Nervous System Functions and Divisions

• The nervous system serves as the body's master controller and communicator.
• Cells communicate through electrical and chemical signals in the nervous system.
• These signals are rapid, specific, and cause almost immediate responses.

Functions of the Nervous System

• Sensory input is information gathered via sensory receptors about internal and external changes.
• Integration is the processing and interpretation of sensory input.
• Motor output involves the activation of effector organs, such as muscles and glands, to produce a response.

Divisions of the Nervous System

• The nervous system has two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of the brain and spinal cord, located in the dorsal body cavity.
• The CNS is the integration and control center; it interprets sensory input and dictates motor output.
• The PNS is the portion of the nervous system outside the CNS.
• The PNS consists mainly of nerves extending from the brain and spinal cord.
• Spinal nerves carry impulses to and from the spinal cord.
• Cranial nerves carry impulses to and from the brain.

Functional Divisions of the PNS

• The PNS has two functional divisions: sensory (afferent) and motor (efferent).
• The sensory division conveys impulses from skin, skeletal muscles, and joints to the CNS via somatic sensory fibers.
• The sensory division conveys impulses from visceral organs to the CNS via visceral sensory fibers.
• The motor division transmits impulses from the CNS to effector organs.
• This division includes muscles and glands.
• There are two divisions of the motor division: the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

• Somatic motor nerve fibers conduct impulses from the CNS to skeletal muscle.
• It is a voluntary nervous system, allowing for conscious control of skeletal muscles.

Autonomic Nervous System

• It is comprised of visceral motor nerve fibers.
• This system regulates smooth muscle, cardiac muscle, and glands.
• It is an involuntary nervous system.
• It has two functional subdivisions: sympathetic and parasympathetic.
• Sympathetic and parasympathetic divisions work in opposition to each other.

Nervous Tissue

• Nervous tissue has two main cell types: neuroglia (glial cells) and neurons (nerve cells).
• Neuroglia (glial cells) are small cells that surround and wrap delicate neurons.
• Neurons (nerve cells) are excitable cells that transmit electrical signals.

Neuroglia of the CNS

• There are four main types of neuroglia that support CNS neurons: astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
• Astrocytes are the most abundant, versatile, and highly branched glial cells.
• They cling to neurons, synaptic endings, and capillaries.
• Astrocytes support and brace neurons.
• They help exchange materials between capillaries and neurons.
• Astrocytes guide the migration of young neurons.
• They control the chemical environment around neurons.
• They respond to nerve impulses and neurotransmitters.
• They influence neuronal functioning and participate in information processing in the brain.
• Microglial cells are small, ovoid cells with thorny processes that touch and monitor neurons.
• They migrate toward injured neurons.
• These cells transform and phagocytize microorganisms and neuronal debris.
• Ependymal cells range in shape from squamous to columnar and may be ciliated.
• Cilia circulate cerebrospinal fluid (CSF).
• They line the central cavities of the brain and spinal column.
• They form a permeable barrier between CSF in cavities and tissue fluid bathing CNS cells.
• Oligodendrocytes are branched cells.
• Processes wrap CNS nerve fibers and form insulating myelin sheaths around thicker nerve fibers.

Neuroglia of PNS

• The two major neuroglia seen in the PNS are satellite cells and Schwann cells.
• Satellite cells surround neuron cell bodies in the PNS.
• Their function is similar to that of astrocytes in the CNS.
• Schwann cells (neurolemmocytes) surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers.
• Schwann cells have a similar function as oligodendrocytes.
• They are vital to the regeneration of damaged peripheral nerve fibers.

Neurons

• Neurons (nerve cells) are structural units of the nervous system.
• They are large, highly specialized cells that conduct impulses.
• Neurons have extreme longevity, and can last a person's lifetime.
• They are amitotic, with few exceptions.
• Neurons have a high metabolic rate and need a continuous supply of oxygen and glucose.
• All neurons have a cell body and one or more processes.

Neuron Cell Body

• The neuron cell body is also called the soma.
• It is the biosynthetic center of the neuron, and synthesizes proteins, membranes, and chemicals.
• Its rough ER is also called chromatophilic substance or Nissl bodies.
• It contains a spherical nucleus with a nucleolus.
• Some neuron cell bodies contain pigments.
• In most neurons, the plasma membrane is part of the receptive region that receives input information from other neurons.
• Most neuron cell bodies are located in the CNS.
• Clusters of neuron cell bodies are called nuclei in the CNS.
• Clusters of neuron cell bodies are called ganglia in the PNS.

Neuron Processes

• Neuron processes are armlike processes that extend from the cell body.
• The CNS contains both neuron cell bodies and their processes.
• The PNS contains chiefly neuron processes.
• Bundles of neuron processes in the CNS form tracts.
• Bundles of neuron processes in the PNS form nerves.
• There are two types of processes: dendrites and axons.

Dendrites

• Motor neurons can contain 100s of short, tapering, diffusely branched processes.
• These dendrites contain the same organelles as in the cell body.
• Dendrites are the receptive (input) region of a neuron.
• Dendrites convey incoming messages toward the cell body as graded potentials (short distance signals).
• In many brain areas, finer dendrites are highly specialized to collect information.
• These dendrites contain dendritic spines, appendages with bulbous or spiky ends.

Axon

• Each neuron has one axon that starts at a cone-shaped area called the axon hillock.
• In some neurons, axons are short or absent; in others, the axon forms almost the entire length of the cell.
• Some axons can be over 1 meter long.
• Long axons are called nerve fibers.
• Axons branch profusely at their end (terminus); numbering as many as 10,000 terminal branches.
• Distal endings are called axon terminals or terminal boutons.
• The axon is the conducting region of a neuron.
• It generates nerve impulses and transmits them along the axolemma (neuron cell membrane) to the axon terminal.
• The terminal region secretes neurotransmitters, which are released into the extracellular space.
• This can then excite or inhibit neurons it contacts.
• An axon can carry on many conversations with different neurons at the same time.
• Axons rely on cell bodies to renew proteins and membranes.
• Axons quickly decay if cut or damaged.
• Axons have efficient internal transport mechanisms; molecules and organelles are moved along axons by motor proteins and cytoskeletal elements.
• Movement occurs in both directions: anterograde and retrograde.
• Anterograde movement is away from the cell body and includes items like mitochondria, cytoskeletal elements, membrane components and enzymes.
• Retrograde movement is toward the cell body and includes organelles to be degraded, signal molecules, viruses, and bacterial toxins.

Myelin Sheath

• The myelin sheath is composed of myelin, a whitish, protein-lipid substance.
• The function of myelin is to protect and electrically insulate the axon and increase the speed of nerve impulse transmission.
• Myelinated fibers have a segmented sheath which surrounds most long or large-diameter axons.
• Nonmyelinated fibers do not contain a sheath and conduct impulses more slowly.

Myelination In The PNS

• It is formed by Schwann cells.
• It wraps around the axon in jelly roll fashion.
• One cell forms one segment of the myelin sheath.
• The outer collar of perinuclear cytoplasm (formerly called neurilemma) is a peripheral bulge containing the nucleus and most of the cytoplasm.
• Plasma membranes have less protein.
• They have no channels or carriers, so they are good electrical insulators.
• Interlocking proteins bind adjacent myelin membranes.
• Myelin Sheath Gaps are gaps between adjacent Schwann cells.
• These are sites where axon collaterals can emerge and were formerly called nodes of Ranvier.
• Nonmyelinated fibers are thin fibers not wrapped in myelin.
• These thin fibers are surrounded by Schwann cells, one cell may surround 15 different fibers but no coiling occurs.

Myelin Sheaths In The CNS

• They are formed by the processes of oligodendrocytes, not whole cells.
• Each cell can wrap up to 60 axons at once.
• A myelin sheath gap is present.
• There is no outer collar of perinuclear cytoplasm.
• Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia.
• White matter is regions of the brain and spinal cord with dense collections of myelinated fibers.
• The myelin sheaths usually form fiber tracts.
• Gray matter is regions of the brain and spinal cord mostly neuron cell bodies and nonmyelinated fibers.

Structural Classification of Neurons

• Neurons structured by the number of processes extending from their cell bodies.
• There are three types: multipolar, bipolar and unipolar.
• Multipolar neurons have 3 or more processes, one axon and the rest dendrites.
• They are the most common and major neuron type in the CNS.
• Bipolar neurons have two processes: one axon and one dendrite.
• They are rare as seen in the retina and olfactory mucosa.
• Unipolar neurons have one T-like process (two axons).
• These are also called pseudounipolar.

Functional Classification of Neurons

• Functional classification of neurons grouped by direction in which nerve impulse travels relative to CNS
• The three types are sensory, motor, and interneurons.
• Sensory neurons transmit impulses from sensory receptors toward the CNS and are almost all unipolar their cell bodies are located in ganglia in the PNS.
• Motor neurons carry impulses from the CNS to effectors and are multipolar; most cell bodies are located in the CNS except some autonomic neurons.
• Interneurons are the association neurons that lie between motor and sensory neurons, shuttle signals through CNS pathways, are mostly entirely within the CNS, and 99% of the body's neurons are interneurons.

The nervous system is the body's control center, using electrical and chemical signals for communication. Its functions include sensory input, integration, and motor output. It comprises the central (brain and spinal cord) and peripheral nervous systems.