Study Notes: The Nervous System (Anatomy & Physiology)

The Nervous System

Overview and Functions

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals between different parts. It is essential for sensation, movement, cognition, and homeostasis.

• Key Functions: Sensory input, integration of information, motor output, regulation of homeostasis.

• Major Divisions: Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Divisions of the Nervous System

The nervous system is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It acts as the main control center.

• Peripheral Nervous System (PNS): Composed of nerves outside the CNS, connecting it to limbs and organs.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Carries sensory information from receptors to the CNS.

• Motor (Efferent) Division: Transmits impulses from the CNS to effector organs (muscles and glands).

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion).

• Sympathetic Division: Prepares the body for 'fight or flight'.

• Parasympathetic Division: Promotes 'rest and digest' activities.

Cells and Tissues of the Nervous System

Types of Nervous Tissue

Nervous tissue consists of two main cell types:

• Neuroglia (Glial Cells): Support, protect, and maintain neurons.

• Neurons: Specialized cells that carry electrical impulses.

Structure of a Neuron

Neurons are the functional units of the nervous system.

• Cell Body: Contains the nucleus and organelles.

• Dendrites: Receive signals and carry them toward the cell body.

• Axon: Transmits impulses away from the cell body.

• Axon Terminals: Transmit signals to the next cell.

Myelinated vs. Unmyelinated Neurons

• Myelinated Neurons: Axons are covered by a myelin sheath, which increases the speed of impulse transmission.

• Unmyelinated Neurons: Lack myelin, resulting in slower impulse transmission.

Action Potential (Nerve Impulse)

Generation and Transmission

An action potential is an electrical impulse generated by neurons in response to stimuli. It is transmitted by the movement of ions across the cell membrane.

• Resting Membrane Potential: The difference in charge across the membrane when the neuron is not transmitting an impulse.

• Depolarization: Sodium ions (Na+) enter the cell, making the inside more positive.

• Repolarization: Potassium ions (K+) exit the cell, restoring the negative charge inside.

• Sodium-Potassium Pump: Maintains ion gradients by pumping Na+ out and K+ in.

The Synapse and Neurotransmitters

Synaptic Transmission

The synapse is the junction between two neurons where signals are transmitted chemically via neurotransmitters.

• No Physical Contact: Neurons are separated by a small gap called the synaptic cleft.

• Neurotransmitters: Chemical messengers released from the presynaptic neuron to stimulate the postsynaptic neuron.

Major Neurotransmitters and Their Functions

• Dopamine: Pleasure, reward, motor control.

• Serotonin: Mood, sleep, appetite.

• GABA: Calming, inhibition of nerve activity.

• Endorphins: Euphoria, pain relief.

• Noradrenaline: Concentration, alertness.

• Adrenaline: Fight or flight response.

• Acetylcholine: Learning, memory, muscle contraction.

• Glutamate: Memory, learning, excitatory signals.

Nerves and Receptors

Types of Nerves

• Sensory (Afferent) Nerves: Carry information from sensory receptors to the CNS.

• Motor (Efferent) Nerves: Transmit commands from the CNS to muscles and glands.

• Autonomic Nerves: Control involuntary functions.

Central Nervous System (CNS)

Structure and Protection

The CNS consists of the brain and spinal cord, protected by the skull, vertebrae, and meninges.

• Meninges: Three layers—dura mater, arachnoid mater, pia mater—surround the CNS.

• Spaces: Subdural (between dura and arachnoid), subarachnoid (between arachnoid and pia, contains CSF), epidural (outside dura, in spinal canal).

Brain Anatomy

The brain is divided into several major regions, each with specific functions.

• Cerebrum: Controls higher functions, divided into lobes.

• Thalamus: Relay center for sensory information.

• Hypothalamus: Regulates homeostasis and endocrine functions.

• Midbrain, Pons, Medulla Oblongata: Control basic life functions.

Lobes of the Brain

• Frontal Lobe: Movement, higher thinking, personality, speech production (Broca's area).

• Parietal Lobe: Sensory interpretation, spatial relationships.

• Temporal Lobe: Processing smell, taste, sound, memory, language.

• Occipital Lobe: Vision, visual processing.

Brain Hemispheres

Left and Right Hemispheres

The brain is divided into two hemispheres, connected by the corpus callosum.

• Contralateral Control: Each hemisphere controls the opposite side of the body.

• Specialization: Right hemisphere: spatial, artistic, facial perception. Left hemisphere: language, logic.

• Communication: Essential for coordinated function; quality of connection affects intelligence.

Ventricles and Cerebrospinal Fluid (CSF)

Brain Ventricles

The brain contains four ventricles filled with CSF:

• Right and left lateral ventricles

• Third ventricle

• Fourth ventricle

Functions of CSF

Cerebrospinal fluid (CSF) is vital for CNS function.

• Shock Absorption: Protects brain from injury.

• Nutrition: Supplies essential nutrients.

• Intracranial Pressure: Maintains stable pressure.

• Waste Removal: Cleanses toxins and waste.

• Temperature Regulation: Keeps CNS temperature stable.

• Immune Function: Contains immune cells for defense.

Reflex Arc

Mechanism of Reflexes

Reflexes are rapid, automatic responses to stimuli, involving a direct pathway from sensory input to motor output via the spinal cord.

• Sensory Receptor: Detects stimulus.

• Sensory Neuron: Transmits signal to CNS.

• Relay Neuron: Processes signal in spinal cord.

• Motor Neuron: Sends command to effector (muscle).

• Effector: Produces response.

Summary Table: Major Neurotransmitters

Neurotransmitter	Main Function	Example/Application
Dopamine	Pleasure, reward, motor control	Parkinson's disease, addiction
Serotonin	Mood, sleep, appetite	Depression, anxiety
GABA	Calming, inhibition	Epilepsy, anxiety
Endorphins	Euphoria, pain relief	Runner's high, pain management
Noradrenaline	Concentration, alertness	Attention, stress response
Adrenaline	Fight or flight	Acute stress, emergency response
Acetylcholine	Learning, memory, muscle contraction	Alzheimer's disease, muscle function
Glutamate	Memory, learning, excitation	Neuroplasticity, excitotoxicity

Key Equations

Sodium-Potassium Pump

The sodium-potassium pump maintains the resting membrane potential:

• For every ATP hydrolyzed, 3 Na+ ions are pumped out and 2 K+ ions are pumped in.

$\text{ATP} + 3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} \rightarrow \text{ADP} + 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}}$

References

• Ross and Wilson Anatomy and Physiology in Health and Illness, Waugh & Grant (2022)

• Principles of Anatomy and Physiology, Tortora & Derrickson (2021)

• Human Anatomy & Physiology, Marieb & Hoehn (2019)

• National Institute of Neurological Disorders and Stroke (2023)

• National Health Service (2024)