The Nervous System- Nervous Tissue

The Nervous system has three major functions :  Sensory – monitors internal & external environment through presence of receptors  Integration – interpretation of sensory information (information processing); complex (higher order) functions  Motor – response to information processed through stimulation of effectors. Such as  muscle contraction  glandular secretion

Enteric Nervous System gutgut Central Nervous System (CNS) brainbrain spinal cordspinal cord Peripheral Nervous System (PNS) cranial nerves (12 pr)cranial nerves (12 pr) spinal nerves (31 pr)spinal nerves (31 pr) All the neural tissue outside CNS Afferent division (sensory input) Efferent division (motor output)  Somatic nervous system  Autonomic nervous system

General Organization of the nervous system Brain & spinal cord

Histology of neural tissue 2 types of neural cells in the nervous system:  Neurons - For processing, transfer, and storage of information.  Neurons are either sensory, interneurons or motor neurons.  Neuroglia – For support, regulation & protection of neurons

CNS neuroglia: astrocytes oligodendrocytes microglia ependymal cells PNS neuroglia: Schwann cells (neurolemmocytes) satellite cells Histology of neural tissue ………contd.

Astrocytes 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 create myelin sheath around axons of neurons in the CNS. Myelinated axons transmit impulses faster than unmyelinated axons

Microglia “brain macrophages” phagocytize cellular wastes & pathogens

Ependymal cells line ventricles of brain & central canal of spinal cord produce, monitor & help to circulate CSF (cerebrospinal fluid)

Schwann cells 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 support groups of cell bodies of neurons within ganglia of the PNS

All neurons share certain basic structural features. A typical neuron is composed of 3 basic parts: 1.Cell body 2. Dendrites 3. Axon Neuron structure

Neuron Structure……..Cell Body  The cell body is the neuron’s control center.  It is responsible for receiving, integrating & sending nerve impulses.  It consists of – Plasma membrane Cytoplasm Nucleus with prominent nucleolus Nissl bodies (large granular body found in neurons, release chemicals, namely proteins ) Free ribosomes

Neuron Structure………Dendrites  Shorter & smaller process of the neuron.  Most neurons usually have more than 1 dendrites. But Some neurons have only one dendrite.  Branch off the cell body.  Dendrites conduct nerve impulses towards the cell body, they receive input & transfer input to the cell body for processing.

Larger & longer process of the neuron. Also Known as nerve fiber. Extend from the cell body. Most neurons have only 1 axon except Anaxonic. The axon transmits nerve impulse away from the cell body toward another cell. Neuron Structure………Axon

Neuron structure

 is the process by which part of an axon is wrapped with a myelin sheath. It is actually a protective fatty coating. It has a glossy- white appearance.  Most axons of the nervous system are surrounded by a myelin sheath (myelinated axons)  The presence of myelin speeds up the transmission of action potentials along the axon  The myelin sheath supports, protects & insulates an axon.  In the insulated portion of an axon, no voltage change occur across the membrane. Voltage change occurs at the nodes. Myelination

 Myelin will get laid down in segments (internodes) along the axon, leaving unmyelinated gaps known as “nodes of Ranvier” 1.Regions of the nervous system containing groupings of myelinated axons make up the “white matter”. Found in the PNS & tracts/pathways in the CNS. 2.“Grey matter” is mainly comprised of groups of neuron cell bodies, dendrites, synapses (connections between neurons) unmyelinated axons & neuroglia. Found in ganglia of the PNS & nuclei/centers in the CNS. Myelination…… contd.

Neural Tissue Organization

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

Anaxonic neurons No anatomical clues to determine axons from dendrites Functions unknown Bipolar neurons Two processes coming off cell body– one dendrite & one axon only found in eye, ear & nose

Multipolar neurons multiple dendrites & single axon most common type Unipolar (pseudounipolar) neurons  Single process coming off cell body, giving rise to dendrites (at one end) & axon (making up rest of process)

Classification of neurons Functional 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

SYNAPSES  Synapse are the site of functional contact between two excitable cells.  Synapses are electrical or chemical.  A chemical synapse produces only one-way information transfer-from a presynaptic neuron to a postsynaptic neuron.  Axon terminals contain synaptic vesicles filled with neurotransmitter molecules.  An excitatory neurotransmitter depolarizes and an inhibitory hyperpolarizes.

Conduction across synapses Most synapses within the nervous system are chemical synapses & involve the release of a neurotransmitter. In order for neural control to occur, “information” must not only be conducted along nerve cells, but must also be transferred from one nerve cell to another across a synapse.

The Structure of a Typical Synapse

Synapses SSynapses are the site of functional contact between two excitable cells. TTypes of Synapses (based on contacts) – 1. Axo-dendritic (one between the axon of one neuron and the dendrites of another) 2. Axo-somatic (one between the axon of one neuron and the body of another) 3. Axo-axonic (one between the axon of one neuron and the axon of another neuron) SSynapses may be of 2 types, electrical or chemical. MMost synapses within the nervous system are chemical synapses & involve the release of a neurotransmitter. AA chemical synapse produces only one-way information transfer from a presynaptic neuron to a postsynaptic neuron. AAxon terminals contain synaptic vesicles filled with neurotransmitter molecules. AAn excitatory neurotransmitter depolarizes and an inhibitory one hyperpolarizes.

Types of Synapses

Multiple Sclerosis Autoimmune disease Destruction of myelin sheath Scar tissue may form

Epilepsy Rapid synchronous firing of neurons Seizure Treatments: Drugs Implants Brain surgery