Nervous System: Part 1

Student Objectives

• Define key terms related to the nervous system.

• Discuss the primary functions of the nervous system.

• Identify two major divisions of the nervous system and list the components of each.

• Identify the three types of neurons and their general functions.

• Identify the three parts of a neuron and the functions of each.

• Define nerves and differentiate between the kinds of nerves.

• Identify the components of a reflex arc and describe the functions of each.

• Describe the structure and function of the coverings and fluid spaces of the brain and spinal cord.

Key Terms

• Neuron: The basic functional cell of the nervous system, specialized for transmitting electrical impulses.

• Neuroglia (Glial cells): Supporting cells in the nervous system that do not conduct impulses but provide structural and metabolic support to neurons.

• CNS (Central Nervous System): Composed of the brain and spinal cord; responsible for integration and control.

• PNS (Peripheral Nervous System): Consists of nerves and ganglia outside the CNS; connects the CNS to limbs and organs.

• Effector: An organ (muscle or gland) that responds to a stimulus from the nervous system.

• Reflex Arc: The neural pathway involved in a reflex action.

• Myelin Sheath: Insulating layer around many axons that increases the speed of impulse transmission.

• Synapse: The junction between two neurons where information is transmitted.

• Action Potential: The electrical signal that travels along the neuron.

Primary Functions of the Nervous System

Overview

The nervous system is the master control and communication system of the body. It coordinates all body activities by transmitting signals between different parts of the body.

• Cells communicate via electrical and chemical signals.

• Responses are rapid, specific, and usually immediate.

Three Overlapping Functions

• Sensory Input: Information gathered by sensory receptors about internal and external changes.

• Integration: Processing and interpretation of sensory input to determine an appropriate response.

• Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Example: Touching a hot surface activates sensory receptors (sensory input), the brain processes the information (integration), and the hand withdraws (motor output).

Organization of the Nervous System

Major Divisions

• Central Nervous System (CNS):

  • Consists of the brain and spinal cord.

  • Acts as the integration and control center.

  • Interprets sensory input and dictates motor output.

• Peripheral Nervous System (PNS):

  • Consists mainly of nerves (spinal and cranial) and ganglia.

  • Connects the CNS to the rest of the body.

  • Spinal nerves: to and from the spinal cord.

  • Cranial nerves: to and from the brain.

Functional Divisions of the PNS

• Sensory (Afferent) Division:

  • Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to the CNS.

  • Visceral sensory fibers: convey impulses from visceral organs to the CNS.

• Motor (Efferent) Division:

  • Transmits impulses from the CNS to effector organs (muscles and glands).

  • Subdivided into:

    • Somatic Nervous System: Voluntary control of skeletal muscles.

    • Autonomic Nervous System: Involuntary control of smooth muscle, cardiac muscle, and glands. Includes sympathetic and parasympathetic subdivisions.

Types of Neurons

Structural Classification

• Multipolar Neurons: Three or more processes (one axon, multiple dendrites). Most common type in CNS.

• Bipolar Neurons: Two processes (one axon, one dendrite). Found in special sensory organs (e.g., retina, olfactory mucosa).

• Unipolar Neurons: One process that splits into two branches (peripheral and central). Mostly sensory neurons in PNS.

Functional Classification

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS. Mostly unipolar.

• Motor (Efferent) Neurons: Carry impulses away from the CNS to effectors. Mostly multipolar.

• Interneurons: Lie between sensory and motor neurons; shuttle signals through CNS pathways. Mostly multipolar.

Structure of a Neuron

Three Main Parts

• Cell Body (Soma): Contains the nucleus and organelles; metabolic center of the neuron.

• Dendrites: Short, branched extensions; receive signals and convey them toward the cell body.

• Axon: Long, singular process; transmits impulses away from the cell body to other neurons or effectors.

Example: Motor neurons may have hundreds of dendrites and a single long axon.

Myelination

• Myelin Sheath: Insulating layer around axons, formed by Schwann cells (PNS) or oligodendrocytes (CNS).

• Increases speed of impulse transmission.

• Nodes of Ranvier: Gaps between myelin segments where action potentials are regenerated.

Types of Nerves

• Mixed Nerves: Contain both sensory and motor fibers; impulses travel both to and from CNS.

• Sensory (Afferent) Nerves: Carry impulses only toward CNS.

• Motor (Efferent) Nerves: Carry impulses only away from CNS.

Example: Spinal nerves are mixed nerves; cranial nerves may be sensory, motor, or mixed.

Reflex Arc

Components and Functions

• Receptor: Site of stimulus action.

• Sensory Neuron: Transmits afferent impulses to CNS.

• Integration Center: Synaptic region in CNS (may be monosynaptic or polysynaptic).

• Motor Neuron: Conducts efferent impulses from integration center to effector.

• Effector: Muscle or gland that responds to efferent impulses.

Types of Reflexes:

• Somatic Reflexes: Activate skeletal muscle.

• Autonomic (Visceral) Reflexes: Activate smooth muscle, cardiac muscle, or glands.

Example: Withdrawal reflex when touching a hot object.

Coverings and Fluid Spaces of the Brain and Spinal Cord

Meninges

• Dura Mater: Tough, outermost layer; consists of periosteal and meningeal layers in the brain.

• Arachnoid Mater: Middle, web-like layer; contains the subarachnoid space filled with cerebrospinal fluid (CSF).

• Pia Mater: Delicate, innermost layer; closely adheres to the brain and spinal cord.

Functions: Protect CNS, enclose blood vessels, contain CSF, and form partitions within the skull.

Cerebrospinal Fluid (CSF)

• Forms a liquid cushion around the brain and spinal cord.

• Reduces brain weight by floating it, protecting from trauma.

• Nourishes the brain and removes waste.

• Produced by choroid plexuses in the ventricles.

Ventricles

• Fluid-filled chambers within the brain, continuous with the central canal of the spinal cord.

• Lined by ependymal cells and filled with CSF.

• Includes paired lateral ventricles, third ventricle, and fourth ventricle.

Clinical Note: Meningitis

• Inflammation of the meninges, which can spread to the CNS and cause encephalitis.

• Diagnosed by analyzing CSF obtained via lumbar puncture.

Summary Table: Organization of the Nervous System

Key Equations

• Action Potential: Change in membrane potential () over time due to ion movement.

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and the summary table.