Nervous System The body’s primary communication and control system Divided according to structural and functional categories

Nervous System Structural Central Nervous System Peripheral Nervous System Functional Sensory (Afferent) Division Motor (Efferent) Division

Structural Organization CNS Brain Spinal cord PNS Cranial nerves Spinal nerves Ganglia

Functional Organization

Functional Organization Sensory (Afferent) Division – receives sensory information (input) from receptors and transmits this information to the CNS Motor (Efferent) Division – transmits motor impulses (output) from the CNS to muscles or glands

Sensory Division Somatic sensory components are the general somatic senses (touch, pain, pressure, vibration, temperature, proprioception) Visceral sensory components transmit nerve impulses from blood vessels and viscera to the CNS (temperature, stretch of the organ wall)

Motor Division Somatic motor component conducts nerve impulses from the CNS to skeletal muscles Voluntary nervous system Autonomic motor component (ANS) innervates internal organs, regulates smooth muscle, cardiac muscle, and glands Visceral motor system or involuntary nervous system

Nervous Tissue Specialized tissue responsible for excitability and conduction of impulses Comprised of: Neurons – nerve cells with processes Neuroglia (glial cells) – cellular connective tissue of the NS

Neuron Structural and functional unit of nervous tissue Excitable cells that initiate and transmit nerve impulses

Neuron Characteristics: High metabolic rate Extreme longevity Typically non-mitotic

Neuron Size: varies from 4-20 um (motor neurons > sensory neurons) Nucleus: Large, pale vesicular, usually central Large prominent nucleolus “Fish eye” or “owl’s eye”

Neuron Cytoplasm (neuroplasm) Basophilic, contains usual cell organelles (Golgi, mitochondria) and pigments (melanin, lipofuscin) Centrosome – absent Neurofibrils Nissl granules

Nissl Granules Chromophilic bodies Give a granular appearance Rough endoplasmic reticulum Degenerate due to fatigue or injury to the neuron (chromatolysis)

Neurofibrils Thread-like structures Form a plexiform pattern in the cell body Arranged in parallel manner in both dendrites and axon Functions: provide internal support, play role in intracellular transport of organelles and vesicles

Neurofibrils Microtubules – thickest, contain a thin central filament with a hollow core Neurofilaments – abundant in axon Microfilaments – thinnest

Neuron Structure Vary in shape and size Cell body (perikaryon, soma) + one or more cell processes Enclosed by neurolemma

Neuron Structure Cell body – serves as the neuron’s control center and is responsible for receiving, integrating, and sending nerve impulses Cell processes Axons Dendrites

Cell Processes Axons Dendrites Only one axon is present in a neuron May be absent, solitary, or numerous Usually multiple in a neuron Thin long process of uniform thickness and smooth surface Short, multiple processes Thickness diminishes as they divide repeatedly Branches are studded with spiny projections

Cell Processes Axons Dendrites Branches of axon are fewer and at right angles to the axon Branch diffusely and are given off at acute angles Contains neurofibrils and no Nissl granules Contain both neurofibrils and Nissl granules No Golgi complexes Forms the efferent component of the impulse Afferent component

Axon a.k.a. axis cylinder One long thin process that arises from a conical elevation on the perikaryon (axon hillock) Cell membrane = axolemma Carries impulse away from cell body Axonal transport Anterograde – perikaryon → axon terminal Retrograde – axon terminal → perikaryon

Dendrites Single or multiple processes containing the extension of cytoplasm of neurons with its cell organelles Provide receptive surface for the neuron Carry impulses towards the soma of the neuron

Types of Neurons Based on number of processes: Unipolar Pseudounipolar Bipolar Multipolar

Types of Neurons Description Location Unipolar Only 1 process (axon) Embryo, rare in adults Pseudounipolar 1 axon bifurcates Dorsal root ganglia Ganglia of some cranial nerves Bipolar Dendrite and axon arise at opposite poles of cell body Olfactory epithelium Vestibular and cochlear ganglia Bipolar cells of retina Multipolar Numerous dendrites Most common Most neurons of cerebrum and cerebellum

Functional Classification Types of Neurons Functional Classification Description Sensory (Afferent) Transmit impulses toward the CNS Motor (Efferent) Carry impulses toward the body surface Interneurons (Association Neurons) Any neurons between a sensory and a motor neuron

Neuroglia/Glial Cells The cellular connective tissue of the nervous tissue Functions: 1. Provide support and nutrition 2. Maintain homeostasis 3. Form myelin 4. Participate in signal transmission

Neuroglia Found in both the CNS and PNS Outnumber neurons Smaller than neurons and capable of mitosis Do not transmit nerve impulses

Neuroglia Astrocytes Oligodendrocytes Schwann cells Ependymal cells Protoplasmic astrocytes Fibrous astrocytes Oligodendrocytes Schwann cells Ependymal cells Microglia

Neuroglia Glial Cell Type Origin Location Function Astrocytes Neural tube CNS Structural support, repair processes Blood–brain barrier, metabolic exchanges Oligodendrocytes Myelin production, electric insulation Schwann cells PNS Ependymal cells Lining cavities of central nervous system Microglia Bone marrow Macrophagic activity

Microglia Smallest glial cells Small elongated cells with short irregular processes Dense elongated nuclei (other glial cells: spherical nuclei)

Microglia Function: phagocytic (represent the mononuclear phagocytic system in nerve tissue)

Astrocytes Largest, most numerous Star-shaped cells with multiple radiating processes Large vesicular nuclei Only astrocytes exhibit vascular end-feet

Astrocytes Functions: Types: Form supporting network for neurons Provide nourishment to the neurons Types: Protoplasmic Fibrous

Protoplasmic Astrocyte Thick processes Abundant granular cytoplasm Some processes attached to neighboring capillaries by perivascular sucker feet (vascular end-feet) Found in grey matter

Fibrous Astrocyte Have long and straight processes seen only in the white matter

Ependymal Cells Low columnar (cuboidal) epithelial cells Lining the ventricles of the brain and central canal of the spinal cord

Ependymal Cells Ciliated in some locations (facilitate movement of CSF) Function: Secrete CSF

Oligodendrocytes Smaller than astrocytes, have fewer and shorter processes Scanty cytoplasm Nucleus usually ovoid or spherical and more deeply staining than astrocytes Found mostly in white matter

Oligodendrocytes Produce the myelin sheath (provides the electrical insulation of neurons in the CNS) Processes wrap around axons (producing a myelin sheath)

Oligodendrocytes Function: provides insulation to the axon that allows electrical signals to propagate more efficiently 1 oligodendrocyte can extend to up to 50 axons

Schwann Cells Nucleus: flattened Abundant cytoplasm Function : same as oligodendrocytes but are located around axons in the PNS Can only wrap around 1 axon