Nervous System Overview and Key Concepts

Functions of the Nervous System

Coordinates body activities via electrical and chemical signals.

Central Nervous System (CNS)

Brain and spinal cord; main control center.

Peripheral Nervous System (PNS)

Nerves branching from CNS.

Afferent Division (Sensory)

Brings information to CNS.

Somatic Senses

Touch, pain, temperature.

Special Senses

Vision, hearing, taste, smell, equilibrium.

Visceral Senses

Internal organs.

Efferent Division (Motor)

Sends signals from CNS to effectors.

Somatic Nervous System

Voluntary control over skeletal muscles.

Autonomic Nervous System

Involuntary control over cardiac/smooth muscle, glands.

Sympathetic Division

'Fight or Flight' response.

Parasympathetic Division

'Rest and Digest' response.

Neurons (Nerve Cells)

Specialized for electrical impulse transmission.

Cell Body (Soma)

Contains nucleus and organelles.

Dendrites

Receive signals.

Axon

Transmits impulses away from soma.

Myelin Sheath

Insulation that speeds up impulse transmission.

Nodes of Ranvier

Gaps in myelin allowing saltatory conduction.

Sensory (Afferent) Neurons

Carry input to CNS.

Motor (Efferent) Neurons

Carry output from CNS to effectors.

Interneurons (Association Neurons)

Process information within CNS.

Multipolar Neurons

Most common; multiple dendrites, one axon.

Bipolar Neurons

One dendrite, one axon (found in special senses).

Unipolar Neurons

Single extension branches into two directions (sensory neurons).

Anaxonic Neurons

No axon; only in CNS.

Astrocytes

Maintain blood-brain barrier.

Oligodendrocytes

Myelinate CNS neurons.

Microglia

Act as phagocytes.

Ependymal Cells

Produce cerebrospinal fluid (CSF).

Schwann Cells

Myelinate PNS neurons.

Satellite Cells

Protect neuron cell bodies.

Resting Membrane Potential

(-70mV) Inside is more negative due to Na⁺/K⁺ pump.

Depolarization

Na⁺ channels open, Na⁺ enters, making inside more positive.

Repolarization

K⁺ channels open, K⁺ exits, restoring negative charge inside.

Hyperpolarization

Overshoot of K⁺ leaving makes it temporarily too negative.

Return to Resting Potential

Na⁺/K⁺ pump restores ion balance.

Myelinated Neurons

Faster due to saltatory conduction (jumps between Nodes of Ranvier).

Unmyelinated Neurons

Slower, continuous impulse conduction.

Electrical Synapse

Direct ion flow through gap junctions (fast, found in heart).

Chemical Synapse

Neurotransmitter release across synaptic cleft (more common).

Action Potential Arrives

Depolarizes presynaptic membrane.

Calcium (Ca²⁺) Channels Open

Triggers vesicles.

Neurotransmitter Release

Via exocytosis into synaptic cleft.

Binding to Receptors

On postsynaptic membrane, triggering ion channels.

Post-Synaptic Response

Can be excitatory (depolarizing) or inhibitory (hyperpolarizing).

Neurotransmitter Removal

By reuptake, enzymatic breakdown, or diffusion.

Excitatory Neurotransmitters

Acetylcholine (ACh), Glutamate.

Inhibitory Neurotransmitters

GABA, Glycine.

Mood & Reward Neurotransmitters

Dopamine, Serotonin.

Fight-or-Flight Neurotransmitters

Norepinephrine, Epinephrine.

Sympathetic Division of ANS

Increases heart rate, dilates pupils, inhibits digestion.

Parasympathetic Division of ANS

Lowers heart rate, constricts pupils, stimulates digestion.

Dual Innervation

Most organs receive signals from both sympathetic and parasympathetic divisions.

Multiple Sclerosis (MS)

Demyelination of CNS neurons → slow nerve signals.

Alzheimer's Disease

Plaques and tangles impair brain function.

Parkinson's Disease

Loss of dopamine-producing neurons.

SSRIs (e.g., Prozac)

Block serotonin reuptake → improves mood.

Cocaine

Blocks dopamine reuptake → excessive stimulation.

Naloxone (Narcan)

Blocks opioid receptors → reverses overdose.

Action Potential Steps

Steps: Depolarization (-55mV threshold reached), Repolarization (K⁺ exits), Hyperpolarization, Return to Resting Potential.