and reasoning are functions localised mainly in the cerebrum, particularly its cortex, which is anatomically and functionally distinct from the cerebellum; because the blow appears to have affected only the cerebellar region at the back of the head, these higher cognitive functions housed in the cerebrum remain intact. (c) Heart rate and breathing are regulated by the medulla oblongata in the hindbrain; because the medulla controls functions essential for immediate survival, any significant damage there, unlike cerebellar damage, can rapidly become life-threatening by disrupting the basic rhythms that keep the body alive. (d) Balance and coordination exercises, such as supervised walking on uneven surfaces, targeted hand-eye coordination tasks and physical therapy aimed at retraining fine motor control, can help the nervous system gradually compensate for cerebellar impairment.
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