Understanding the Nervous System: Structure and Function

1. Nervous system • One of the two main communication systems in the body • Coordinates all the activities of the body carrying messages from one cell to the next • Enables the body to respond and adapt to changes that occur inside and outside the body • Basic structural unit is the NEURON or nerve cell

2. The Nervous System Each neuron consists of: • The cell body. • Dendrites. • The axon.

3. Neurons • Cell body consists of nucleus • Dendrites consist of nerve fibers that carry impulses toward the cell body • Axon is a single nerve fiber that carries impulses away from the cell body • Myelin sheath covers the axon (fat covering) that insulates and maintains the axon • Nodes of Ranvier are areas where no myelin is present

4. The axon of one neuron lies next to the dendrites of another neuron • The spaces between them are called synapses • Impulses coming from one axon “jump” the synapse to get to the dendrite of another neuron which carry it in the right direction • Special chemicals, neurotransmitters, found at the end of each neuron allow impulses to pass from one neuron to another so impulses can follow many different routes

5. Nerve fibers • Nerves are a combination of many nerve fibers found in the brain and spinal cord • 3 types: • Afferent – sensory nerves • Nerves carry messages from body to the brain and spinal cord • Efferent – motor nerves • carries messages from the brain and spinal cord to the body • Associative - interneurons • Carries impulses from the sensory neuron to the motor neuron

6. 3 Divisions Central nervous system • Brain • Spinal cord Peripheral nervous system • Nerves • Autonomic nervous system • Controls involuntary body functions Autonomic nervous system • Includes peripheral nerves and ganglia (group of cell bodies outside the CNS that carry impulses to involuntary muscles and glands)

7. Central nervous system • BRAIN – mass of nerve tissue protected by membranes and skull • Cerebrum • Largest and highest section • Has convolutions (folds) that separates the lobes • 4 lobes: • Frontal • Parietal • Temporal • Occipital

8. Frontal lobe • Links all components of behaviors • Impulse control • Injury here may cause lose of function on one side of the body • Speech may become halted or disorganized • Personality changes can occur • Social rules are disregarded • Sustained attention and “insight” are affected

9. Temporal lobe • Perceives and recognizes verbal material • Most commonly injured • Causes misunderstandings in what is said • Emotional changes such as unexplained panic or tearfulness can occur • Left temporal lobe involved in production of speech, naming and verbal memory • Right temporal lobe involves musical ability, foreign language, visual memory and comprehension of the environment

10. Parietal lobe • Construction of language • Recall of long term memories may be mixed up in time and sequencing • Easily lost or confused about left/right • Difficulty recognizing and naming what they see • May affect ability to read, write or perform calculations • Conscious sensation and voluntary movement is affected • Injury to front part of the lobe may cause loss of body sensation

11. Occipital lobe • Injury usually results in blindness to part or all of the visual field • People may experience “blind spots” or “holes” • May misperceive pictures they see • Recognition of colors may be distorted

12. Cerebellum • Below cerebrum • Responsible for • muscle coordination, • balance and posture, • muscle tone • Diencephalon • Section between the cerebrum and midbrain • Contains 2 structures • Thalamus – acts as a relay center and directs sensory impulses to the cerebrum • Hypothalamus – regulates and control the autonomic nervous system, temperature, appetite, water balance, sleep, etc. Also involved in emotions such as fear, anger, pleasure, pain and affection

13. Midbrain • Below the cerebrum and top of the brain stem • Responsible for conducting impulses between brain parts and certain eye and auditory reflexes • Pons • Below the midbrain and in the brain stem • Responsible for conducting messages to other parts of the brain, chewing, tasting, saliva production and assists with breathing • Medulla oblongata • Lowest part of the brain stem • Connects with the spinal cord and is responsible for breathing, heartbeat, swallowing, coughing and blood pressure

15. Spinal cord • Continues down from the medulla oblongata • Ends at the first or second lumbar vertebrae • Surrounded and protected by vertebrae • Responsible for many reflex actions and carrying afferent and efferent nerves • Meninges • 3 membranes that cover and protect the brain and spinal cord • Dura mater – outer layer • Arachnoid membrane – middle, delicate layer • Pia mater – attached to the brain and contains blood vessels that nourish the nerve tissue

16. The brain has 4 ventricles (hollow spaces that connect with each other and the space under the arachnoid membrane) • Filled with fluid called cerebrospinal fluid • Circulates continuously • Serves as shock absorber to protect brain and spinal cord • Carries nutrients to parts of the CNS and helps remove waste products

17. Peripheral Nervous System • Consists of the somatic and autonomic nervous systems • Somatic nervous system • 12 pairs of cranial nerves • 31 pairs of spinal nerves • Spinal nerves carry messages to and from the spinal cord and are mixed nerves (both afferent and efferent)

18. Autonomic Nervous System • Branch of peripheral nervous system • Maintains balance in the involuntary functions of the body and allows the body to react in times of emergency • 2 divisions: • Sympathetic • Parasympathetic • Work together to maintain homeostasis

19. In times of emergency the sympathetic nervous system prepares the body to act • Increased heart rate • Increased respirations • Increased blood pressure • Decreased digestion • After the emergency, the parasympathetic nervous system counteracts actions of the sympathetic nervous system • Decreased heart rate • Decreased respirations, etc.

21. Diseases/disorders • Traumatic disorders – caused by injury: • Concussion. • Brain contusion. • Congenital disorders – present at birth: • Spina bifida. • Hydrocephalus. • Cerebral palsy. • Cerebrovascular accident (CVA). • Encephalitis. • Epilepsy. • Meningitis. • Multiple sclerosis (MS) • Neuralgia. • Paralysis. • Parkinson’s disease. • Dementia. • Alzheimer’s disease.

22. Epilepsy • Seizure syndrome • Disorder associated with abnormal electrical impulses in the neurons of the brain • Causes • Trauma • Toxins • Idiopathic (spontaneous) • Treated with medications

24. Parkinson’s disease • Chronic, progressive degeneration of brain cells usually in people over 50 y.o. • Tremors, stiffness, muscular rigidity, forward leaning position, shuffling gait, mood swings, etc. • Treatment is supportive and symptomatic

25. Shingles • Herpes zoster • Inflammation of nerve cells caused by herpes virus • Same virus that causes chicken pox • Occurs in thoracic area on one side of the body and follow path of affected nerves • Itching, fever, increased skin sensitivity • Treatment is to relieve pain and itching until inflammation subsides

26. Dementia • General term defining a loss in at least two areas of complex behavior such as language, memory, visual/spatial abilities or judgment • Does not mean that everyone that is forgetful has dementia

27. Alzheimer’s disease • Progressive disease where initial symptom is usually a problem with remembering recently learning information • Nerve endings in the cortex of the brain degenerate and block signals • Cause is unknown and has 3 stages: • 1st stage lasts 2-4 years and involves short-term memory loss, anxiety and poor judgment • 2nd stage lasts 2-10 years and increases memory loss, difficulty recognizing people, motor problems, loss of social skills • 3rd stage lasts 1-3 years and includes inability to recognize self, weight loss, seizures, mood swings, and aphasia (loss of speech).