Nervous System

The Nervous System- nervous tissue

Nervous System

Body’s control center and communication network. It direct the functions of the body’s organs and systems.

Consists of the brain, spinal cord, cranial nerves, peripheral nerves and the sensory and motor structures of the body.

Central Nervous System

• Consists of brain and spinal cord.• The brain plays a central role in the control of

most bodily functions, including awareness, movements, sensations, thoughts, speech, and memory. Some reflex movements can occur via spinal cord pathways without the participation of brain structures.

• The spinal cord is connected to a section of the brain called the brainstem and runs through the spinal canal.

• The brain and spinal cord is composed of gray and white matter.

Parts of brain

• 1.Cerebrum : The largest part of the brain consisting of two hemispheres which controls voluntary actions and are the seats of intelligence, memory, imagination and will.

• 2.Cerebellum : The large mass having ridges and furrows attached to cerebrum, which regulates muscular movement of locomotion.

• 3.Medula Oblongata : The lowermost part of the brain which continues as the spiral cord in vertebral column. It control involuntary action.

Peripheral Nervous System

• Nerve Fibers• Axonal projections travel in bundles through the

body. These bundles are encapsulated in fibroconnective tissue in a manner similar to that seen in muscle tissue. Entire nerve bundles are surrounded by the epineurium. Branching from the epineurium and dividing the nerve bundle into fascicles is the perineurium. Finally each individual axon is surrounded by the endoneurium .

• Ganglions• Usually more than one nerve is needed to reach from the CNS to or

from the peripheral effector organs. These chains of nerve fibers interconnect in structures called ganglions .Within the ganglion very large cells are visible. These are the cell bodies of the neurons Within the cell body you should be able to see the Nissl substance, an accumulation of the basophilic stain of ribosomes.The cell bodies are supported by small surrounding cells called capsule cells or amphicytes. Between this arrangement, you will find a number of bundles of axonal projections. Ganglions of the sympathetic system are easily distinguished from others by the presence of lipofuscin pigment within the cytoplasm of the cell bodies.

• The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain

• The spinal cord begins below the medulla and ends just above the small of the back at the conus medularis. The area within the vertebral column beyond the end of the spinal cord is called the cauda equina.

Functions of Nervous System

• Control of all body functions

• Coordination of different body organs

Properties of Nervous System

• Sensitivity

• Conductivity

• Responsiveness

Neurons

• Basic building blocks of the nervous system.• Information-processing units of the brain

responsible for receiving and transmitting information.

Parts of a Neuron

• Dendrites- treelike extension at the beginning of a neuron that help increase the surface area of the cell body and are cover with synapses.

• Axon- the elongated fiber that extends from the cell body to the terminal endings and transmit the neural signal.

• Synaptic cleft- small gap that separates the presynaptic neuron and the postsynaptic neuron. It creates the physical barrier for the electrical signal carried by one neuron to be transferred to another neuron

Classification of neuronsStructural classification based on number of processes coming off of the cell body:

Anaxonic neurons

• no anatomical clues to determine axons from dendrites

• functions unknown

Multipolar neuron

• multiple dendrites & single axon

• most common type

Bipolar neuron• two processes coming off cell

body – one dendrite & one axon

• only found in eye, ear & nose

Unipolar (pseudounipolar) neuron

• single process coming off cell body, giving rise to dendrites (at one end) & axon (making up rest of process)

Classification of neuronsFunctional classification based on type of information & direction of information transmission:• Sensory (afferent) neurons –

• transmit sensory information from receptors of PNS towards the CNS

• most sensory neurons are unipolar, a few are bipolar

• Motor (efferent) neurons –

• transmit motor information from the CNS to effectors (muscles/glands/adipose tissue) in the periphery of the body

• all are multipolar

• Association (interneurons) –

• transmit information between neurons within the CNS; analyze inputs, coordinate outputs

• are the most common type of neuron (20 billion)

• are all multipolar

CNS component

• Grey matter (or gray matter) is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil, glial cells and capillaries. Grey matter contains neural cell bodies, in contrast to white matter, which does not and mostly contains myelinated axon tracts

• White matter is one of the two components of the central nervous system and consists mostly of glial cells and myelinated axons that transmit signals from one region of the cerebrum to another and between the cerebrum and lower brain centers.

Neuroglia

- Aka Glial cells.- From the greek word ‘glue’, discovered by a german

pathologist Rudolf Vichow in 1846.- Non- neuronal cells that maintain hemeostasis. It provides

support and protection for neurons in the brain.- In human brain there are 10-100 glial cell for every neuron.- They form a major component of the nerve tissue inc. the ff: Supporting cells of CNS (astrocytes, microglia,

oligodendrocytes) Supporting cells of PNS (neurolimma and satellite cells)

2 TYPES OF NEUROGLIA• CNS Neuroglia

Astrocytes

Oligodendrocytes

Microglia

Ependymal cells

• PNS Neuroglia

Schwann cells (neurolemmocytes)

Satellite cells

NAME DESCRIPTION FUNCTION

Astrocytes (CNS)

These cells are star-shaped because of their numerous processes radiating in all directions.

• Create supportive framework for neurons• Create “blood-brain barrier”• Monitor & regulate interstitial fluid surrounding neurons• Secrete chemicals for embryological neuron formation• Stimulate the formation of scar tissue secondary to CNS injury

Oligodendrocytes(CNS)

These cells have fewer cell processes. According to their

distribution:

The intrafascicular cells are found in the myelinated tracts.

The perineuronal cells are seen on the surface of the somata of

neurons.

Create myelin sheath around axons of neurons in the CNS. Myelinated

axons transmit impulses faster than unmyelinated axons

Ependymal cells(CNS)

Also named ependymocytes, form the surface layer of the wall of the

system of interconnected, fluid-filled cavities of the brain and spinal cord,

•Line ventricles of brain & central canal of spinal cord• These cells create and secrete cerebrospinal fluid (CSF) and beat their cilia to help circulate it

Microglia(CNS)

These are the smallest of the glial cells which have a flattened cell

body with a few short, fine processes. They are often related to

capillaries, and are said to be phagocytic in nature.

• “Brain Macrophages”• Phagocytize cellular wastes & pathogens

Schwann cells(PNS)

Principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle.

• Surround all axons of neurons in the PNS creating a neurilemma around them. Neurilemma allows for potential regeneration of damaged axons• Creates myelin sheath around most axons of PNS

Satellite cells(PNS)

Satellite cells are small cells that line the exterior surface

of PNS neurons and help regulate the external

chemical environment.

Support groups of cell bodies of neurons within ganglia of the

PNS

Cerebrum

• The cerebrum, also known as the telencephalon, is the largest and most highly developed part of the human brain.

• It encompasses about two-thirds of the brain mass and lies over and around most of the structures of the brain.

• The outer portion (1.5mm to 5mm) of the cerebrum is covered by a thin layer of gray tissue called the cerebral cortex.

Layer of Cerebral cortex

•  Layer I is the molecular layer, which contains very few neurons;

• layer II the external granular layer; • layer III the external pyramidal layer; • layer IV the internal granular layer;

• layer V the internal pyramidal layer; and• layer VI the multiform, or fusiform layer.

• The frontal lobes are located in the front of the brain and are responsible for voluntary movement and, via their connections with other lobes, participate in the execution of sequential tasks; speech output; organizational skills; and certain aspects of behaviour , mood, and memory.

• The parietal lobes are located behind the frontal lobes and in front of the occipital lobes. They process sensory information such as temperature, pain, taste, and touch. In addition, the processing includes information about numbers, attentiveness to the position of one’s body parts, the space around one’s body, and one's relationship to this space.

• The temporal lobes are located on each side of the brain. They process memory and auditory (hearing) information and speech and language functions.

Histology of the Cerebral Cortex

Stellate (Granule) Cells• These come in a wide assortment of shapes.

• They are typically small (< 10 micrometres) multipolar neurons.

• Their short axons do not leave the cortex.• Stellate cells are the principal

interneurons of the neocortex.

Pyramidal Cells• the principal output neurons.

• these are numerous small projections that are the preferential site of synaptic contact.

• range in size from 10 micrometres in diameter to 70-100 micrometres

Horizontal Cells (of Cajal)• A small fusiform cell found in

the superficial layer of the cerebral cortex with its long axis placed horizontally.

• These are prominent during development but most

disappear after birth.

Fusiform Cells

• These are found in the deepest cortical layer.

• They are spindle-shaped with a tuft of dendrites emerging from each end of the spindle.

• They are, however, otherwise like pyramidal cells with an axon that leaves the cortex.

Cells of Martinotti

• These cells are found in all cortical layers but are more abundant in the deeper cortical layers.

• They are unusual in the fact that they have an axon that ascends to the surface.

Cerebellum• “little brain”• Attached to the brainstem by

several large connections called cerebellar peduncles

• Consists of gray nuclei and white nerve tracts

• plays an important role in motor control

• may also be involved in some cognitive functions

THREE LAYERS OF CEREBERALL CORTEX

A. Molecular Layer - outermost layer- mainly a synaptic area

with relatively few nerve cell bodies

- Cell bodies: stellate cells, basket cells

- axons and dendrites of deeper cells (Purkinje and golgi).

B. Purkinje Layer-situated between molecular and granule layerCell bodies: only Purkinje cells

Purkinje cells - single axon, very large, flask-shaped cells with

huge dendritic arborizations extending upward to the molecular layer

-dendrites spread out in a fan-like fashion

C. Granule Cell Layer-innermost layer-contains a dense

accumulation of very small neurons.

Cell bodies: granule cells, Golgi cells

-climbing fibers and mossy fibers are also found in this layer

Meninges

• Dura Matter• Arachoid Matter• Pia Matter

The Meninges - Connective Coverings of the Brain

• The brain is enclosed in three layers of connective tissue. The outer most (dura mater) consists of dense connective tissue. Underlying the dura is the arachnoid layer, often described as a "roof with pillars" made of dense connective tissue. Spaces within the arachnoid are filled with cerebrospinal fluid. The inner most layer, the pia mater, consists of loose connective tissue on the surface of the brain and lining channels which penetrate the brain carrying the vascular system (Pia Mater).