Human Neural System Overview
Coordination: Process where organs interact and complement each other to maintain homeostasis. Endocrine (chemical) and Neural (point-to-point electrical) systems jointly integrate body functions.
Central vs Peripheral Neural System
- Central Neural System (CNS): Brain and spinal cord; site of information processing
- Peripheral Neural System (PNS): All nerves associated with CNS
- Afferent fibres: Carry impulses from tissues/receptors to CNS 1992
- Efferent fibres: Carry impulses from CNS to peripheral tissues
PNS Divisions
| Division | Function | Key PYQs |
|---|---|---|
| Somatic Neural System | Relays impulses from CNS to voluntary skeletal muscles | 2005 |
| Autonomic Neural System (ANS) | Relays impulses from CNS to involuntary organs and smooth muscles | 2005 |
ANS Subdivisions
- Sympathetic: Accelerates heartbeat, inhibits digestion
- Parasympathetic: Conserves energy, stimulates digestion 1990, 1996, 1999
Historical PYQ Detail:
• Vagus nerve (Cranial Nerve X): Major parasympathetic nerve regulating heart rate and gastrointestinal movements 1989, 1992, 1997, 2004
• Abducens nerve (VI): Controls eyeball movement 2005
• Vagus nerve (Cranial Nerve X): Major parasympathetic nerve regulating heart rate and gastrointestinal movements 1989, 1992, 1997, 2004
• Abducens nerve (VI): Controls eyeball movement 2005
Neuron: Structural and Functional Unit
Neuron Anatomy
- Cell Body & Dendrites: Contain cytoplasm, typical organelles, and Nissl's granules (composed of free ribosomes and Rough Endoplasmic Reticulum) 1997, 2003, NEET 2018. Dendrites transmit impulses towards the cell body
- Axon: Long fibre transmitting impulses away from cell body to synapse/neuromuscular junction. Distal branches terminate as synaptic knobs, containing synaptic vesicles filled with neurotransmitters NEET 2013, 2024 Re
Disease Correlates:
• Parkinson's: Linked to dopamine deficiency 2005
• Alzheimer's: Linked to acetylcholine deficiency 2009
• Parkinson's: Linked to dopamine deficiency 2005
• Alzheimer's: Linked to acetylcholine deficiency 2009
Classification of Neurons
| Type | Structure | Location / Occurrence |
|---|---|---|
| Multipolar | One axon, two or more dendrites | Cerebral cortex 2022 Re, 2024 Re |
| Bipolar | One axon, one dendrite | Retina of eye 2022 Re, 2024 Re |
| Unipolar | Cell body with one axon only | Embryonic stage |
Classification of Nerve Fibres (Axons)
| Type | Characteristics | Location |
|---|---|---|
| Myelinated | Enveloped by Schwann cells and oligodendrocytes forming myelin sheath NEET 2017, OS 2017, 2024 Re. Gaps in sheath are nodes of Ranvier 2002 | Spinal and cranial nerves 2022 Re |
| Unmyelinated | Enclosed by Schwann cell but lacks myelin sheath | Autonomous and Somatic neural systems 2022 Re |
Generation and Conduction of Nerve Impulse
Resting State (Polarised Membrane)
- Axonal membrane is comparatively more permeable to K⁺ and nearly impermeable to Na⁺ and negatively charged axoplasmic proteins 2004, 2011
- Intracellular fluid (Axoplasm): High concentration of K⁺ NEET 2013 and negatively charged proteins; low Na⁺
- Extracellular fluid: High concentration of Na⁺; low K⁺
- Resting Potential: Outer surface is positively charged; inner surface is negatively charged 2007
- Mechanism: Gradients actively maintained by Sodium-Potassium Pump, transporting 3 Na⁺ outwards for 2 K⁺ inwards
Action Potential (Depolarisation and Conduction)
- Depolarisation: Stimulus makes membrane freely permeable to Na⁺. Rapid influx of Na⁺ 2000, 2008 reverses polarity (inner side becomes positive, outer becomes negative). This electrical potential difference is the action potential (nerve impulse)
- Conduction: Current flows on inner surface from depolarised site (A) to next site (B), and on outer surface from B to A to complete circuit
- Repolarisation: Na⁺ permeability is extremely short-lived. K⁺ rapidly diffuses outside, restoring resting potential
Charge Sequence during impulse:
Inner membrane charge goes from Negative → Positive → back to Negative 2007
Inner membrane charge goes from Negative → Positive → back to Negative 2007
Transmission of Impulses
Synapse Structure and Types
A synapse connects the axon of one neuron to the dendrite/soma of the next 1999
| Type | Characteristics | Key Features |
|---|---|---|
| Electrical Synapses | Pre- and post-synaptic membranes in extreme proximity | Direct current flow. Always faster than chemical synapses, but rare in human body 2022 |
| Chemical Synapses | Membranes separated by fluid-filled synaptic cleft | Use neurotransmitters 2022 |
Mechanism of Chemical Transmission
- Action potential arrives at axon terminal
- Synaptic vesicles move towards and fuse with pre-synaptic membrane
- Neurotransmitters released into synaptic cleft
- Transmitters bind to specific receptors located on post-synaptic membrane NEET 2013, NEET 2017
- Ion channels open, generating new potential (excitatory or inhibitory)
Central Neural System (Brain)
Meninges and Forebrain
- Cranial Meninges: Outer dura mater, middle thin arachnoid, inner pia mater (contacts brain tissue)
- Cerebrum: Divided into left and right cerebral hemispheres connected by nerve tract called corpus callosum NEET 2018, 2015, 2024
- Cerebral Cortex (Grey Matter): Highly folded outer layer containing neuron cell bodies. Contains motor areas, sensory areas, and association areas (responsible for complex functions like memory, communication, and intersensory associations)
- White Matter: Inner region consisting of myelinated tracts
Thalamus & Hypothalamus
- Thalamus: Wrapped by cerebrum; major coordinating centre for sensory and motor signaling
- Hypothalamus: Located at base of thalamus. Critically controls thermoregulation (body temperature) 2009, 2010, 2014, NEET 2019, 2024 and urges for eating and drinking. Contains neurosecretory cells
Limbic System (Lobe)
Inner cerebral hemispheres + amygdala + hippocampus. Along with hypothalamus, regulates sexual behaviour, expression of emotions (excitement, pleasure, rage, fear), and motivation NEET 2018, 2023
Midbrain and Hindbrain
Midbrain
- Located between thalamus/hypothalamus and pons
- Canal called cerebral aqueduct passes through it
- Dorsal portion consists of four round lobes called corpora quadrigemina
Hindbrain
Comprises Pons, Cerebellum, and Medulla
- Pons: Consists of fibre tracts that interconnect different regions of brain 2024. Exception/Crucial detail: Contains pneumotaxic centre which alters respiratory rate 2022 Re
- Cerebellum: Highly convoluted surface to provide additional space for neurons; regulates posture and balance 2024
- Medulla (Oblongata): Connects to spinal cord. Contains crucial centres controlling respiration, cardiovascular reflexes, and gastric secretions NEET 2018, 2024
Brain Stem: Forms connection between brain and spinal cord. Consists of Midbrain, Pons, and Medulla oblongata 2023, 2024