I



Nervous System – Neurology – study of

A. Functions – control center

1. sensory – senses change within the body and outside environment

2. integrate or interpret – interprets change

3. response – motor function - initiates action in the form of muscular contraction or glandular secretions

B. Organization – 2 principal divisions

1. Divisions -

a. Central nervous system – CNS – brain and spinal cord – control center for entire nervous system

- sensory info is integrated and correlated, thoughts and emotions are generated and muscles are stimulated to contract and glands to secrete

b. Peripheral nervous system – PNS – nerves from brain cranial and spinal cord – spinal carry inward from receptors and outward from CNS

1.SNS – somatic nervous system – sensory neurons from cutaneous and sensory receptors in head, body wall, and extremities to CNS and motor neurons from CNS to skeletal muscles only - voluntary

2. ANS –autonomic nervous system – contains sensory neurons that convey homeostatic info from receptors to viscera of CNS and motor neurons from CNS to smooth muscle, cardiac muscle and glands – involuntary

- sympathetic – neurons involve

expediture of energy

➢ speeds heart beat

- parasympathetic – neurons – restore and conserve body energy

➢ slows heart beat

C. Histology –

1. Cell types – only 2

a. Neuroglia – glial cells – 6 types (exhibit 9.1)

- support and protect neurons

- smaller than neurons and out number them by 5-10 times

- common source of tumors (gliomas) –

▪ 40-45% of all brain tumors

b. Neurons – conduct impulses from one part of body to another

(fig 8.2 pg 217)

- cell body – well defined nucleus and nucleolus surrounded by cytoplasm along with another organelles – no mitotic apparatus in adults

- dendrites – short, thick highly branched extensions of cytoplasm

o (neuron usually has several)

o receive impulses and conduct toward cell body

- axon – single long thin extension that sends impulses to another neuron or tissue

o vary from 1mm – 1meter

o has side branches called synaptic terminals or knobs that secrete neurotransmitters

- myelin – multi layered lipid and protein covering of axons (called myelin sheath)

- electrically insulates the axon and increases the speed of conduction

- some are unmyelinated – no sheaths

- 2 types of neuroglea produce myelin sheaths – page 218

- neurolemmocytes (schwann cells) form sheaths around axons during fetal development and 1st term of life – PNS only ** can regenerate

- Oligodendrocytes myelinated axons of the CNS cannot regrow ( no neurolemma)

❖ Myelin increases from birth to maturity and greatly increases the speed of impulse conduction

❖ since it is still in progress during infancy, the infant has slow reflexes and lack coordination

❖ MS (pg 224) and Tay-Sachs disease

2. Grouping of neural tissue –

a. Nerve fiber- any process projecting from the cell body

• axon or dendrite

b. nerve – group of many fibers in PNS

• Sciatic nerve of thigh

• Most nerves have motor and sensory fibers

c. ganglia – group of cell bodies (neurons) and synapses in the PNS

d. tract – bundle of fibers in the CNS

• May run long distances up and down the spinal cord or connect parts of the brain

• Ascending tract carries impulses upward

• Descending tracts carry impulses downward

e. white matter – groups of myelinated axons from many neurons

• looks white

f. gray matter – neuron cell bodies & dendrites or or unmyelinated axons (bundles)

• found covering outer surface of the brain and in the deeper regions called nuclei (similar to ganglion but has unmyelinated dendrites) ex. Horns of spinal cord

3. Classification of neurons-

a. structural –based on # of processes extending from cell body

- multipolar – several dendrites – 1 axon

most neurons in the brain and spinal cord are like this

- bipolar – 1 dendrite and 1 axon

found in retina of eye, inner ear and nose

- unipolar – have only 1 process that

divides into a central branch which functions as axon and a peripheral branch - dendrite

- originate as bipolar but fuse during development

- found in the dorsal root ganglia (sensory) of spinal nerves

b. functional- based on the direction in which they transmit nerve impulses

- sensory (afferent) – transmit form receptors in skin, sensory organs muscles, joints, and viscera to the brain and spinal cord

- motor (efferent) – convey impulses from brain and spinal cord to effectors which may be muscles or glands

- association (interneurons) – carry impulses from sensory neurons to motor neurons and are located in the brain and spinal cord only – makes up most neurons of humans

I. Functions –

A. Nerve Impulses – like tiny electrical currents that pass along neurons – these result from ion movement in and out of plasma membranes of neurons

1. Ion Channels – highly selective with respect to which ions pass through – some are always open but most are not

➢ ion passage is controlled by protein molecules (gate) that open and close in response to a change in voltage of the plasma membrane or differences in ion concentration on either side of the membrane

➢ others open and close in response to chemicals (hormones, neuro-transmitters)

2. Membrane potentials – in a resting neuron there is a difference in electrical charges on the outside and inside of the plasma membrane. One cause is the varying numbers of K+ and Na+ on either side. More K+ in and 15x more Na+ out. The other factor is the presence of large charged ions trapped inside (phosphates and proteins)

pg 220 membrane potentials

3. Excitability – ability of nerve cells to respond to stimulus and convert into nerve impulses

- stimulus - anything capable of reducing resting membrane potential

▪ *depolarization – loss and reversal of polarization due to rapid openings of Na+channels

▪ *repolarization – recovery of resting membrane potential due to slower opening of K+ channels and crossing of Na+ (*takes 1/1000 of a second)

(see fig 8.6 pg 221)

- action potential – rapid change in membrane potential (read pp. 220-221)

4. All or none principle – any stimulus strong enough to initiate a nerve impulse is called threshold

- if stimuli is strong enough to generate a nerve action potential, the impulse travels along the entire neuron at maximum strength

- EX. gun powder trail

5. Continuous and salutatory conduction

- step by step depolarization of each adjacent area of an axon or a dendrite is called continuous conduction (unmyelinated)

- salutatory conduction – in myelinated fibers the myelin insulates so ion movement is inhibited

- thus nerve impulse jump from node to node much faster (see figure 8.8b pg 223)

6. Speed of nerve impulse –determined by temperature , diameter of fiber and presence of myelin

- warm faster – ice slows pain

- large fibers - faster

- myelinated faster

B. Conduction across synapsis – neuromuscular and neuroglandular junction

- across synapse – most diseases of brain and psychistric disorders involve disruption of synaptic communication

1. pre-synaptic neuron

2. post-synapic neuron (pg 224 fig 8.9)

- neurotransmitters are made by neurons from

amino acids

- when an impulse arrives at synaptic bulb of the pre-synaptic neuron , depolarization occurs, and calcium channels open – liberates(sets free)

neurotransmitters

- this causes excitatory transmission – creates

impulse – or inhibitory transmission which

prevents impulse only one way – post synaptic

neuron is an integrator – responds 3 ways

1. facilitation – if excitatory (depolarization) is greater than inhibitory effect but less than threshold a subsequent stimuli can generate an impulse – building up effect

2. If excitatory is greater than inhibitory and threshold, one or more impulses will be generated

3. If inhibitory is greater than excitatory then no impulse will be generated

C. Altering conduction across synapsis

- disease, drugs, and pressure can alter conduction

- myasthenia gravis – antibodies attack of acetylcholine receptors on skeletal muscle

- alkalosis – increase of Ph above 7.45 and increases excitability of neurons causes light headiness, numbness, tingling, spasms, convulsions

- acidosis – decreases in Ph below 7.35, depression, weakness and coma

D. Regeneration

- limited ability

- around 6 months old – they lose ability to respond

- PNS – some myelinated axons and dendrites (neurolemmas present)

- CNS – oligodendrocytes – no regeneration

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