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

Nervous System

• The body’s primary communication and control system

• Divided according to structural and functional categories

Nervous System

Structural Organization• CNS

BrainSpinal cord

• PNSCranial nervesSpinal nervesGanglia

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

1. Microtubules – thickest, contain a thin central filament with a hollow core

2. Neurofilaments – abundant in axon3. 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 DendritesOnly one axon is present in a neuron

May be absent, solitary, or numerousUsually multiple in a neuron

Thin long process of uniform thickness and smooth surface

Short, multiple processesThickness diminishes as they divide repeatedlyBranches are studded with spiny projections

Cell Processes

Axons DendritesBranches 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 granulesNo 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:1.Unipolar2.Pseudounipolar3.Bipolar4.Multipolar

Types of NeuronsDescription Location

Unipolar Only 1 process (axon) Embryo, rare in adults

Pseudounipolar 1 axon bifurcates Dorsal root gangliaGanglia of some cranial nerves

Bipolar Dendrite and axon arise at opposite poles of cell body

Olfactory epitheliumVestibular and cochlear gangliaBipolar cells of retina

Multipolar Numerous dendritesMost common

Most neurons of cerebrum and cerebellum

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 nutrition2. Maintain homeostasis3. 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– Protoplasmic astrocytes– Fibrous astrocytes

• Oligodendrocytes• Schwann cells• Ependymal cells• Microglia

NeurogliaGlial Cell Type Origin Location Function

Astrocytes Neural tube

CNS Structural support, repair processes

Blood–brain barrier, metabolic exchanges

Oligodendrocytes Neural tube

CNS Myelin production, electric insulation

Schwann cells Neural tube

PNS Myelin production, electric insulation

Ependymal cells Neural tube

CNS Lining cavities of central nervous system

Microglia Bone marrow

CNS 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: – 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