Nervous System Chapter 7



Nervous System Chapter 7

Functions of Nervous System

1. Uses millions of sensory receptors to monitor changes in/out of body.

2. Processes and decides what to do at each moment.

3. Effects a response by activating muscles or glands

Structural classification of nervous system

1. Central nervous system – brain, spinal cord- interpret incoming sensory information and issue instructions. (CNS)

2. Peripheral nervous system – nerves that extend from brain and spinal cord. (PNS)

Functional classification – concerning only w/PNS two subdivisions

1. Sensory, afferent division – nerves that carry impulses to CNS from sensory receptors.

2. Motor, efferent division – nerves that carry impulses away from the CNS to organs, muscles and glands.

a. Somatic nervous system – voluntary nervous system, allows control of skeletal muscles.

b. Autonomic nervous system – involuntary nervous system – such as activity of smooth and cardiac muscles and glands

1. sympathetic

2. parasympathetic

Nervous Tissue: structure and function

Two types of cells: supporting cells and neurons

1. supporting cells in CNS – neuroglia

a. astrocytes – star shaped – many projection connected to nerve cells anchoring it to blood capillaries. (Protect brain from substances in blood.)

b. Microglia – spider like – phagocytes – dispose of debris.

c. Ependymal cells – line cavities of brain and spinal cord. Their cilia beat and move cerebrospinal fluid.

d. Oligodendrocytes – wrap around nerve fibers producing fat insulating coverings called myelin sheaths.

2. Supporting cells in PNS – two major varieties:

a. schwann cells – form myelin sheaths around nerve fibers that extend from CNS.

b. Satellite cells – act as generally protective, cushioning cells.

Difference between neuroglia and neurons

1. neuroglia not able to transmit nerve impulses. Neurons transmit impulses .

2. neuroglia never lose ability to divide, neurons do.

3. Neurons – nerve cells

a. cell body – contains nucleus

b. Process or fiber – one or more extending from cell body.

1. axon – generate nerve impulses and conduct them away from cell body.

2. Dendrites – conduct impulses toward cell body

• may have hundreds of dendrites but only one axon.

3. axonal terminals – thousands of endings on the axon.

4. Neurotransmitter – released by axonal terminals into extracellular space when impulse reaches terminal.

5. Synaptic cleft – tiny gap between neurons.

6. Synapse – junction between neurons.

• neurons do not touch each other. (synaptic cleft is between them.)

7. myelin – whitish fatty material, protects and insulates fibers, increasing transmission rate of nerve impulses.

8. Schwann cells – wrap around axon forming myelin sheath.

9. Neurilemma – part of schwann cell external to myelin sheath.

10. Node of Ranvier – gaps between schwann cells.

11. Ganglia – collection of cell bodies.

12. Tracts – nerves running through CNS.

13. White matter – dense collection of myelinated fibers.

14. Gray matter – unmyelinated fibers.

Functional Classification – grouped according to the direction the nerve impulse is traveling relative to the CNS. There are three types: 1. Sensory 2. motor 3. association neurons.

1. Sensory (afferent) – carry impulses toward the central nervous system. Cell bodies of these are always found in ganglion outside the CNS.

a. Sensory receptors – dendrite endings of the neuron are associated with special receptors. 1. Complex receptors of special sense organs. 2. cutaneous sense organs – found in skin (pain receptors – bare dendrite endings – most numerous) 3.proprioceptors – in muscles and tendons-

constantly advise the brain on your movement

2. Motor – (efferent) – carry impulses away from the CNS. The cell bodies of the motor neurons are always found in the CNS.

3. Association Neurons –( interneurons) – They connect the motor and sensory neurons. Their cell bodies are always located in the CNS.

Structural Classification – based on the number of processes extending form the cell body. 1. Multipolar –several processes; most common, all motor and association neurons.

2. bipolar neurons – two processes – an axon and a dendrite. 3. Unipolar – single process from the cell body.

Physiology Nerve Impulses

Two major functions:

1. irritability – respond to stimulus and convert to impulse.

2. Conductivity – ability to transmit the impulse to other neurons and muscles and glands.

Irritability

1. plasma membrane of resting neuron is polarized. (fewer ions + inside neuron than in fluid around). K+ (potassium) – major ion inside, Na + (sodium) major ion outside.

• When inside neuron is negative and outside is +, the neuron is inactive.

2. neuron is stimulated (light, touch, etc.)

3. “sodium gates” in membrane of neuron open.

4. Sodium rushes into neuron.

5. Polarity changes- inside is now more +, outside is less +. (depolarization)

6. This activates neuron to transmit nerve impulses (action potential).

7. Impulse always runs entire length of axon. (never part way)

8. K+ rush out into tissue fluid. Restores cell to polarized condition (repolarization)

• repolarization must occur before another impulse can be conducted.

Conductivity

1. action potential reaches axonal endings.

2. Axon terminal releases a neurotransmitter chemical.

3. Diffuses across synapse and binds to receptors on another neuron.

4. When enough neurotransmitter is released the process described in 1-8 under irritability will occur.

Central Nervous System

Brain – 4 major sections

1. cerebral hemisphere

2. diencephalon

3. brain stem

4. cerebellum

Regions of cerebral hemisphere to know:

1. parietal lobe – somatic (body) sensory area. – left side of sensory cortex receives impulses from right side of body.

2. occipital lobe – visual area

3. temporal lobe – auditory area, olfactory area.

4. frontal lobe – primary motor area. (mouth, face, hands)

- Broca’s area – involved in ability to speak

- Higher intellectual reasoning – anterior part of frontal lobe.

- Speech area – junction of temporal, parietal, and occipital lobe.

- Language comprehension – also in frontal lobe

5. Cerebral cortex- outermost gray matter of cerebrum

6. Corpus callosum- deeper cerebral white matter. Connects the cerebral hemisphere.

7. Basal nuclei- buried deep within white matter; islands of gray matter- help regulate voluntary muscle activity

Diencephalon- interbrain- sits on the top of brain stem; enclosed by cerebral hemisphere. Major structures are: thalamus, hypothalamus, epithalmus.

*Thalamus- relay station for sensory impulses passing upward to the sensory cortex.

*Hypothalamus- makes up floor of Diencephalon. Plays a role in the regulation of body temperature, metabolism and H2O balance thirst and appetite- regulates pituitary gland.

*Epithalamus- forms roof of 3rd ventricle- important parts: pineal body, choroid plexus. Choroid plexus- forms cerebral spinal fluid.

Brain stem- includes- midbrain, pons, and medulla oblongata- provides pathway for ascending and descending tracts- control vital activities such as breathing and blood pressure.

*Midbrain- contains cerebral aqueduct which connects 3rd and 4th ventricle.

*Pons- rounded structures- area that controls breathing.

*Medulla Oblongata- most inferior part of brain stem- controls functions such as heart rate, blood pressure, breathing, swallowing, vomiting.

Protection of the Central Nervous System

Meninges- protective covering in CNS

1. dura mater- outermost layer – double layered where it surrounds the brain.

2. Arachnoid mater – weblike middle layer – creates subarachnoid space – fills with cerebrospinal fluid.

3. Pia mater – inner most layer – clings tightly to brain and spinal cord.

- arachnoid villi – protrude through the dura mater. Specialized projections out of the arachnoid membrane.

- Cerebrospinal fluid – similar to plasma formed from blood by choroid plexuses. – provides a watery cushion.

Pathway of Cerebrospinal fluid through the brain – from two lateral ventricles (cerebral hemispheres) into 3rd ventricle, then through the cerebral aqueduct of the midbrain to the 4th ventricle. Some fluid reaching the 4th ventricle continues down central canal of spinal cord. – most moves from 4th ventricle into subarachnoid space. Spinal fluid then returns to blood through dural sinuses.

Spinal Cord - about 17 inches long, glistening white, continuation of the brain stem.

- Provides a two way pathway to and from the brain

- It is enclosed in the vertebral column

- Extends from the foramen magnum to the 1st or 2nd lumbar vertebrae.

- It is protected by meninges.

- The meningeal coverings extend past the 2nd lumbar vertebrae. This is a good area to remove CSF if needed for testing.

- 31 pairs of spinal nerves arise from the cord

- the lowest spinal nerves travel to the end of the vertebral column before they exit.

- Cauda equina – collection of spinal nerves at the inferior end of the vertebral canal (looks like horses tail). Pg 218

- Spinal cord has gray matter and white matter.

- Gray matter is in the inner part (shaped like a butterfly) * dorsal (posterior) horns – two posterior projections of the butterfly shape. They contain association neurons or interneurons. * Ventral (anterior) horns – two anterior projections of the butterfly shape. They contain the cell bodies of the motor neuron of the somatic (voluntary) nervous system. This gray matter region surrounds the central canal which contains CSF. Coming off of the spinal cord are the dorsal root and the ventral root. The dorsal root has an enlarged area called the dorsal root ganglion. The cell bodies of the sensory neurons are found in the dorsal root ganglion. The dorsal and the ventral root fuse to form the spinal nerves.

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