Neural Tissue Chapter 12

I. Overview of the Nervous System A. Provides swift, but brief responses to stimuli As opposed to what?? B. Consists of: 1. Neural tissue: a. Neurons- functional units

b. Neuroglia (aka Glial Cells) i. Separate & protect neurons ii. Provide a supportive framework for neural tissue iii. Act as phagocytes iv. Help regulate interstitial fluid v. Far outnumber neurons (account for ~ ½ the volume of the nervous system). 2. Blood vessels 3. Connective tissues

C. Organs of the nervous system 1. Brain 2. Spinal cord 3. Sensory receptors for eyes, ears,… 4. Nerves

D. Major anatomical divisions of the nervous system: 1. Central Nervous System (CNS) a. Consists of: i. Brain ii. Spinal cord b. Responsible for: i. Integrating, processing, & coordinating sensory data and motor commands ii. Intelligence iii. Memory iv. Learning v. Emotion

2. Peripheral Nervous System (PNS) a. Consists of neural tissue outside the CNS (nerve fibers (axons)) i. Cranial nerves- connected to the brain ii. Spinal nerves- connected to the spinal cord iii. Peripheral nerves- aka ‘nerves’ b. Responsible for: i. Delivering sensory information to the CNS ii. Carrying motor commands to peripheral tissues & systems

c. Functional divisions of the PNS i. Afferent division- brings sensory information to the CNS from receptors in peripheral tissues & organs ii. Efferent division- carries motor commands from the CNS to effectors (muscles & glands)  Somatic nervous system (SNS)- controls skeletal muscle *Conscious- voluntary *Subconscious- involuntary (reflex)

 Autonomic nervous system (ANS or visceral motor system)- provides automatic, involuntary regulation of smooth muscle, cardiac muscle & glands *Sympathetic division- fight or flight activities * Parasympathetic division- activities that conserve energy

II. Neurons A. Representative Neuron Structure (Multipolar, most common type in CNS) 1. Soma- cell body a. Large nucleus & noticeable nucleolus b. Many mitochondria c. Clusters of ribosomes & RER (Nissl bodies) give the cell a grayish appearance, hence the name “gray matter” d. Lack centrioles (can’t divide)

2. Dendrites- sensitive processes a. Extend from soma b. Highly branched w/ dendritic spines c. Neurons receive info from other neurons via synaptic connections at the dendritic spines

3. Axon- long, cytoplasmic process a. Connects to the soma at a axon hillock (thickened region) b. May have side branches (collaterals) to communicate w/ multiple cells c. End in a series of fine extensions (telodendria) * Telodendria end at synaptic terminals (synaptic bulbs) * Synaptic terminals are a part of a synapse

4. Synapse- site of intercellular communication a. There are 2 cells involved: i. Presynaptic cell  Has the synaptic terminal  Sends a message ii. Postsynaptic cell  Receives the message

b. Involves the release of chemicals (neurotransmitters) by the synaptic terminal c. May occur on a dendrite, the soma, or along the axon

B. Neuron Classification 1. Structural Classification a. Anaxonic Neurons i. Small ii. Dendrites & axons look alike iii. Located in the brain and special sense organs iv. Function is poorly understood!

b. Bipolar Neurons i. Highly branched dendrites at one end, one axon at the other end with the soma in the middle ii. Short (30 mm end-to-end) iii. Rare- found in special sense organs iv. Relay info about sight, smell, hearing

c. Unipolar Neurons (Pseudounipolar Neuron) i. Dendrites and axon are continuous, with the soma off to one side. ii. Most sensory neurons of the PNS are unipolar iii. Axons may extend a meter or more!

d. Multipolar Neurons i. Several dendrites and a single axon w/ one or more branches ii. Most common type in the CNS iii. One function- controls skeletal muscle iv. Can be as long as unipolar neurons

2. Functional Classification a. Sensory Neurons i. Form the afferent division of the PNS (deliver info from sensory receptors to the CNS)  Somatic sensory neurons- monitor exterior conditions  Visceral sensory neurons- monitor internal conditions ii. Unipolar iii. Axons are known as afferent fibers iv. ~10 million in the body

b. Motor Neurons (efferent neurons) i. Form the efferent division of the PNS (carry instructions from the CNS to peripheral effectors)  Somatic motor neurons- (in SNS) innervate skeletal muscle  Visceral motor neurons- (in ANS) innervate all other peripheral effectors ii. ~ ½ million in the body iii. Axons are known as efferent fibers

c. Interneurons (association neurons) i. Responsible for distribution of sensory info & coordination of motor activity ii. May be situated between sensory & motor neurons iii. Located entirely w/in brain & spinal cord iv. ~20 billion in the body

III. Neuroglia A. Neuroglia of the CNS: 1. Ependymal Cells a. Epithelial cells (cuboidal to columnar in shape) b. Line the ventricles of the brain and central canal of the spinal cord (both are filled with cerebrospinal fluid (CSF))

2. Astrocytes a. Largest, most numerous glial cells b. Functions: i. Maintaining blood-brain barrier - isolates the CNS from the general circulation ii. Creating a framework for the CNS (microfilaments) iii. Repairing damaged neural tissue-stabilize tissue & prevent further damage iv. Guiding neuron development (in embryos) v. Controlling the interstitial environment

3. Oligodendrocytes a. Myelinate CNS axons (myelin= multilayered membraneous sheath) a. Increases speed of communication b. Myelinated areas appear white, hence “white matter” c. Tie clusters of axons together

4. Microglia a. Remove cell debris, wastes, and pathogens by phagocytosis

B. Neuroglia of the PNS: 1. Soma are clustered together in masses called ganglia 2. Soma & axons are completely insulated by processes of glial cells: a. Satellite cells (amphicytes)- surround soma in peripheral ganglia b. Schwann cells (neurilemmocytes)- form a sheath around every peripheral axon (even unmyelinated)

3. Demyelination a. Progressive destruction of myelin sheaths in the CNS & PNS b. Results in a gradual loss of sensation & motor control that leaves affected areas numb & paralyzed i. Heavy metal poisoning ii. Diphtheria iii. Multiple sclerosis (MS)

Neural Responses to Injury Read page 407 in your text to learn how a damaged axon is able to (sometimes) repair itself.