The Nervous System: Structure, Function, and Organization

The Nervous System

Overview and Functions

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is essential for sensory perception, integration of information, and motor output.

• Sensory Input: Gathering information from sensory receptors that monitor changes (stimuli) inside and outside the body.

• Integration: Processing and interpreting sensory input to determine if action is needed.

• Motor Output: Activating muscles or glands in response to integration.

Additional info: These functions are analogous to a feedback loop, where sensory input is received, processed, and an appropriate response is generated.

Organization of the Nervous System

Structural Classification

• Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the integration and command center.

• Peripheral Nervous System (PNS): Composed of nerves extending from the brain and spinal cord (cranial and spinal nerves); acts as communication lines among sensory organs, CNS, and effectors.

Functional Classification

• Sensory (Afferent) Division: Carries information to the CNS. Includes somatic sensory fibers (from skin, muscles, joints) and visceral sensory fibers (from organs).

• Motor (Efferent) Division: Carries impulses from the CNS to effector organs. Subdivided into:

  • Somatic Nervous System: Voluntary control of skeletal muscles.

  • Autonomic Nervous System: Involuntary control of smooth/cardiac muscles and glands; further divided into sympathetic and parasympathetic divisions.

Nervous Tissue: Structure and Function

Cell Types

• Neurons: Specialized cells that transmit nerve impulses.

• Neuroglia (Glial Cells): Supporting cells that provide support, insulation, and protection for neurons. They do not conduct impulses and retain the ability to divide.

Types of Neuroglia

• CNS Glial Cells:

  • Astrocytes: Anchor neurons to capillaries, regulate exchanges, and control the brain's chemical environment.

  • Microglia: Phagocytic cells that monitor neuron health and dispose of debris.

  • Ependymal Cells: Line brain and spinal cord cavities; cilia circulate cerebrospinal fluid.

  • Oligodendrocytes: Produce myelin sheaths around CNS axons.

• PNS Glial Cells:

  • Schwann Cells: Form myelin sheaths in the PNS.

  • Satellite Cells: Protect and cushion neuron cell bodies.

Neurons: Structure and Function

Neuron Anatomy

• Cell Body: Contains the nucleus and metabolic center.

• Processes: Dendrites (conduct impulses toward cell body) and axons (conduct impulses away from cell body).

• Axon Terminals: Contain neurotransmitter vesicles; separated from the next neuron by a synaptic cleft (synapse).

• Myelin Sheath: Fatty covering that insulates axons and speeds impulse transmission. Formed by Schwann cells in PNS and oligodendrocytes in CNS.

• Nodes of Ranvier: Gaps in the myelin sheath along the axon.

Neuron Classification

• Functional:

  • Sensory (Afferent) Neurons: Carry impulses to CNS; receptors include cutaneous sense organs and proprioceptors.

  • Motor (Efferent) Neurons: Carry impulses from CNS to effectors.

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

• Structural:

  • Multipolar: Many processes; most common.

  • Bipolar: One axon and one dendrite; found in special sense organs.

  • Unipolar: Single process; sensory neurons in PNS ganglia.

Functional Properties

• Irritability: Ability to respond to a stimulus and convert it to a nerve impulse.

• Conductivity: Ability to transmit the impulse to other neurons, muscles, or glands.

Nerve Impulse Transmission

Resting Membrane Potential

The resting neuron membrane is polarized, with fewer positive ions inside than outside. Sodium (Na+) is the major positive ion outside, and potassium (K+) is the major positive ion inside.

Action Potential

• A stimulus increases membrane permeability to Na+, causing depolarization.

• If threshold is reached, an action potential is generated and propagated along the axon (all-or-none response).

• Myelinated fibers conduct impulses faster.

Repolarization

• Membrane becomes impermeable to Na+ and permeable to K+; K+ exits the cell, restoring negative internal charge.

• The sodium-potassium pump restores original ion distribution using ATP: $3 \text{Na}^+ \text{ out}, 2 \text{K}^+ \text{ in}$

Synaptic Transmission

1. Action potential reaches axon terminal, opening calcium channels.

2. Calcium influx causes neurotransmitter vesicles to fuse with the membrane.

3. Neurotransmitter is released into the synaptic cleft.

4. Neurotransmitter binds to receptors on the next neuron, generating a graded potential.

5. If enough neurotransmitter is released, an action potential is triggered in the postsynaptic neuron.

6. Neurotransmitter is quickly removed by reuptake or enzymatic breakdown.

Additional info: Transmission is electrical along the neuron and chemical at the synapse.

Reflexes and Reflex Arcs

Types of Reflexes

• Somatic Reflexes: Stimulate skeletal muscles (e.g., withdrawal from pain).

• Autonomic Reflexes: Regulate smooth muscle, heart, and glands (e.g., digestion, blood pressure).

Reflex Arc Components

1. Sensory receptor

2. Sensory neuron

3. Integration center (CNS)

4. Motor neuron

5. Effector organ

Two-neuron arcs (e.g., knee-jerk) and three-neuron arcs (e.g., withdrawal) exist.

Central Nervous System (CNS)

Brain Regions

• Cerebral Hemispheres: Paired superior parts; contain cortex (gray matter), white matter, and basal nuclei.

• Diencephalon: Includes thalamus (relay station), hypothalamus (autonomic control, emotion, pituitary regulation), and epithalamus (pineal gland, CSF formation).

• Brain Stem: Midbrain (reflex centers), pons (breathing), medulla oblongata (vital centers: heart, breathing, swallowing).

• Cerebellum: Coordinates balance and skeletal muscle activity.

Protection of the CNS

• Meninges: Dura mater (outer), arachnoid mater (middle, CSF-filled), pia mater (inner).

• Cerebrospinal Fluid (CSF): Cushions brain and spinal cord; formed by choroid plexuses, circulates in ventricles and subarachnoid space.

• Blood-Brain Barrier: Selective permeability protects brain from harmful substances.

Peripheral Nervous System (PNS)

Nerve Structure

• Nerves are bundles of neuron fibers outside the CNS, protected by connective tissue layers: endoneurium, perineurium, epineurium.

• Mixed Nerves: Contain both sensory and motor fibers.

• Sensory Nerves: Carry impulses toward CNS.

• Motor Nerves: Carry impulses away from CNS.

Cranial and Spinal Nerves

• Cranial Nerves: 12 pairs, mostly serving head and neck; only the vagus extends to thoracic/abdominal cavities.

• Spinal Nerves: 31 pairs, named for their origin; divide into dorsal and ventral rami, forming plexuses (cervical, brachial, lumbar, sacral).

Autonomic Nervous System (ANS)

Divisions and Functions

• Somatic Nervous System: Single neuron from CNS to skeletal muscle; voluntary control.

• Autonomic Nervous System: Two-neuron chain (preganglionic and postganglionic); involuntary control of smooth/cardiac muscle and glands.

• Sympathetic Division: "Fight or flight"—prepares body for stress (increases heart rate, dilates bronchioles, etc.).

• Parasympathetic Division: "Rest and digest"—conserves energy, maintains daily functions (digestion, defecation, diuresis).

Neurotransmitters

• Parasympathetic: Acetylcholine (cholinergic fibers).

• Sympathetic: Norepinephrine (adrenergic fibers).

Developmental Aspects of the Nervous System

• The nervous system forms early in embryonic development; vulnerable to infections, drugs, and oxygen deprivation.

• Major congenital diseases: cerebral palsy, anencephaly, hydrocephalus, spina bifida.

• Brain growth ends in young adulthood; neuron loss and decreased brain mass occur with age, especially with cardiovascular disease.