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Short Notes Neural Control and Coordination - Short Notes for NEET
Neuron - Structural and Functional Unit
Structure of Neuron
| Part | Description |
|---|---|
| Cell Body (Soma/Cyton) | • Contains nucleus and cell organelles • Nissl's granules: Rough ER (protein synthesis) • Highly metabolically active |
| Dendrites | • Short, branched projections • Receive signals from other neurons • Conduct impulses toward cell body • Increase surface area for receiving signals |
| Axon | • Long, single projection • Conducts impulses away from cell body • Uniform diameter • May be myelinated or unmyelinated • Ends in axon terminals/synaptic knobs |
| Myelin Sheath | • White, fatty insulating layer • Formed by Schwann cells (PNS) or Oligodendrocytes (CNS) • Speeds up nerve conduction • Gaps: Nodes of Ranvier (saltatory conduction) |
| Axon Terminals | • Bulb-like structures at axon end • Contain synaptic vesicles with neurotransmitters • Form synapses with other neurons/effectors |
Types of Neurons
Based on Function
| Type | Function | Direction |
|---|---|---|
| Sensory (Afferent) | Transmit impulses from receptors to CNS | Receptors → CNS |
| Motor (Efferent) | Transmit impulses from CNS to effectors | CNS → Muscles/Glands |
| Interneurons (Association) | Connect sensory and motor neurons; found in CNS | Within CNS |
Based on Structure
- Multipolar: Many dendrites, one axon (most common in CNS - motor neurons)
- Bipolar: One dendrite, one axon (retina, olfactory epithelium)
- Unipolar (Pseudounipolar): Single process divides into two branches (sensory neurons)
Based on Myelination
- Myelinated neurons: Axon covered with myelin sheath; faster conduction (up to 120 m/s)
- Unmyelinated neurons: No myelin sheath; slower conduction (0.5-2 m/s)
Nerves
- Definition: Bundle of axons enclosed in connective tissue sheath
- Types:
- Sensory (Afferent) nerves: Only sensory axons
- Motor (Efferent) nerves: Only motor axons
- Mixed nerves: Both sensory and motor axons (most common)
Human Nervous System - Organization
Overview
- Central Nervous System (CNS): Brain + Spinal Cord
- Peripheral Nervous System (PNS): Cranial nerves (12 pairs) + Spinal nerves (31 pairs)
- Visceral Nervous System/Autonomic Nervous System (ANS): Sympathetic + Parasympathetic
Central Nervous System (CNS)
Brain
- Protection:
- Skull (cranium): Bony protection
- Meninges:Three membranes
- Dura mater: Outer, tough
- Arachnoid: Middle, web-like
- Pia mater: Inner, vascular, adheres to brain
- Cerebrospinal Fluid (CSF): Between arachnoid and pia mater, cushions brain
- Weight: ~1.4 kg in adult
Parts of Brain
| Part | Components | Functions |
|---|---|---|
| Forebrain | Cerebrum: • Largest part • Two cerebral hemispheres connected by corpus callosum • Outer cerebral cortex (grey matter - cell bodies) • Inner white matter (myelinated axons) • Divided into lobes: Frontal, Parietal, Temporal, Occipital Diencephalon: • Thalamus: Relay center for sensory info • Hypothalamus: Controls body temp, hunger, thirst, sleep; produces hormones; ANS control | Cerebrum: • Frontal lobe: Motor control, speech, reasoning, personality • Parietal lobe: Sensory perception, spatial awareness • Temporal lobe: Hearing, memory, emotion • Occipital lobe: Vision • Memory, learning, consciousness Hypothalamus: • Homeostasis • Links nervous and endocrine systems |
| Midbrain | • Small region • Corpora quadrigemina: 4 rounded swellings (vision and hearing reflexes) • Cerebral peduncles: Tracts connecting forebrain and hindbrain | • Controls eye movements • Relays auditory and visual signals • Maintains posture |
| Hindbrain | Pons: • Bridge connecting parts of brain Cerebellum: • Second largest part • Highly folded • Two hemispheres Medulla Oblongata: • Connects brain to spinal cord • Contains vital centers | Pons: • Regulation of respiration Cerebellum: • Coordination of voluntary movements • Balance and posture • Motor learning Medulla: • Respiratory center • Cardiovascular center (heart rate, BP) • Reflex centers (vomiting, coughing, sneezing, swallowing) |
Spinal Cord
- Location: Extends from medulla through vertebral column to lumbar region (L1-L2)
- Length: ~45 cm in adults
- Protection: Vertebral column, meninges, CSF
- Structure:
- Grey matter:Inner, H-shaped; contains cell bodies and dendrites
- Dorsal horn: Sensory neurons
- Ventral horn: Motor neurons
- White matter: Outer; myelinated axons forming tracts
- Grey matter:Inner, H-shaped; contains cell bodies and dendrites
- Functions:
- Conduction pathway between brain and PNS
- Reflex center (spinal reflexes)
Peripheral Nervous System (PNS)
Cranial Nerves (12 Pairs)
| Number | Name | Type | Main Function |
|---|---|---|---|
| I | Olfactory | Sensory | Smell |
| II | Optic | Sensory | Vision |
| III | Oculomotor | Motor | Eye movement, pupil constriction |
| IV | Trochlear | Motor | Eye movement |
| V | Trigeminal | Mixed | Facial sensation, chewing |
| VI | Abducens | Motor | Eye movement |
| VII | Facial | Mixed | Facial expression, taste (anterior 2/3 tongue) |
| VIII | Vestibulocochlear | Sensory | Hearing and balance |
| IX | Glossopharyngeal | Mixed | Swallowing, taste (posterior 1/3 tongue) |
| X | Vagus | Mixed | Parasympathetic to thoracic/abdominal organs; swallowing, speech |
| XI | Accessory | Motor | Head and shoulder movement |
| XII | Hypoglossal | Motor | Tongue movement |
Spinal Nerves (31 Pairs)
- All are mixed nerves
- Distribution:
- Cervical: 8 pairs (C1-C8)
- Thoracic: 12 pairs (T1-T12)
- Lumbar: 5 pairs (L1-L5)
- Sacral: 5 pairs (S1-S5)
- Coccygeal: 1 pair
- Formation:
- Dorsal root: Contains sensory fibers; has dorsal root ganglion (cell bodies of sensory neurons)
- Ventral root: Contains motor fibers
- Roots unite to form spinal nerve
Autonomic Nervous System (ANS) / Visceral Nervous System
- Definition: Part of PNS that controls involuntary functions of internal organs
- Not under conscious control
- Regulates: Heart rate, digestion, respiration rate, pupil diameter, urination
Divisions of ANS
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Origin | Thoracolumbar (T1-L2) | Craniosacral (Cranial nerves + S2-S4) |
| Ganglia Location | Close to spinal cord (paravertebral) | Near or within target organs |
| Preganglionic Fiber | Short | Long |
| Postganglionic Fiber | Long | Short |
| Neurotransmitter (Preganglionic) | Acetylcholine (ACh) | Acetylcholine (ACh) |
| Neurotransmitter (Postganglionic) | Norepinephrine (noradrenaline) | Acetylcholine (ACh) |
| Function | "Fight or Flight" • Increases heart rate • Dilates pupils • Dilates bronchi • Increases BP • Inhibits digestion • Stimulates glucose release • Diverts blood to muscles | "Rest and Digest" • Decreases heart rate • Constricts pupils • Constricts bronchi • Decreases BP • Stimulates digestion • Stimulates salivation • Promotes urination/defecation |
| Active During | Stress, emergency, exercise | Relaxation, normal conditions |
Key Points
- Sympathetic and parasympathetic systems work antagonistically on most organs
- Dual innervation: Most organs receive both sympathetic and parasympathetic innervation
- Exceptions: Sweat glands, adrenal medulla, most blood vessels - only sympathetic
Generation and Conduction of Nerve Impulse
Resting Membrane Potential
- Definition: Electrical potential across neuron membrane when not conducting impulse
- Value: -70 mV (inside negative relative to outside)
- Cause:
- Na+-K+ pump: Active transport - pumps 3 Na+ out, 2 K+ in (uses ATP)
- Differential permeability: Membrane more permeable to K+ than Na+
- Result: More positive charges outside, more negative inside
- Negatively charged proteins inside cell
- State: Neuron is polarized
Action Potential - Nerve Impulse
Phases of Action Potential
| Phase | Event | Membrane Potential |
|---|---|---|
| 1. Resting State | • Polarized • Na+ and K+ channels closed | -70 mV |
| 2. Stimulus | • Adequate stimulus applied • Reaches threshold (-55 mV) | -55 mV (threshold) |
| 3. Depolarization | • Voltage-gated Na+ channels open • Rapid influx of Na+ • Inside becomes positive • "All-or-none" principle - once threshold reached, full depolarization occurs | +30 to +40 mV (peak) |
| 4. Repolarization | • Na+ channels close • Voltage-gated K+ channels open • K+ flows out • Membrane potential returns to negative | Returns toward -70 mV |
| 5. Hyperpolarization | • K+ channels slow to close • Membrane becomes more negative than resting • Undershoot | -80 to -90 mV |
| 6. Return to Resting | • K+ channels close • Na+-K+ pump restores ionic gradients | -70 mV |
Propagation of Nerve Impulse
Unmyelinated Axons - Continuous Conduction
- Action potential propagates as wave of depolarization
- Each segment of membrane depolarizes adjacent segment
- Slow conduction (0.5-2 m/s)
Myelinated Axons - Saltatory Conduction
- Myelin sheath insulates axon - prevents ion flow
- Nodes of Ranvier: Gaps in myelin, high concentration of voltage-gated channels
- Action potential "jumps" from node to node (saltatory = jumping)
- Fast conduction (up to 120 m/s)
- Energy efficient (fewer Na+-K+ pumps needed)
Synapse - Junction Between Neurons
Types of Synapses
- Electrical synapse:
- Direct connection via gap junctions
- Ions flow directly between cells
- Bidirectional, fast, no delay
- Rare in mammals (cardiac muscle, smooth muscle)
- Chemical synapse:(Most common)
- Small gap (synaptic cleft, ~20-40 nm)
- Uses neurotransmitters
- Unidirectional
- Slight delay (~0.5 ms)
Structure of Chemical Synapse
- Presynaptic neuron:
- Axon terminal (synaptic knob)
- Contains synaptic vesicles with neurotransmitters
- Voltage-gated Ca²⁺ channels
- Synaptic cleft: Space between neurons
- Postsynaptic neuron:
- Membrane with neurotransmitter receptors
Transmission Across Chemical Synapse
| Step | Process |
|---|---|
| 1 | Action potential arrives at axon terminal (presynaptic) |
| 2 | Depolarization opens voltage-gated Ca²⁺ channels |
| 3 | Ca²⁺ influx into presynaptic terminal |
| 4 | Ca²⁺ triggers fusion of synaptic vesicles with presynaptic membrane |
| 5 | Exocytosis - neurotransmitter released into synaptic cleft |
| 6 | Neurotransmitter diffuses across cleft, binds to receptors on postsynaptic membrane |
| 7 | Opens ligand-gated ion channels on postsynaptic membrane |
| 8 | Excitatory: Na+ enters → depolarization → EPSP (Excitatory Post-Synaptic Potential) Inhibitory: Cl- enters or K+ exits → hyperpolarization → IPSP (Inhibitory Post-Synaptic Potential) |
| 9 | If EPSP reaches threshold → action potential in postsynaptic neuron |
| 10 | Neurotransmitter removed: enzymatic degradation or reuptake into presynaptic terminal |
Important Neurotransmitters
| Neurotransmitter | Location/Function | Effect |
|---|---|---|
| Acetylcholine (ACh) | • Neuromuscular junction • Parasympathetic NS • CNS (memory, learning) | Usually excitatory (muscle contraction) Inhibitory in heart |
| Norepinephrine | • Sympathetic NS • CNS (alertness, arousal) | Excitatory |
| Dopamine | • CNS (motor control, reward) • Deficiency → Parkinson's disease | Inhibitory/modulatory |
| Serotonin | • CNS (mood, sleep, appetite) • Low levels → depression | Inhibitory |
| GABA (Gamma-Aminobutyric Acid) | • Major inhibitory neurotransmitter in CNS | Inhibitory |
| Glutamate | • Major excitatory neurotransmitter in CNS | Excitatory |
Reflex Arc and Reflex Action
- Reflex: Rapid, automatic, involuntary response to stimulus
- Reflex arc components:
- Receptor: Detects stimulus
- Sensory neuron: Carries impulse to CNS
- Integration center: Spinal cord or brain; contains interneurons
- Motor neuron: Carries impulse from CNS to effector
- Effector: Muscle or gland that responds
- Examples:
- Knee-jerk reflex: Monosynaptic (no interneuron)
- Withdrawal reflex: Polysynaptic (interneurons present)
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FAQs on Short Notes Neural Control and Coordination - Short Notes for NEET
| 1. What are the main components of the human nervous system? |  |
Ans. The human nervous system is primarily composed of two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, which process and integrate information. The PNS includes all the nerves that branch out from the CNS, connecting it to the rest of the body, facilitating communication between the brain, spinal cord, and various organs and muscles.
| 2. How does the synapse function in neural communication? | |
Ans. A synapse is the junction between two neurons, where communication occurs. When an electrical impulse reaches the end of a neuron (the presynaptic terminal), it triggers the release of neurotransmitters from vesicles into the synaptic cleft. These neurotransmitters then bind to receptors on the adjacent neuron (the postsynaptic terminal), leading to the generation of a new electrical impulse in that neuron, thereby facilitating the transfer of signals across the nervous system.
| 3. What is the role of neurotransmitters in the nervous system? | |
Ans. Neurotransmitters are chemical messengers that transmit signals across synapses between neurons. They play a crucial role in various functions, including mood regulation, muscle contraction, and the modulation of pain. Different neurotransmitters, such as dopamine, serotonin, and acetylcholine, have specific functions and effects on target cells, influencing psychological and physiological processes within the body.
| 4. What is the significance of the reflex arc in neural control? | |
Ans. The reflex arc is a neural pathway that mediates a reflex action, providing a quick and automatic response to a stimulus. It typically involves a sensory neuron that detects the stimulus, an interneuron that processes the information, and a motor neuron that triggers the response in a muscle or gland. This mechanism allows for rapid reactions to potentially harmful stimuli, thereby playing a critical role in protecting the body from injury.
| 5. How does the endocrine system complement neural control? | |
Ans. The endocrine system complements neural control by regulating long-term processes through hormones. While the nervous system provides fast, short-term responses to stimuli via electrical impulses, the endocrine system releases hormones into the bloodstream to influence various bodily functions over a longer duration. This dual system ensures that the body can respond swiftly to immediate threats while maintaining homeostasis and regulating growth, metabolism, and reproductive processes over time.
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