Chapter 9 — Neural Control and Coordination
Key Concepts Quick Recap
| Term | Definition |
|---|
| Neuron | Structural and functional unit of nervous system |
| Dendrites | Short branched processes; receive impulses towards cell body |
| Axon | Long fibre; conducts impulse away from cell body |
| Myelin sheath | Insulating layer of Schwann cells; speeds up conduction (saltatory) |
| Nodes of Ranvier | Gaps in myelin sheath; site of saltatory impulse conduction |
| Synapse | Junction between two neurons; impulse transmitted by neurotransmitters |
| Resting membrane potential | -70 mV; axon polarised (K⁺ inside; Na⁺ outside) |
| Action potential | Depolarisation: Na⁺ rushes in; repolarisation: K⁺ rushes out |
| Reflex arc | Receptor → Afferent nerve → Spinal cord → Efferent nerve → Effector |
I. Choose the Best Answer (MCQs)
1. The myelin sheath is formed by: (a) Astrocytes (b) Schwann cells (c) Oligodendrocytes (d) Microglia
Answer(b) Schwann cells In the peripheral nervous system (PNS), myelin sheath is formed by Schwann cells. In the central nervous system (CNS), oligodendrocytes form myelin. Myelin is a lipid-protein layer that increases speed of nerve impulse transmission by allowing saltatory conduction (jumping from node to node).
2. The resting membrane potential of a neuron is approximately: (a) +70 mV (b) 0 mV (c) -70 mV (d) -120 mV
Answer(c) -70 mV At rest: Na⁺/K⁺ ATPase pump maintains high K⁺ inside and high Na⁺ outside. Membrane is more permeable to K⁺ → K⁺ leaks out → inside becomes negative. Result: resting potential = –70 mV (inside negative relative to outside).
3. Neurotransmitter at the neuromuscular junction (motor end plate) is: (a) Dopamine (b) Serotonin (c) Acetylcholine (d) GABA
Answer(c) Acetylcholine Acetylcholine (ACh) is the neurotransmitter released at the neuromuscular junction by motor neurons. It binds nicotinic receptors on muscle fibre → depolarisation → muscle contraction. Acetylcholinesterase destroys ACh rapidly (ending the signal). Curare blocks ACh receptors → paralysis. Organophosphate poisoning inhibits acetylcholinesterase → constant stimulation.
4. Which part of the brain maintains balance and coordinates muscular activities? (a) Cerebrum (b) Hypothalamus (c) Cerebellum (d) Pons
Answer(c) Cerebellum Cerebellum: "little brain" at the back; two hemispheres with folded cortex (folia). Functions: coordination of voluntary movements, maintenance of posture and balance, fine motor control. Damage → ataxia (loss of coordination), intention tremor.
5. The reflex centre for knee-jerk reflex is located in: (a) Cerebrum (b) Medulla oblongata (c) Cerebellum (d) Spinal cord
Answer(d) Spinal cord Knee-jerk (patellar) reflex is a monosynaptic spinal reflex — involves only one synapse (between sensory and motor neuron in spinal cord). The reflex arc does NOT go through the brain, making it extremely fast. This is why you can still blink, salivate, etc. even with severe brain injury.
6. The hypothalamus controls: (a) Voluntary movement only (b) Hunger, thirst, temperature, and hormone release (c) Only visual information processing (d) Only memory
Answer(b) Hunger, thirst, temperature, and hormone release Hypothalamus controls: body temperature (thermoregulation), hunger (satiety centre), thirst, circadian rhythms, emotional responses, and secretes releasing/inhibiting hormones to control the anterior pituitary. It is the master controller of the endocrine system via its link to the pituitary gland.
7. Which part of the brain connects the two cerebral hemispheres? (a) Thalamus (b) Corpus callosum (c) Pons (d) Cerebellum
Answer(b) Corpus callosum Corpus callosum is a large bundle of nerve fibres (white matter) that connects the left and right cerebral hemispheres, allowing them to communicate. "Split-brain" patients (corpus callosum cut) show fascinating dissociation between hemisphere functions.
II. Short Answer Questions
Q1. Describe the structure of a neuron.
Answer
A neuron is the structural and functional unit of the nervous system.
Parts of a neuron:
1. Cell body (Perikaryon/Soma):
Answer
- Contains nucleus, Nissl bodies (rough ER for protein synthesis), mitochondria.
- All metabolic activities of neuron occur here.
2. Dendrites:
Answer
- Short, branched extensions from cell body.
- Receive impulses from other neurons (afferent).
- Covered with dendritic spines for synaptic connections.
3. Axon (Nerve fibre):
Answer
- Single, long process from cell body (via axon hillock).
- Conducts impulses away from cell body (efferent).
- Covered by myelin sheath (Schwann cells in PNS).
- Nodes of Ranvier: gaps in myelin → saltatory conduction.
- Ends in axon terminals (synaptic boutons).
Types of neurons by function:
- Sensory (afferent): Carry impulse from receptors → CNS.
- Motor (efferent): Carry impulse from CNS → effectors.
- Interneurons (association): Connect sensory and motor neurons in CNS.
Q2. Explain how a nerve impulse is generated and transmitted across a synapse.
Answer
Generation of nerve impulse (Action potential):
Resting state (–70 mV):
- Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in.
- High K⁺ inside, high Na⁺ outside → outside positive.
Depolarisation:
- Stimulus → Na⁺ channels open → Na⁺ rush in → membrane potential rises to +35 mV.
Repolarisation:
- Na⁺ channels close; K⁺ channels open → K⁺ rush out → returns to –70 mV.
Refractory period:
- Brief period when neuron cannot generate another impulse (absolute refractory period).
Transmission across a synapse: 1. Action potential reaches axon terminal. 2. Ca²⁺ channels open → Ca²⁺ enters. 3. Synaptic vesicles fuse with presynaptic membrane → neurotransmitters released. 4. Neurotransmitters diffuse across synaptic cleft. 5. Bind to receptors on postsynaptic membrane. 6. Ion channels open → new action potential generated in postsynaptic neuron. 7. Neurotransmitter degraded (acetylcholinesterase) or reuptaken.
Common Mistakes
- Saying myelin is made by Schwann cells in CNS — Schwann cells are PNS only; oligodendrocytes are CNS.
- Confusing afferent (sensory, towards CNS) and efferent (motor, away from CNS).
- Saying action potential travels because Na⁺ is pumped out — Na⁺ RUSHES IN during depolarisation; K⁺ rushes OUT during repolarisation. The pump only maintains the resting state.
- Forgetting corpus callosum connects the two cerebral hemispheres.
Exam Checklist
- Neuron parts: cell body → dendrites (receive) + axon (send).
- Myelin: Schwann cells (PNS), oligodendrocytes (CNS); Nodes of Ranvier.
- Action potential: -70 mV → Na⁺ in → +35 mV → K⁺ out → -70 mV.
- Synapse: Ca²⁺ → vesicle release → neurotransmitter → postsynaptic receptor.
- Brain: Cerebrum (intelligence), Cerebellum (balance), Medulla (vital centres), Hypothalamus (homeostasis).
- Knee-jerk reflex: spinal cord; does NOT involve brain (monosynaptic).
