NERVOUS TISSUE

DR Mubashar iqbal

Nervous System

• The master controlling and communicating system of the body

• Functions • Sensory input –the stimuli goes to CNS • Integration – interpretation of sensory input • Motor output – response to stimuli coming from CNS

Central nervous system (CNS) • Brain and spinal cord • Integration and command center

Peripheral nervous system (PNS) • Paired spinal and cranial nerves • Carries messages to and from the spinal cord and brain

PROTECTIVE LAYERS OF THE CNS( OR) MENINGES

• Nervous tissue is very delicate, bones, connective tissue layers, and a watery cerebrospinal fluid (CSF) surround and protect the brain and the spinal cord.

The CN-tissue are three membrane called meninges.

1.Dura Mater (dura mean hard):

The outermost meningeal layer is the dura mater, a tough, strong, and thick layer of dense connective tissue fibers. dura mater also lodges a number of sensory nerves and blood vessels.

• 2.Arachnoid Mater:

Deep to the dura mater is a more delicate thin layer connective tissue, the arachnoid mater. The dura mater and arachnoid mater surround the brain and spinal cord on their external surfaces.

3.Pia Mater:• The innermost meningeal layer is the delicate

connective tissue pia mater. This layer contains numerous blood vessels and adheres directly to the surfaces of the brain and spinal cord.

• Between the arachnoid mater and the pia mater is the subarachnoid space. Delicate,weblike strands of collagen and elastic fibers attach the arachnoid mater to the pia mater.

• Circulating in the subarachnoid space is the cerebrospinal fluid (CSF) that bathes and protects both the brain and spinal cord.

CEREBROSPINAL FLUID

• Clear, colorless fluid cushions and protects brain and spinal cord

• Continually produced by choroid plexuses in brain ventricles

• Important for homeostasis and brain metabolism• Reabsorbed into venous blood via arachnoid villi.• CSF circulates through the ventricles and around the

outer surfaces of the brain and spinal cord in the subarachnoid space. CSF also fills the central canal of the spinal cord.

MORPHOLOGY OF NEURON• The structural and functional cells of the nervous tissue

are the neurons. Although neurons vary in size and shape, a general structure of these cells can be described. Each neuron consists of soma or cell body, numerous dendrites, and a single axon.

• The cell body or soma contains the nucleus, nucleolus, numerous different organelles, and the surrounding cytoplasm or perikaryon.

• Projecting from the cell body are numerous cytoplasmic extensions called dendrites that form a dendritic tree.

• Surrounding the neurons are the smaller and more numerous supportive cells collectively called neuroglia. These cells form the non neural components of the CNS.

TYPES OF NEURONS IN THE CNS• This anatomic classification is based on the number of

dendrites and axons that originate from the cell body.

1.Multipolar neurons:

These are the most common type in the CNS and include all motor neurons and inter neurons of the brain, cerebellum, and spinal cord. Projecting from the cell body of a multipolar neuron are numerous branched dendrites and a single axon.

2. Bipolar neurons: These are not as common and are purely sensory neurons. Single dendrite and a single axon are associated with the cell body.

Bipolar neurons are found in the retina of the eye, in the organs of hearing and equilibrium in the inner ear, and in the olfactory epithelium in the upper region of the nose

• Unipolar neurons: Most neurons in the adult organism that exhibit only

one process leaving the cell body were initially bipolar during embryonic development. The two neuronal processes fuse during later development and form one process. The unipolar neurons (formerly called pseudounipolar neurons) are also sensory.Unipolar neurons are found in numerous sensory ganglia of cranial and spinal nerves.

Myelin Sheath and Myelination of Axons• Highly specialized cells present in both the CNS and

the PNS wrap around the axon numerous times to build up successive layers of modified cell membrane and form a lipid-rich, insulating sheath around the axon called the myelin sheath.

• Interspersed along the length of a myelinated axon are small gaps or spaces in the myelin sheath between individual cells that myelinated the axons. These gaps are called nodes of Ranvier. Axons in both the CNS and the PNS can be either myelinated or remain unmyelinated.

• In the PNS, all axons are surrounded by specialized Schwann cells that either myelinate the axons or envelope the unmyelinated axons. Schwann cells myelinate individual peripheral axons and extend along their length, from their origin to their termination in the muscle or gland

• There are no Schwann cells in the CNS. Instead, neuroglial cells called oligodendrocytes myelinate the axons in the CNS. Oligodendrocytes differ from Schwann cells in that the cytoplasmic extensions of one oligodendrocyte envelopes and myelinates numerous axons.

White and Gray Matter• The brain and the spinal cord contain gray matter and

white matter.• The gray matter of the CNS consists of neurons, their

dendrites, and the supportive cells called neuroglia. This region represents the site of connections or synapses between a multitude of neurons and dendrites. Gray matter covers the surface of the brain (cerebrum) and cerebellum.

• White matter in the CNS is devoid of neuronal cell bodies and consists primarily of myelinated axons, some unmyelinated axons, and the supportive neuroglial oligodendrocytes. The myelin sheaths around the axons impart a white color to this region of the CNS.

NEUROGLIA(Supporting Cells in the CNS)• Highly branched, supportive, non neuronal cells in

the CNS that surround the neurons, their axons, and dendrites. These cells do not become stimulated or conduct impulses, but are morphologically and functionally different from the neurons.

• The CNS contains approximately tenfold more neuroglial cells than neurons. The four types of neuroglia cells are astrocytes, oligodendrocytes, microglia, and ependymal cells.

1. Astrocytes• Are the largest and most numerous in gray matter

Consist of two types, fibrous astrocytes and protoplasmic astrocytes

• Both types abut on capillaries and neurons, and form blood brain barrier. Form glial limiting membrane that surrounds the brain and spinal cord

• Support metabolic exchange and contribute to energy metabolism of CNS. Control chemical environment around neurons by clearing neurotransmitters.

2. Oligodendrocytes

• Surround and myelinate numerous axons at one time, in contrast to Schwann cells.

3. Microglia• Part of the mononuclear phagocyte system and found

throughout CNS. Phagocytic cells in the CNS, similar to connective tissue macrophages

3.Ependymal Cells• Line the ventricles in the brain and central canal of

the spinal cord• Ciliated cells move the CSF through the central canal

of spinal cord.

SPINAL CORD• Thoracic region of spinal cord contains anterior, posterior,

and lateral gray horns. Lateral horns contain motor neurons of sympathetic division of autonomic nervous system

• Anterior horns of gray matter contain motor neurons. Axons from anterior horns form anterior roots of spinal nerves

• White matter contains closely packed ascending and descending axons

• Posterior columns of white matter contain fasciculus gracilis and fasciculus cuneatus

• Gray matter inside the spinal cord is H-shaped and contains neurons and interneurons

• Gray commissure connects two sides of the gray matter and contains the central canal

CEREBRAL CORTEX• Molecular layer (I): most superficial and covered by pia

mater; contains neuroglial cells and horizontal cells of Cajal

• External granular layer (II): contains neuroglial cells and small pyramidal cells

• External pyramidal layer (III): medium-sized pyramidal cells predominant type

• Internal granular layer (IV): thin layer with small granule, pyramidal cells, and neuroglia

• Internal pyramidal layer (V): contains neuroglial cells and largest pyramidal cells

• Multiform layer (VI): deepest layer, adjacent to white matter with various cell types