Essentials of Human Anatomy Nervous System I Dr Fadel Naim Ass. Prof. Faculty of Medicine IUG

Introduction The function of the nervous system, along with the endocrine system, is to communicate The nervous system is made up of the brain the spinal cord the nerves

Functions of Nervous System Sensory Function sensory receptors gather information information is carried to the CNS Motor Function decisions are acted upon impulses are carried to effectors Integrative Function sensory information used to create sensations memory thoughts decisions

Organization of the Nervous System Organized to detect changes in internal and external environments, evaluate the information, and initiate an appropriate response Subdivided into smaller “systems” by location: Central nervous system (CNS) Structural and functional center of entire nervous system Consists of the brain and the spinal cord Integrates sensory information, evaluates it, and initiates an outgoing response Peripheral nervous system (PNS) Nerves that lie in “outer regions” of nervous system Cranial nerves—originate from brain Spinal nerves—originate from spinal cord

Divisions of Peripheral Nervous System Sensory Division picks up sensory information and delivers it to the CNS Motor Division carries information to muscles and glands Divisions of the Motor Division Somatic – carries information to skeletal muscle Autonomic – carries information to smooth muscle, cardiac muscle, and glands

Divisions Nervous System

Organization of the Nervous System “Systems” according to the types of organs they innervate Somatic nervous system (SNS) Somatic motor division—carries information to the somatic effectors (skeletal muscles) Somatic sensory division—carries feedback information to somatic integration centers in the CNS

Organization of the Nervous System Autonomic nervous system (ANS) Efferent division of ANS—carries information to the autonomic or visceral effectors (smooth and cardiac muscles and glands) Sympathetic division—prepares the body to deal with immediate threats to the internal environment Parasympathetic division—coordinates the body’s normal resting activities Visceral sensory division—carries feedback information to autonomic integrating centers in the CNS

Organization of the Nervous System Afferent and efferent divisions Afferent division—consists of all incoming sensory pathways Efferent division—consists of all outgoing motor pathways

Sensory Division Somatic sensory components: General somatic senses: touch pain pressure vibration, temperature proprioception. Special senses: Taste Vision Hearing Balance smell

Sensory Division Visceral sensory components transmit nerve impulses from blood vessels and viscera to the CNS visceral senses primarily include: temperature stretch (of the organ wall).

Motor Division The somatic motor component (somatic nervous system; SNS): conducts nerve impulses from the CNS to skeletal muscles also known as the voluntary nervous system The autonomic motor component (autonomic nervous system; ANS): internal organs, regulates smooth muscle, cardiac muscle, and glands. Innervates Internal organs Regulates smooth muscle Regulates cardiac muscle Regulates glands also known as the visceral motor system or involuntary nervous system

Nerve Cells Nervous Tissue Two distinct cell types Neurons Glial cells excitable cells initiate and transmit nerve impulses Glial cells nonexcitable cells support and protect the neurons

The Neuron structural and functional unit of the nervous system. formed of nerve cell processes the dendrites the axon.

Characteristics of Neurons Neurons have a high metabolic rate. Neurons have extreme longevity. Neurons typically are non-mitotic.

Neuron Structure Neurons come in all shapes and sizes All neurons share certain basic structural features. typical neuron: Cell body (soma) Dendrites Axon

Neuron Structure – Cell Body The cell body the neuron’s control center responsible for: receiving integrating sending nerve impulses. Consists of: Plasma membrane Cytoplasm Nucleus with prominent nucleolus Chromatophobic substance (Nissil bodies): RER Free ribosomes

Cells of the Nervous System Components of neurons Axon A single process extending from the axon hillock, sometimes covered by a fatty layer called a myelin sheath Conducts nerve impulses away from the cell body of the neuron

Cells of the Nervous System Components of neurons Dendrites Each neuron has one or more dendrites, which branch from the cell body Conduct nerve signals to the cell body of the neuron Distal ends of dendrites of sensory neurons are receptors

Classifications of Neurons Neurons vary widely in morphology and location. classified based on structure function. Structural classification: number of processes extending from the cell body. unipolar neuron has a single process bipolar neurons have two processes multipolar neurons have three or more processes

Classification of Neurons – Structural Differences Unipolar one process ganglia Bipolar two processes eyes, ears, nose Multipolar many processes most neurons of CNS

Light micrograph of the gray matter of the spinal cord (×270). Observe the multipolar neuron (mN) cell bodies and their processes.

Classification of Neurons – Functional Differences Sensory Neurons afferent carry impulse to CNS most are unipolar some are bipolar Interneurons link neurons multipolar in CNS Motor Neurons multipolar carry impulses away from CNS carry impulses to effectors

Nerves Nerves are organs of the PNS. Sensory (afferent) nerves convey sensory information to the CNS. Motor (efferent) nerves convey motor impulses from the CNS to the muscles and glands. Mixed nerves: both sensory and motor Axons terminate as they contact other neurons, muscle cells, or gland cells. An axon transmits a nerve impulse at a specialized junction with another neuron called synapse.

Peripheral Nerves Organization – coverings: Epineurium wraps entire nerve Perineurium wraps fascicles of tracts Endoneurium wraps individual axons

Repair of Nerve Fibers Mature neurons are incapable of cell division; therefore, damage to nervous tissue can be permanent Neurons have limited capacity to repair themselves Nerve fibers can be repaired if the damage is not extensive, the cell body and neurilemma are intact, and scarring has not occurred

Regeneration of PNS Axons PNS axons are vulnerable to cuts and trauma. A damaged axon can regenerate if some neurilemma remains. PNS axon regeneration depends upon three factors. amount of damage neurolemmocyte secretion of nerve growth factors stimulates outgrowth of severed axons distance between the site of the damaged axon and the effector organ

Regeneration Of Neurones Regeneration in human nerves is a slow process. The rate of growth is about 1.5 mm per day in human, while in rabbits it is about 3 mm. per day. If: no infection partially cut axon neurolemma is intact regeneration starts in the nerve cell by resuming its normal shape and contents. The neurofibrils (axon sprouts) of the central stump will grow. They enter the neurolemmal tube and extend to the distal cut end of the nerve.

The Neuroglia no C.T. in the CNS instead there are the neuroglia. the supporting tissue between the neurones of the C.N.S.

Functions Of Neuroglia Support the neurones similar to the function of C.T. in other organs form the myelin sheaths around axons act as insulator between neurones nutritive function for neurones defend against inflammations help in regeneration of neurones secrete the C.S.F

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