Essentials of Human Anatomy Nervous System I
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
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 sensorycomponents • 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 • 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 • theaxon.
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
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