Introduction to the Nervous System

Introduction to the Nervous System

• The nervous system is divided into two major parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of the brain and spinal cord.
• The PNS includes the cranial nerves, spinal nerves, and autonomic nervous system.

Functions of the Nervous System

• The nervous system controls motor, sensory, autonomic, cognitive, and behavioral activities.
• It responds to internal and external environments, maintaining homeostasis.
• It directs all psychological, biological, and physical activity through complex chemical and electrical messages.

Cells of the Nervous System

• Neurons are the basic functional unit of the brain, supported and protected by glial cells (50 times more numerous).
• Dendrites receive electrochemical signals.
• Axons transmit electrical impulses away from the cell body.
• Myelin sheaths increase conduction speed.

Neurotransmitters

• Neurotransmitters communicate signals between neurons, or neurons to target cells (e.g., muscle, endocrine cells).
• They are manufactured and stored in vesicles.
• Release of neurotransmitters occurs when an electrical action potential reaches the nerve terminal, crossing the synapse and binding to receptors on the postsynaptic membrane.
• Neurotransmitters can either excite or inhibit activity in the target cells.
• Imbalances in neurotransmitters are implicated in many neurological disorders.

Major Neurotransmitters (Examples)

• Acetylcholine (important in the parasympathetic nervous system)
• Serotonin (regulates mood and sleep)
• Dopamine (affects behavior and movement)
• Norepinephrine (important in the sympathetic nervous system)
• Gamma-aminobutyric acid (inhibitory)
• Enkephalins/Endorphins (inhibit pain)

Central Nervous System (CNS) Details

• The brain accounts for 2% of total body weight (1400 g in a young adult; 1200 g in an older adult).
• The cerebrum is divided into three major areas: cerebrum, brain stem, and cerebellum.
• The cerebrum has wrinkled surfaces (gyri) increasing surface area for complex activities.
• The cerebrum is made up of gray matter (billions of neuron cell bodies visible).
• White matter (myelinated nerve fibers) connects various parts of the brain and spinal cord, forming tracts.
• Specialized lobes in the cerebrum (frontal, parietal, temporal, occipital) have specific functions: concentration, abstract thought, memory, speech, judgment, personality, sensory analysis, body position awareness, auditory processing, language, etc.

Structures and Functions in the CNS

• **Corpus Callosum:**Thick nerve fibers connecting the two hemispheres of the brain.
• Thalamus: Relay station for all sensations (except smell); involved in memory and pain processing.
• Hypothalamus: Regulates endocrine system and pituitary gland, controlling metabolic functions, temperature, and behavior.
• Brain Stem: Composed of midbrain, pons, and medulla oblongata; connects the brain and the periphery, controls respiration, reflexes, etc.
• Cerebellum: Located below the cerebrum. Coordinates movement, balance, and posture.

Protection of the Brain

• The skull protects the brain from injury.
• Meninges (dura mater, arachnoid, pia mater) are protective membranes surrounding the brain and spinal cord.
• Cerebrospinal fluid (CSF) fills spaces around the brain and spinal cord, providing shock absorption and nourishment.

Cranial Nerves

• Twelve pairs of nerves emerge from the brain and pass through openings in the skull.
• Some are sensory, some motor, and some mixed.
• Cranial nerves innervate the head, neck, and special sense structures.

Spinal Cord

• Continuous with the medulla, extending from the foramen magnum to the lower lumbar spine.
• Protected by vertebrae and meninges.
• Contains dorsal and ventral roots.
• Dorsal roots receive sensory information; ventral roots carry motor impulses.
• Spinal nerves emerge and branch to innervate the body.

Autonomic Nervous System (ANS)

• The ANS controls involuntary body functions.
• Two major divisions: sympathetic (fight-or-flight) and parasympathetic (rest-and-digest).
• Controls organs like the heart, lungs, blood vessels, digestive organs, and more.

Motor and Sensory Pathways

• The nervous system involves complex pathways to control and receive sensations.
• Motor pathways transmit signals that control voluntary and involuntary movements.
• Sensory pathways receive, integrate, and send various types of sensations.

Upper and Lower Motor Neurons

• Upper motor neurons connect the brain to motor neurons in the spinal cord.
• Lower motor neurons connect the spinal cord to muscles.
• Damage to either type affects movement differently.