Nervous

NERVOUS SYSTEM

Important details only • Key words • Easy to memorize

1. Functions of the Nervous System

• Sensory input — Detects stimuli inside and outside the body.

• Integration — Processes information and decides the response.

• Motor output — Activates muscles or glands.

2. Organization of the Nervous System

• Central Nervous System (CNS) — Brain and spinal cord; command center.

• Peripheral Nervous System (PNS) — Cranial and spinal nerves; communication lines.

• Sensory (afferent) division — Carries impulses toward the CNS.

• Motor (efferent) division — Carries impulses away from the CNS.

• Somatic nervous system — Voluntary control of skeletal muscles.

• Autonomic nervous system — Involuntary control of smooth muscles, cardiac muscle, and glands.

• Sympathetic division — Prepares the body for action.

• Parasympathetic division — Supports resting body functions.

3. Nervous Tissue

Neurons — Nerve cells that transmit impulses.

• Cell body — Contains nucleus; metabolic center.

• Dendrites — Carry impulses toward the cell body.

• Axon — Carries impulses away from the cell body.

• Axon terminals — Release neurotransmitters.

• Synapse — Junction where signals pass between cells.

• Synaptic cleft — Gap between communicating cells.

• Myelin sheath — Insulates axons and speeds impulse transmission.

• Nodes of Ranvier — Gaps in the myelin sheath.

Neuroglia (Supporting Cells)

• Astrocytes — Support neurons; regulate the brain’s chemical environment.

• Microglia — Remove debris; defend CNS cells.

• Ependymal cells — Line brain and spinal cord cavities; help circulate cerebrospinal fluid (CSF).

• Oligodendrocytes — Form myelin in the CNS.

• Schwann cells — Form myelin in the PNS.

• Satellite cells — Protect and cushion neuron cell bodies in the PNS.

4. Neuron Classification

By function

• Sensory neurons — Receptors to CNS.

• Motor neurons — CNS to muscles or glands.

• Interneurons — Connect sensory and motor neurons within the CNS.

By structure

• Multipolar — Many processes; most common; all motor neurons and interneurons.

• Bipolar — One axon and one dendrite; special sense organs.

• Unipolar — One short process; sensory neurons in PNS ganglia.

5. Nerve Impulse

• Irritability — Ability to respond to a stimulus.

• Conductivity — Ability to transmit an impulse.

• Resting state — Membrane is polarized; inside is negative.

• Depolarization — Na⁺ enters the neuron; inside becomes more positive.

• Action potential — Electrical impulse triggered by sufficient stimulation.

• Propagation — Impulse travels along the axon.

• Repolarization — K⁺ leaves the neuron; inside becomes negative again.

• Sodium-potassium pump — Uses ATP; moves 3 Na⁺ out and 2 K⁺ in.

• All-or-none response — Impulse occurs fully or not at all.

6. Synaptic Transmission

• Action potential reaches axon terminal.

• Ca²⁺ channels open.

• Vesicles release neurotransmitters.

• Neurotransmitters cross the synaptic cleft.

• Bind to receptors on the next cell.

• Neurotransmitters are removed by reuptake or enzymes.

• Electrical transmission — Along the neuron.

• Chemical transmission — Between neurons at the synapse.

7. Reflex Arc

Reflex — Rapid, predictable, involuntary response to a stimulus.

Five elements:

• Sensory receptor

• Sensory neuron

• Integration center (CNS)

• Motor neuron

• Effector (muscle or gland)

• Somatic reflex — Activates skeletal muscles.

• Autonomic reflex — Regulates smooth muscle, heart, and glands.

• Two-neuron reflex — Example: knee-jerk reflex.

• Three-neuron reflex — Includes an interneuron; example: withdrawal reflex.

8. Major Brain Regions

Know each region and its main function.

• Cerebral hemispheres — Higher functions, sensation, and voluntary movement.

• Cerebral cortex — Outer gray matter.

• Cerebral white matter — Fiber tracts connecting brain areas.

• Basal nuclei — Modify and regulate voluntary movements.

• Diencephalon — Thalamus, hypothalamus, and epithalamus.

• Brain stem — Midbrain, pons, and medulla oblongata.

• Cerebellum — Balance, coordination, and timing of muscle activity.

Cerebral Lobes

• Frontal lobe — Voluntary movement, judgment, problem-solving, speech.

• Parietal lobe — Processes body sensations.

• Temporal lobe — Hearing and smell areas.

• Occipital lobe — Vision.

Important Brain Structures

• Thalamus — Relays sensory impulses to the cerebral cortex.

• Hypothalamus — Regulates temperature, water balance, metabolism, emotions, and pituitary activity.

• Epithalamus — Contains pineal gland; choroid plexus produces CSF.

• Midbrain — Visual and auditory reflex centers.

• Pons — Helps control breathing.

• Medulla oblongata — Controls heart rate, blood pressure, breathing, swallowing, and vomiting.

• Reticular activating system (RAS) — Alertness, consciousness, sleep-wake cycles.

• Corpus callosum — Connects the two cerebral hemispheres.

• Broca’s area — Motor speech.

• Primary motor area — Voluntary skeletal muscle movement.

• Primary somatic sensory area — Receives body sensations.

• Visual area — Occipital lobe.

• Auditory area — Temporal lobe.

9. Protection of the CNS

• Meninges — Protective membranes around the brain and spinal cord.

• Dura mater — Tough outer layer.

• Arachnoid mater — Middle, web-like layer.

• Pia mater — Inner layer attached to the brain and spinal cord.

• Cerebrospinal fluid (CSF) — Cushions and protects the CNS.

• Ventricles — CSF-filled cavities in the brain.

• Choroid plexus — Produces CSF.

10. Spinal Cord

• Main function — Carries impulses between brain and body; reflex center.

• 31 pairs of spinal nerves — Arise from the spinal cord.

• Cauda equina — Collection of spinal nerves at the lower end of the spinal canal.

• Gray matter — Contains neuron cell bodies.

• Dorsal horns — Receive sensory information.

• Ventral horns — Contain somatic motor neurons.

• White matter — Myelinated tracts.

• Ascending tracts — Carry sensory impulses toward the brain.

• Descending tracts — Carry motor impulses from the brain.

• Dorsal root ganglion — Contains sensory neuron cell bodies.

• Dorsal root — Sensory input.

• Ventral root — Motor output.

11. Peripheral Nervous System

• Cranial nerves — Carry impulses to and from the brain.

• Spinal nerves — Carry impulses to and from the spinal cord.

• Nerve plexus — Network of spinal nerves.

• Cervical plexus — Neck region.

• Brachial plexus — Shoulder and upper limb.

• Lumbar plexus — Lower abdominal region and anterior thigh.

• Sacral plexus — Pelvis and lower limb.

• Sciatic nerve — Major nerve supplying the lower limb.

12. Autonomic Nervous System (ANS)

Controls involuntary activities of cardiac muscle, smooth muscle, and glands.

• Preganglionic neuron — Extends from the CNS to an autonomic ganglion.

• Postganglionic neuron — Extends from the ganglion to the target organ.

13. Important Nervous System Disorders

• Concussion — Mild traumatic brain injury.

• Contusion — Brain tissue injury with destruction.

• Cerebrovascular accident (CVA/stroke) — Interrupted brain blood supply causing tissue damage.

• Hemiplegia — Paralysis on one side of the body.

• Aphasia — Impaired language or speech ability.

• Transient ischemic attack (TIA) — Temporary restriction of brain blood flow.

Most important to memorize: CNS vs. PNS, afferent vs. efferent, neuron parts, neuroglia, Na⁺/K⁺ during nerve impulses, reflex arc, brain structures and functions, meninges, spinal cord roots, and sympathetic vs. parasympathetic divisions.