Overview of the nervous system

Major Divisions of the Nervous System

The nervous system is divided into two primary components: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS)

The CNS consists of the brain (cerebrum, cerebellum, and brainstem) and spinal cord. It governs emotional responses, sensory perception, reasoning, voluntary and involuntary movement, coordination, learning, memory, and the regulation of homeostasis.

Brain Regions

Region	Components	Functions
Hindbrain	Medulla, pons, cerebellum	Vital reflexes, heart rate, breathing, balance, motor coordination
Midbrain	Tectum, tegmentum	Sensory processing, motivation, movement control
Forebrain	Cerebral hemispheres, limbic system, basal ganglia	Higher cognition, emotion, memory, voluntary movement

Protection of the CNS

Structure	Function
Cerebrospinal Fluid (CSF)	Cushions brain, provides buoyancy, transports nutrients and waste
Meninges	Protective membranes surrounding brain and spinal cord
Blood-Brain Barrier (BBB)	Selective endothelial barrier preventing toxins from entering CNS; allows small lipophilic molecules (O₂, CO₂) and transported nutrients (glucose)

Peripheral Nervous System (PNS)

The PNS connects the CNS with the rest of the body and is divided into two main branches.

Somatic Nervous System

Controls voluntary movements and skeletal muscle activity. Sensory information enters the CNS through dorsal roots, whereas motor commands exit through ventral roots. Degeneration of motor pathways such as the ventral roots contributes to disorders like ALS (motor neuron disease).

Autonomic Nervous System (ANS)

Regulates involuntary physiological functions including heart rate, digestion, and breathing. It contains two opposing divisions:

Division	Function	Physiological Effects
Sympathetic	“Fight or flight” activation	↑ heart rate, ↑ respiration, glucose release, ↓ digestion
Parasympathetic	“Rest and digest” restoration	↓ heart rate, ↑ digestion, energy conservation

Hormonal Communication and the Hypothalamus

The hypothalamus is a central regulator of homeostasis and communicates with the endocrine system via the pituitary gland.

Hypothalamus–Pituitary Interaction

• The hypothalamus regulates hormone release.
• The pituitary secretes hormones into the bloodstream, producing slower but longer-lasting effects than neural signals.

Major Pituitary Hormones

Hormone	Target	Function
GH	Body tissues	Growth
ACTH	Adrenal cortex	Stress response (cortisol release)
TSH	Thyroid	Regulates metabolism
FSH & LH	Ovaries/testes	Reproduction
Prolactin	Mammary glands	Milk production
Oxytocin	Uterus, brain	Bonding, childbirth
Vasopressin (ADH)	Kidneys	Water balance

Hypothalamus and Homeostasis

Regulates hunger, thirst, temperature, reproduction, stress, circadian rhythms, and the HPA axis, which triggers cortisol release during stress.

Major Systems of the Brain

Cerebral Cortex

The cortex is divided into four lobes:

Lobe	Function
Frontal	Planning, decision-making, movement, working memory, inhibition, attention
Parietal	Sensory integration, touch, spatial processing, proprioception
Temporal	Hearing, language comprehension, memory, emotion
Occipital	Vision

Language Areas

• Broca’s area (frontal): speech production; damage → Broca’s aphasia (non-fluent, effortful speech).
• Wernicke’s area (temporal): speech comprehension; damage → Wernicke’s aphasia (fluent but nonsensical speech).

Limbic System

The limbic system processes emotion, motivation, and memory formation.

Structure	Function
Amygdala	Fear, threat detection, emotional learning
Hippocampus	Formation of episodic memories; spatial, temporal, and contextual encoding
Cingulate gyrus	Emotional awareness, pain processing, empathy

Basal Ganglia

Responsible for movement control, motivation, and habit formation. Works through direct (movement initiation) and indirect (movement inhibition) pathways.

Key Structures

Structure	Function
Caudate nucleus & Putamen (Striatum)	Receive dopaminergic input; essential in movement and habit learning
Nucleus accumbens	Reward, reinforcement, addiction
Globus pallidus	Regulates voluntary movement

Disruption in these circuits can cause Parkinson’s disease, Huntington’s disease, and other motor impairments.

Midbrain Dopaminergic Pathways

Origin	Projection	Function
Ventral tegmental area (VTA)	Nucleus accumbens & frontal cortex	Reward, motivation, reinforcement learning, addiction
Substantia nigra	Striatum	Voluntary movement; degeneration causes Parkinson’s

Neurons and Neural Communication

Neuron Structure

Component	Role
Dendrites	Receive incoming signals
Soma (Cell body)	Integrates information
Axon	Conducts action potentials
Terminal boutons	Release neurotransmitters

Glial cells support neurons; oligodendrocytes form myelin in the CNS.

Resting Membrane Potential

Neurons maintain a –70 mV internal environment due to differential ion distribution.

Ion Distribution

Ion	Inside	Outside
Na⁺	Low	High
K⁺	High	Low
Cl⁻	Low	High

The Na⁺/K⁺ pump maintains this gradient, exporting 3 Na⁺ ions and importing 2 K⁺ ions using ATP.

Action Potentials

An action potential is generated when the axon hillock reaches threshold (~–50 mV).

Phases of an Action Potential

Phase	Description
Depolarisation	Na⁺ channels open; sodium rushes in → membrane becomes positive
Repolarisation	Na⁺ channels close; K⁺ channels open → potassium exits
Hyperpolarisation	Excess K⁺ leaves → membrane becomes more negative than resting

Refractory Periods

Type	Properties
Absolute	No firing possible; channels inactive
Relative	Firing possible but requires stronger stimulus

Conduction Speed

Myelin sheaths enable saltatory conduction, where signals jump between Nodes of Ranvier. Loss of myelin (e.g., MS) slows or blocks neural transmission.

Synaptic Transmission

When an action potential reaches the terminal, neurotransmitters are released into the synaptic cleft and bind to receptors on the postsynaptic neuron.

Post-Synaptic Potentials

PSP Type	Effect
EPSP	Small depolarisation → increases firing likelihood
IPSP	Small hyperpolarisation → decreases firing likelihood

Neurons sum all EPSPs and IPSPs at the axon hillock.
If the combined input crosses threshold → the neuron fires.
If inhibition dominates → firing is prevented.
This process is called neural integration