Study Notes: Nervous System (BIOL 1161)

Nervous System Overview

Introduction to the Nervous System

The nervous system is the master controlling and communicating system of the body. It coordinates rapid and specific responses to internal and external stimuli through electrical and chemical signals.

• Electrical and chemical signals are used for communication between cells.

• Responses are typically rapid and almost immediate.

Functions of the Nervous System

The nervous system performs three main functions:

• Sensory input: Information is gathered by sensory receptors about changes inside and outside the body.

• Integration: The processing and interpretation of sensory input.

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

Example: Seeing a glass of water (sensory input), the brain processes the information (integration), and the arm muscles move to pick up the glass (motor output).

Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord located in the dorsal body cavity. It serves as the integration and control center, interpreting sensory input and dictating motor output.

Peripheral Nervous System (PNS)

The PNS is the portion of the nervous system outside the CNS. It consists mainly of nerves that extend from the brain and spinal cord.

• Spinal nerves: Connect to and from the spinal cord.

• Cranial nerves: Connect to and from the brain.

Functional Divisions of the PNS

• Sensory (afferent) division: Conveys impulses from sensory receptors to the CNS.

  • Somatic sensory fibers: From skin, skeletal muscles, and joints.

  • Visceral sensory fibers: From visceral organs.

• Motor (efferent) division: Transmits impulses from the CNS to effector organs (muscles and glands).

  • Somatic nervous system: Voluntary control of skeletal muscles.

  • Autonomic nervous system (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. Subdivided into:

    • Sympathetic division

    • Parasympathetic division

Histology of Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is highly cellular with little extracellular space. There are two principal cell types:

• Neuroglia (glial cells): Small cells that support, protect, and wrap neurons.

• Neurons (nerve cells): Excitable cells that transmit electrical signals.

Types of Neuroglia

• Astrocytes: Most abundant, versatile, and highly branched glial cells. Functions include:

  • Support and brace neurons

  • Facilitate exchanges between capillaries and neurons

  • Guide migration of young neurons

  • Control chemical environment

  • Respond to nerve impulses and neurotransmitters

  • Influence neuronal functioning

• Microglial cells: Small, ovoid cells with thorny processes. They monitor neurons, migrate toward injured neurons, and can transform to phagocytize microorganisms and neuronal debris.

• Ependymal cells: Range from squamous to columnar, may be ciliated. They line the central cavities of the brain and spinal column and help circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Branched cells whose processes wrap CNS nerve fibers, forming insulating myelin sheaths.

• Satellite cells: Surround neuron cell bodies in the PNS, similar in function to astrocytes.

• Schwann cells (neurolemmocytes): Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers. Vital for regeneration of damaged peripheral nerve fibers.

Neurons: Structure and Function

General Properties of Neurons

• Structural units of the nervous system

• Large, highly specialized cells that conduct impulses

• Extreme longevity (up to 100 years or more)

• Amitotic (do not divide), with few exceptions

• High metabolic rate—require continuous supply of oxygen and glucose

• All have a cell body and one or more processes

Neuron Cell Body (Perikaryon or Soma)

• Biosynthetic center: synthesizes proteins, membranes, and other chemicals

• Contains rough ER (chromatophilic substance or Nissl bodies)

• Spherical nucleus with nucleolus

• Some contain pigments

• Plasma membrane is part of the receptive region

• Most neuron cell bodies are in the CNS (nuclei), while ganglia are in the PNS

Dendrites

• Short, tapering, diffusely branched processes

• Receptive (input) region of neuron

• Convey incoming messages toward cell body as graded potentials (short distance signals)

• Specialized dendrites in the brain collect information with dendritic spines

Axon: Structure and Function

• One axon per cell, arising from the axon hillock

• Long axons are called nerve fibers

• Occasional branches (axon collaterals)

• Distal endings called axon terminals or terminal boutons

• Conducting region of neuron, generates and transmits nerve impulses along the axolemma to the axon terminal

• Neurotransmitters released into extracellular space to excite or inhibit other neurons

• Lacks rough ER and Golgi apparatus; relies on cell body for renewal

Axonal Transport

• Anterograde: Movement away from cell body (e.g., mitochondria, cytoskeletal elements, enzymes)

• Retrograde: Movement toward cell body (e.g., organelles to be degraded, signal molecules, viruses, bacterial toxins)

Myelin Sheath

Structure and Function

• Composed of myelin (whitish, protein-lipoid substance)

• Segmented sheath around most long or large-diameter axons (myelinated fibers)

• Protects and electrically insulates axon

• Increases speed of nerve impulse transmission

• Nonmyelinated fibers conduct impulses more slowly

Myelination in the PNS

• Plasma membranes of myelinating cells have less protein

• No channels or carriers; good electrical insulators

• Interlocking proteins bind adjacent myelin membranes

• Nodes of Ranvier: Gaps between adjacent Schwann cells; sites where axon collaterals can emerge

• Nonmyelinated fibers: Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling

Myelin Sheaths in the CNS

• Formed by multiple, flat processes of oligodendrocytes (not whole cells)

• Can wrap up to 60 axons at once

• Nodes of Ranvier are present

• No outer collar of perinuclear cytoplasm

• Thinnest fibers are unmyelinated, covered by long extensions of adjacent neuroglia

• White matter: Regions with dense collections of myelinated fibers (fiber tracts)

• Gray matter: Mostly neuron cell bodies and nonmyelinated fibers

Classification of Neurons

Structural Classification

Neuron Type	Processes	Location	Example
Multipolar	Many dendrites, one axon	Most abundant in CNS	Motor neurons, interneurons
Bipolar	One dendrite, one axon	Special sensory organs (retina, olfactory)	Retinal cell
Unipolar (Pseudounipolar)	Single process splits into two branches	Mainly in PNS (sensory neurons)	Sensory neuron in dorsal root ganglion

Functional Classification

• Sensory (afferent) neurons: Transmit impulses from sensory receptors toward CNS; almost all are unipolar; cell bodies in ganglia in PNS.

• Motor (efferent) neurons: Carry impulses from CNS to effectors; multipolar; most cell bodies in CNS (except some autonomic neurons).

• Interneurons (association neurons): Lie between motor and sensory neurons; shuttle signals through CNS pathways; most are entirely within CNS; 99% of body's neurons.

Membrane Potential and Neural Signaling

Membrane Potential Changes

Neurons use changes in membrane potential as communication signals. These changes occur when ion concentrations across the membrane change or membrane permeability to ions changes.

• Graded potentials: Incoming signals operating over short distances.

• Action potentials: Long-distance signals of axons.

Changes in membrane potential are used to receive, integrate, and send information.

Depolarization

• Decrease in membrane potential (toward zero and above)

• Inside of membrane becomes less negative than resting membrane potential

• Increases probability of producing a nerve impulse

*Additional info: Further details on hyperpolarization, action potential generation, and synaptic transmission would be covered in subsequent notes or chapters.*