Physiology of the Brain, Spinal Cord, and Peripheral Nervous System Author: SB Thapa For NHPC Exam –PCL General Medicine

 Introduction

The physiology of the nervous system is the study of how nerve tissues and structures function to maintain coordination,consciousness, sensation, and homeostasis. Understanding these processes helps healthcare students diagnose neurological conditionsand interpret patient symptoms effectively.


Part 1: Physiology of the Brain

The brain functions as the control center of the entire body. It receives sensory input, integrates information,and initiates motor and autonomic responses. It is responsible for thought, emotion, behavior, and memory.



1. Cerebral Cortex Functions

The cerebral cortex performs higher brain functions. It is divided into sensory, motor, and association areas.- Sensory Areas: Interpret sensations (visual, auditory, tactile).  - Motor Areas: Control voluntary muscle movement.  - Association Areas: Involved in reasoning, judgment, and emotion.[Diagram Placeholder: Functional areas of cerebral cortex]



2. Functional Lobes of the Brain

- Frontal Lobe: Motor control, speech (Broca’s area), personality, planning.  - Parietal Lobe: Perception of touch, temperature, pressure, and spatial awareness.  - Temporal Lobe: Hearing, language comprehension (Wernicke’s area), and memory.  - Occipital Lobe: Vision processing center.



3. Thalamus and Hypothalamus Functions

- Thalamus: Relay center for sensory signals to the cortex; filters incoming information.  - Hypothalamus: Regulates hunger, thirst, body temperature, circadian rhythm, and autonomic activities.It links the nervous and endocrine systems via the pituitary gland.



4. Brainstem Physiology

The brainstem (midbrain, pons, and medulla oblongata) controls basic life functions:- Midbrain: Visual and auditory reflexes, muscle tone regulation.  - Pons: Relays signals between cerebrum and cerebellum; regulates breathing rhythm.  - Medulla Oblongata: Controls vital centers – respiration, heartbeat, vasomotor tone.Damage to medulla causes respiratory or cardiac failure.


5. Cerebellum Physiology

The cerebellum coordinates voluntary movement, maintains balance, and ensures posture.It compares intended movement with actual performance for smooth coordination.Lesions cause ataxia (incoordination) and tremors.[Diagram Placeholder: Cerebellar pathways]



6. Higher Brain Functions

These include consciousness, sleep, language, emotion, and memory.- Consciousness: Maintained by the reticular activating system (RAS).  - Sleep: Controlled by hypothalamic and brainstem centers; alternates between REM and non-REM sleep.  - Memory: Short-term (seconds to minutes) and long-term (stored in hippocampus and cortex).  - Emotion: Limbic system and hypothalamus integrate emotional responses.



7. Neurotransmitters of the Brain

Neurotransmitters are chemicals that transmit signals between neurons.| Neurotransmitter | Function | Clinical Relevance ||------------------|-----------|--------------------|| Acetylcholine | Learning, muscle activation | Alzheimer’s (deficiency) || Dopamine | Movement, pleasure | Parkinson’s (low), Schizophrenia (high) || Serotonin | Mood, sleep | Depression (low) || GABA | Inhibitory | Epilepsy (low) || Norepinephrine | Alertness, stress | Anxiety, hypertension |



Part 2: Physiology of the Spinal Cord

The spinal cord serves as a two-way communication highway between the brain and the body.It mediates reflexes and conducts sensory and motor information.



1. Reflex Arc

A reflex is an automatic, rapid, predictable response to a stimulus.Components of the reflex arc:1. Receptor (detects stimulus)  2. Sensory neuron (afferent pathway)  3. Integration center (spinal cord)  4. Motor neuron (efferent pathway)  5. Effector (muscle or gland)  [Diagram Placeholder: Simple reflex arc]



2. Ascending and Descending Pathways

- Ascending tracts carry sensory information to the brain (pain, temperature, touch).    Examples: Spinothalamic tract, dorsal column pathway.  - Descending tracts transmit motor commands from brain to muscles.    Example: Corticospinal tract controls voluntary movement.


3. Segmental Organization

Each spinal segment gives rise to a pair of spinal nerves controlling specific body regions (dermatomes and myotomes).Damage causes loss of function below the lesion level.



Summary of Spinal Cord Functions

- Conducts sensory and motor impulses.  - Integrates spinal reflexes.  - Provides communication between brain and periphery.



Part 3: Physiology of the Peripheral Nervous System (PNS)

The PNS links the CNS to organs and tissues. It includes the somatic (voluntary) and autonomic (involuntary) systems.



1. Nerve Impulse Conduction

Neurons transmit impulses via action potentials.Steps:1. Resting potential (–70 mV) maintained by sodium-potassium pump.  2. Depolarization (Na⁺ influx).  3. Repolarization (K⁺ efflux).  4. Hyperpolarization and return to resting state.Myelin sheath enables saltatory conduction, increasing speed.[Diagram Placeholder: Action potential curve]



2. Sensory and Motor Divisions

- Sensory Division: Carries impulses from receptors to CNS (touch, pain, temperature).  - Motor Division: Carries commands from CNS to muscles or glands.  



3. Autonomic Nervous System (ANS) Physiology

The ANS regulates involuntary functions like heart rate and digestion.It is divided into:- Sympathetic system: Activates during stress (fight or flight).  - Parasympathetic system: Active during rest (rest and digest).  Both maintain homeostasis through opposite effects.




4. Neuroeffector Transmission

At neuroeffector junctions, neurotransmitters are released to activate target tissues:- Acetylcholine at parasympathetic endings.  - Norepinephrine at sympathetic endings.  



Clinical Correlations

• Spinal shock: Temporary loss of reflexes after spinal injury.  • Peripheral neuropathy: Nerve damage causing weakness or numbness (e.g., diabetes).  • Autonomic dysfunction: Abnormal blood pressure regulation and sweating.  • Multiple sclerosis: Demyelination causing slow conduction and muscle weakness.  

Exam Focus and Key Points

✔ Structure and function of reflex arc.  ✔ Functions of thalamus, hypothalamus, cerebellum.  ✔ Mechanism of action potential and neurotransmitters.  ✔ Differences between sympathetic and parasympathetic functions.  ✔ Clinical conditions involving CNS and PNS disorders.



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