NERVOUS TISSUE

NERVOUS TISSUE
The body has 75 trillion cells whose functions and responses must be coordinated. This is accomplished by two systems of the body.
a) Nervous System--swift but brief responses to stimuli
b) Endocrine system--slower but effects last longer (affects things such as growth, sexual development, pregnancy, etc.)
The nervous system has 3 functions: sensory (receiving information from the outside world), motor (responding to the outside world by initiating muscle contractions or glandular secretions) and integrative (processing information through analysis & storage so that the optimal response can be chosen).
We can divide the nervous system in several ways:
--CNS: central nervous system consisting of brain & spinal cord
--PNS: peripheral nervous system consisting of cranial & spinal nerves (depending on their place of origin) and ganglia
--Sensory (affarent) Division: brings sensory info to the CNS
--Motor (efferent) Division: carries motor commands to the muscles & glands
--SNS: somatic nervous system that controls skeletal muscle
--ANS: autonomic nervous system that involuntarily controls cardiac and smooth muscle & glands
Nervous tissue (such as the images of the human brain below)has 2 primary types of cells, neurons & neuroglia.

NEURONS

NEURONS

1) Neuroglia--cells which support & protect the neurons and actually outnumber them
2) Neurons
Neurons are excitable cells which may vary form less than 1 mm to more than 5 feet in length (or more). Most neurons have 3 general regions:

NEURON

a) dendrites-Dendrites are highly branched processes that extend from the cell body involved in monitoring the surrounding area for local chemical, mechanical, or electrical information.
NEURON


b) the cell body or soma-The soma contains the nucleus & organelles. Clusters of soma in the CNS are called nuclei while clusters outside the CNS are called ganglia.
c) axon: Axons are present on the vast majority of neurons and can be quite long. They transmit electrical impulses.

Myelination
A neuron's electrical message (the action potential) can speed along an axon at 3'/sec (2 mph). This speed can be increased 20x by a coating of myelin over the axon secreted by oligodendrocytes or Schwann cells. Fat, myelinated axons transmit messages at up to 280 mph. Why can't all neurons be fat & myelinated? First of all, not all messages are urgent. Secondly, this would make the spinal cord the width of a garbage can & a spinal nerve as thick as a garden hose.

SYNAPTIC TRANSMISSION
Eventually, a neuron's action potential reaches the synaptic terminals. Although the action potential is electrical, most intercellular communication is chemical. The synaptic knobs may have millions of vesicles of the neurotransmitter. For example, neuromuscular synapses (where neurons contact muscle cells). The neurotransmitter ACh (acetyl choline) is packaged into vesicles, each with thousands of ACh molecules.
NEURON

The communication between neurons and other cells (muscles, glands, and other neurons) is essential to the body. Many drugs and poisons effect the body through the interaction of neurotransmitters with receptors at synapses or through altering the neuron's ability to conduct electricity. Here are some examples of drugs and poisons which affect ACh and other neurotransmitters.

EFFECTS OF DRUGS AND POISONS ON ACh
1. BOTULINUS TOXIN--blocks ACh release,paralyzes voluntary muscles
2. BARBITUATES--decreases ACH release, muscular weakness
3. INSECTICIDES, NERVE GAS--prevents ACh inactivation by cholinesterase; sustained muscle contraction

4. CURARE--prevents ACh binding to receptors; paralysis of voluntary muscles
5. NICOTINE--binds to ACh receptors, facilitiating them

EFFECTS OF DRUGS AND POISONS ON OTHER NEUROTRANSMITTERS
1. ANTI-PSYCHOTIC DRUGS(thorazine)--block dopamine receptors; long term reduction of dopamine in muscle control regions leads to motor problems
2. MINOR TRANQUILIZERS (valium, librium)--bind to GABA receptors, enhancing its inhibitory effect
3. ANTIDEPRESSANTS (Elavil)--prevents the reuptake of NE and serotonin form the synaptic cleft; prolongs elevating effects
4. ANTIHYPERTENSIVE--act in PNS, block NE's stimulation of smooth muscle by binding to NE receptors
5. LSD--binds to serotonin receptors, blocking the inhibition of some pathways; sensory information is no longer filtered--overload
6. AMPHETAMINES--resemble NE and dopamine, used at pleasure center; body begins to make less NE and dopamine
7. COCAINE--blocks reabsorption of dopamine (the most responsible for the feeling of euphoria), NE, and serotonin and their effects are felt for longer periods; eventually the neurotransmitters are washed away and degraded--since they weren't reabsorbed, the brain's neurotransmitter supply decreases; it becomes harder to feel pleasure without cocaine and may eventually become impossible
8. STRYCHNINE--blocks glycine (an inhibitor) receptors of motor neurons resulting in muscle spasms
9. CAPSAICIN (from jalapeno and Hungarian peppers)--causes massive release of substance P from pain receptors in tongue and mouth
10. NOVOCAIN--reduces membrane permeability to sodium; prevents stimulation of sensory neurons
11. ALCOHOL--alters cell membrane conduction of impulses; as blood alcohol levels increase more and more brain centers are affected (which is why the effect of alcohol varies with the amount consumed)

THE SPINAL CORD
SPINAL

SPINAL
The spinal cord is not just a highway to the brain; it initiates many important processes on its own. It makes many simple but urgent decisions, saving time; your hand can be moving away from a hot stove before your brain realizes you've touched it. The spinal cord is about 18" long and only reaches the lower back from which point the final spinal nerves form the cauda equina (the "horse's tail).
SPINAL

SPINAL
There are three layers around both the brain and spinal cord to provide cushioning:
a) dura mater: the tough outer layer
b) arachnoid: a procedure known are a spinal tap removes 5-10 ml of cerebrospinal fluid from the space between the arachnoid and the pia mater known as the subarachnoid space
c) pia mater: contains blood vessels, and is fused to the brain or spinal cord
Below is a picture of the meninges around a sheep's brain.

BRAIN

Between the meninges, spaces are filled with cerebrospinal fluid that functions as a shock absorber, transport of nutrients, wastes and gases.

The tissue of the spinal cord can be divided into gray and white matter.

GRAY AND WHITE

The gray matter contains the cell bodies of neurons organized into groups called nuclei. Sensory nuclei in the posterior horns receive & relay sensory info from receptors. Motor nuclei in the anterior horns issue motor commands to the periphery.
NUCLEI
The white matter contains axons organized into tracts; the axons of a tract are all moving in the same direction and are of the same kind (sensory or motor).
--Ascending tracts send sensory information to the brain:
--the posterior column carries touch information
--the spinothalamic tract carries pain and temperature information
--the spinocerebellar tract carries information about body position
TRACTS
--Descending tracts carry motor commands from the brain:
--the corticospinal tract carries voluntary commands to skeletal muscle
--the reticulospinal tract carries involuntary commands to skeletal muscle
TRACTS

SPINAL NERVES
From the spinal cord, spinal nerves reach the various regions of the body. These spinal nerves carry sensory information from these body regions to the CNS and carry commands from the CNS to these regions. Each spinal nerve supplies an area of the skin referred to as its dermatome.

NERVES

NERVES

REFLEXES
Reflexes are rapid adjustments to preserve the normal conditions of the body. They are fixed "wiring" and are not conscious or voluntary. Each stimulus results in the same response. In general, there are 5 steps in a reflex.
1) A receptor is activated. Receptors are constantly monitoring the internal conditions in the body and the external environment. Too much stress on a muscle, a bright light, and food in the mouth will be detected by receptors and result in reflex responses to these stimuli.
2) A sensory neuron is activated which conducts an impulse to the CNS.
3) The neurotransmitters of the synaptic knob may stimulate a motor neuron (in a monosynaptic reflex) or an interneuron (in a polysynaptic reflex). Interneurons are neurons which analyze incoming sensory info & coordinate motor outputs. Thousands of interneurons may be organized into pools and their connections can be obviously complex.
REFLES
4) Activation of a motor neuron which carries message to muscles or glands.
5) The response of a peripheral effector (e.g. muscle or gland). The response typically counteracts the original stimulus as part of a negative feedback loop.

REFLEXES CLASSIFIED BY:
1) Development
A) Innate reflexes are genetically determined. They may be simple (blinking, withdrawal from pain) or complex (suckling, tracking objects with eyes).
B) Acquired reflexes can be more complex, such as skiing or driving.
2) Nature of Motor Response
A) Somatic reflexes involve skeletal muscle
B) Visceral reflexes other systems, such as the digestive system, respiratory system, cardiovasculary system, and reproductive system.
3) Processing Site
A) Cranial reflexes are processed in the brain.
B) Spinal reflexes are processed in the spinal cord.

EXAMPLES OF COMMON REFLEXES
a) In a stretch reflex, the stimulus is the stretch of a muscle detected by receptors known as spindle fibers and the response is the contraction of that muscle. We not only use stretch reflexes when a doctor hits us with a reflex hammer, they are continually making adjustments to posture.
b) Golgi tendon reflexes prevent damage to muscle through excessive stress. If one tries to create so much tension in a muscle that it might damage the muscle, this reflex can tell the muscle to relax. For example, an arm might suddenly "give out" in an arm-wrestling match or when trying to bench press too much weight in a weight room.
c) Withdrawl reflexes are triggered by painful stimuli. If a limb is involved, many body parts must be involved to maintain balance.