Nervous SystemA&P-II Unit-IV

Muhammad IqbalLecturer

KMU

Objectives

At the end of this unit the learners will be able to:• Define Nervous system.• Briefly discuss organization of Nervous system. • Define nerve & its types.• Describe the structure and function of Neuron and neuroglia. • Identify the principal parts of the brain i.e. Hind brain, Mid

brain & Fore brain. • Discuss the structure & function of: --Brain stem (Mid brain, pons & medulla oblongata) --Cerebellum• Describe the structure of cerebrum in terms of: -- Lobes (sulci and gyrus) --Functional areas

Objectives• Describe the three meninges covering the brain &

spinal cord.• Identify the ventricles of the brain.• Briefly explain the formation, circulation,

reabsorption & functions of cerebrospinal fluid (CSF).

Brain

Nervous System• Definition: The network of nerve cells and fibers that

transmits nerve impulses between parts of the body.• With a mass of only 2 kg, the nervous system is one of

the smallest and yet the most complex of the 11 body systems. It is an intricate , highly organized network of billions of neurons and even more neuroglia.

• The structures that make up the nervous system include the brain, cranial nerves, and their branches, the spinal cord, spinal nerves and their branches, ganglia, enteric plexuses and sensory receptors.

• The skull encloses the brain, which contains about 100 billion (1011) neurons.

• Twelve pairs (right and left) of cranial nerves emerging from the base of the brain.

• The spinal cord connects to the brain through the foramen magnum of the skull and is protected by the bones of the vertebral column.

• It contains about 100 million neurons. 31 pairs of spinal nerves leave the spinal cord for specific region on the right or left side of the body.

• Ganglia (swelling or knot) are small masses of nervous tissue, consisting basically of neuron cell bodies that are located outside the brain and spinal cord.

• Ganglia are closely associated with cranial and spinal nerves. In the walls of organs of the GIT, extensive networks of neurons, called enteric plexuses, help regulate the digestive system.

Functions of the Nervous System

• The nervous system carries out different functions like smell, speech, storing, remembering past events. It controls body movements and regulates the operation of internal organs. All these diverse activities are grouped into three basic functions such as sensory, integrative, and motor.

• Sensory function: sensory receptors detect internal stimuli, such as an increase in blood acidity, external stimuli, such as a housefly sitting on your body part. Neurons called sensory or afferent neurons (af=toward; ferrent=carried) carry this sensory information into the brain and spinal cord through cranial and spinal nerves.

• Integrative Function: The nervous system integrates (processes) sensory information by analyzing and by making decisions for appropriate responses. An important integrative function is perception, the conscious awareness of sensory stimuli. Perception occurs in the brain.

• Motor Function: Once the sensory information is integrated, the nervous system gives an appropriate motor response such as muscle contraction or gland secretion.

• The neurons that serve this function are called motor or efferent neurons.

• Motor neurons carry information from the brain to toward the spinal cord or effectors (muscles and glands). Stimulation of the effectors by motor neurons causes muscles to contract and glands to secrete.

• The motor part of the ANS consists of two branches:

• Sympathetic division—“fight-or-flight” • Parasympathetic division— “rest-and-digest” These two divisions have opposing actions. For

example, sympathetic neurons increase heart rate while parasympathetic neurons do opposite.

Organization cont…

• The operation of the ENS, the “brain of gut,” is involuntary.

• The ENS consists of about 100 million neurons in enteric plexuses that extend most of the length of the GI tract.

• Many of the neurons of the enteric plexuses function independently of the ANS and CNS to some extent, although they also communicate with the CNS via sympathetic and parasympathetic neurons.

• Sensory neurons of the ENS monitor chemical changes within the GI tract as well as the stretching of its walls.

• Enteric motor neurons control the secretions and contraction of GI tract smooth muscle to propel food through the GI tract.

PNS includes all nervous tissues outside the CNS. Its components are:

• Cranial nerves and their branches • Spinal nerves and their branches• Ganglia, and sensory receptors.

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Properties of NeuronSpecial characteristics of neurons– Longevity – can live and function for a lifetime– Amitotic— Do not divide– High metabolic rate – require abundant oxygen and

glucose

Properties of Neuron

– Excitability—the ability to respond to the stimulus and convert it into an action potential.

– Conductivity: The transmission of nerve impulse.– Secretion: When the electrical signal reaches the end

of a nerve fiber, the neuron secretes a chemical neurotransmitter that stimulates the next cell.

Action potential (nerve impulse) is an electrical signal that propagates (travels) along the surface of the membrane of a neuron.

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Parts of the Neuron

• Most neurons have three parts1. A Cell Body2. Dendrites3. An Axon• Cell Body or Perikaryon:– Contains nucleus and organelles

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Neuron Processes

• Dendrites: short, often highly branched.– Receptive regions of the neuron

• Axons: Long cytoplasmic process capable of propagating a nerve impulse– Neuron has only one axon– Transmits impulse away from soma toward another

neuron, muscle fiber or gland – Axon hillock: cone shaped elevation of Axon– Initial segment: the closest part to the axon hillock

– Axoplasm: Cytoplasm of axon– Axolemma: plasma membrane of axon– Axon terminals or telodendria: Terminal branches of axon

– synaptic end bulbs or synaptic knobs: Bulb shaped structures on the tip of axon terminals with synaptic vesicles filled with neurotransmitters

– Synapse: The site of communication between two neurons or a neuron and an effector.

A nerve fiber is a general term for neuronal process as

Dendrites and an axon.

Structural Classification of Neurons Multipolar – possess more than two processes• Numerous dendrites and one axon• Most neurons in the brain and spinal cord are of this type

Bipolar – possess two processes • One main dendrite and one axon• Rare neurons – found in some special sensory organs

Unipolar – possess a single short process

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Neurons Classified by Structure

Figure 12.10a–c

Neuroglia

• Neuroglia (glia=glue) are cells of the nervous system that perform various supportive functions.

• They are smaller but 5 to 50 times more than neurons.

• In contrast to neurons, they divide and multiply.• Brain tumors arise from neuroglia, called glioma.• There are six types of neuroglia.• Four types found in the CNS are astrocytes,

oligodendrocytes, microglia, and ependymal cells.• Other two types found in the PNS are Schwann, and

satellite cells.

Neuroglia of the CNS

Astrocytes (astro=star; cyte=cell)• These star shaped cells are the largest and most

numerous of the neuroglia.• The processes of astrocytes make contact with

blood capillaries, neurons, and the pia matter.

• Functions of Astrocytes: • Astrocytes provide a supportive framework.• Together with the endothelial cells of blood

capillaries they form a blood brain barrier (BBB) to isolate harmful materials from CNS.

• They help in nourishing neurons.• Regulate the chemical composition of tissue fluid.• Take up excess neurotransmitters.

Oligodendrocytes: (oligo=few; dendro=tree)• They resemble astrocytes but have fewer processes.• Their processes form myelin sheath around axons in

the brain and spinal cord.

Microglia:• They are small cells with thinner processes.• They function as phagocytes (macrophages).

Ependymal Cells (epen=above; dym=garment):• They are cuboidal to columnar cells.• Arranged in a single layer.• Possess microvilli and cilia. Functions:• They line cavities of brain and spinal cord.• Secrete cerebrospinal fluid (CSF) and circulate it.

Neuroglia of PNS

Schwann Cells:• They surround the axons in the PNS.• Each cell encircles (myelinates) only one axon.• They form myelin sheath around the axons.• They aid in regeneration of damaged axons of PNS. Satellite cells:• These flat cells surround the cell bodies of neurons of PNS

and form ganglia.• They regulate the exchange of materials between neuronal

cell bodies and interstitial fluid.

ANATOMICAL ORGANIZATION of the NERVOUS SYSTEM

NervousSystem

CNS PNS ANS

BRAIN

SPINALCORD

CRANIALNERVES

SPINALNERVES

SYMPATHETIC

PARA-SYMPATHETIC

Division of Nervous System

Nervous system Division