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The Nervous SystemThe Spinal Cord & Spinal Nerves

Chapter 9

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Parts of the nervous system are grouped according to structure or function

Structural Divisions

1. Central Nervous System (CNS):• Brain• Spinal Cord

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Peripheral Nervous System (PNS):

Cranial Nerves – carry impulses to and from the BRAIN

Spinal Nerves – carry messages to and from the SPINAL CORD

**Together, the CNS & PNS include all of the nervous tissue in the body

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Functional Divisions

Functionally, the nervous system is divided according to whether control is voluntary or involuntary

A tissue or organ that carries out a command from the nervous system is called an EFFECTOR

Effectors are our muscles and glands

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The Somatic Nervous System

Controlled voluntarily (by conscious will) and all the effectors are skeletal muscles

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The Autonomic Nervous System

Is an involuntary system and controls:

Smooth muscleCardiac MuscleGlands

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Neuroglia Cells of the nervous system that serve to

support & protect. Types with specialized functions include:

1. Protection of nervous tissue2. Supporting nervous tissue & binding to other

structures3. Aiding in the repair of cells4. Acting as phagocytes to remove pathogens5. Regulating fluids in between & around cells**Neuroglia continue to multiply throughout life

(unlike typical nerve cells)**Disadvantage is the potential of tumor growth

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Neurons

Functional cells of the nervous system Highly specialized cells Main portion (cell body) contains typical

nucleus & other organelles Distinguishing feature NOT found in any other

cells:Long, threadlike fibers that extend out from the

cell body & carry impulses across the cell called AXONS & DENDRITES

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Types of Neurons

Sensory Neurons – conduct impulses TO the spinal cord and brain

Motor Neurons – carry impulses from the Central Nervous System to the muscles and glands

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The Synapse

Neurons do not work alone

Impulses must be transferred between neurons to convey information

The synapse becomes the “point of junction” or “gap” where impulses are transferred

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Neurotransmitters

Chemicals contained in the end of the axon

They are released into the synapse to stimulate the next cell

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The Reflex Arc A complete pathway through the nervous

system Basic parts include:

1. Receptor – detects a stimulus

2. Sensory neuron – transmits the impulse TO the CNS

3. CNS – impulses are coordinated & a response is organized

4. Motor neuron – carries the impulse AWAY from the CNS

5. Effector – a muscle or gland that carries out a response

**The Patellar Reflex or “Knee-Jerk” Reflex

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The Myelin Sheath

An insulating layer that forms around nerves

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Disorders Involving the Spinal CordMultiple Sclerosis (MS) –

• Myelin sheath is damaged• Fibers degenerate• Although not fully understood, strong evidence

indicates a person’s own immune system attacks the myelin sheath:

Prior viral or bacterial infection Genetic make-up Environmental factors

• Different rate of progression• Episodes of relapse or remission

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Tumors

Commonly found in tissue in or around the spinal cord

Symptoms caused by pressure on the cord and spinal nerves

Symptoms include: pain, numbness, weakness and loss of function

Diagnosed through the use of MRI Treated by surgery and/or radiation (for

malignant tumors)

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Amyotrophic Lateral Sclerosis (ALS) Also known as “Lou Gherig’s Disease” Motor neurons are destroyed Progressive destruction that causes

muscle atrophy and loss of motor control Eventually, the person is unable to

swallow, talk or breathe Death follows soon after

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Poliomyelitis (Polio)

Viral disease of the nervous system Occurs commonly in children Gastrointestinal tract infection passes the virus into

the bloodstream – eventually spreading to the CNS Virus multiplies in motor neurons within the spinal

cord Leads to paralysis and death, if left untreated Vaccines have eliminated the disease in many

countries – Jonas Salk (1954)

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Spinal Cord Injuries

Occur when vertebrae are broken or dislocated (car accidents, diving accidents, etc.)

Because brain & spinal tissue cannot repair itself, severing of the spinal cord causes paralysis of all muscles supplied by nerves BELOW the level of the injury

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Paraplegia

Loss of sensation and movement in the lower portion of the body

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Quadriplegia

Loss of sensation and movement from the neck down