Chapter 8-The Nervous System

8.1 The Nervous System has Anatomical and Functional Divisions

Nervous System

Monitors the internal and external environments

Integrates sensory information

Coordinates voluntary and involuntary responses of many other organ systems

Neuron: a specialized cell transmitting nerve impulses

Central Nervous Tissue

Brain and spinal cord

Peripheral Nervous System

All neural tissue outside the CNS

Integrates and coordinates the processing of sensory data and the transmission of motor commands

Sensory Input Function

PNS

Sensory Receptors (located at the neds of peripheral neurons): detect changes occurring in their surroundings

Once stimulated, sensory receptors transmit a sensory impulse to the CNS

Sensory Impulse: carried on a sensory neuron

Motor (Output From CNS) (Efferent Neurons

Integrative Function

CNS

Involves interpretation of an incoming sensory impulse

i.e. decision is made concerning what's going to happen next, based on sensory impulse

Integration occurs in interneurons

A motor impulse begins

Motor Functions

PNS

Involves response of a body part

Motor Impulses are carried from CNS to responsive body parts called effectors

Motor impulses carried on a motor neuron

Muscle Effector: contracts

Gland Effector: secretes a hormone

Sensory (Input into CNS) (Afferent Neurons)

Autonomic (Effectors: Smooth muscle; Cardiac muscle; Glands) (Unconscious Control)

Somatic (Effectors: Skeletal Muscle) (Conscious Control)

Sympathetic (Fight or Flight) (NT: Norepinephrine) (Adrenergic-Adrenaline)

Parasympathetic (Homeostasis) (NT: Acetylcholine)

8.2 Distinguish between neurons and neuroglia on the basis of structure and functions

Neuron: The structural & functional unit of the nervous system; a nerve cell

Neuron Structure

Cell Body: Central portion of neuron, contains usual organelles, except centroles

Neuron Processes/ Nerve Fibers

Dendrites: many per neuron, short and branched, receptive portion of a neuron, carry impulses toward cell body

Axons: one per neuron, long, thin process. carry impulses away from cell body

Axons in PNS

Large axons are surrounded by a myelin sheath produced by many layers of Schwann Cells (neuroglial cell)

myelin- lipoprotein

Interruptions in the myelin sheath between Schwann cells- Nodes of Ranvier

Small axons do not have a myelin sheath

All axons in the PNS are associated with Schwann Cells

Axons in CNS

Myelin is produced by an oligodendrocyte rather than Scwann Cells

White Matter: A bundle of myelinated nerve fibers

  1. PNS: Nerve
  1. CNS: Tract/Column

Gray Matter: A bundle of cell bodies (or unmyelinated nerve fibers)

PNS: Ganglia/Ganglion

Nucleus

Regeneration of Nerve Fibers

Cell body injury= death of neuron

Damage to an axon may allow for regeneration

Neuroglial Cells= accessory cells of nervous system form supporting network for neurons; "nerve glue"

PNS: Schwann cells: Produces myelin

Satellite cells: nourish neurons

CNS= 4 types; provide bulk of brain and spinal cord tissue

Oligodendrocyte: looks like eyeball, Produces myelin

Astrocyte star-shaped, Nourishes neurons

Microglia: looks like spider, Phagocytosis

Ependymal cells: epithelial like layer, lines spaces in CNS (1. brain=ventricles. 2. spinal cord=central canal)

Classification of Neurons

Functional Classifications

Sensory Neuron: PNS, afferent neurons=carry sensory impulses from sensory to CNS, input information to CNS

Location of receptors

Somatic

External: touch, pressure, temp, sight, smell, hearing, &touch

Proprioceptors(position and movement)

Visceral receptors: digestive, respiratory, cardiovascular, urinary, reproductive, taste, deep pressure & pain

Interneurons (Association): CNS

link other neurons together (sensory to interneuron to motor neuron)

Motor Neurons: PNS, Efferent:carry motor impulses away from CNS and to effectors

output information from CNS

Effectors: muscles and glands

Somatic:Skeletal muslces. Visceral: Muscle and glands, Sympathetic: adrenergic, Parasympathetic: Cholingeric)

Structural Classifiction

Multipolar: Most common, many extensions, many dendrites lead toward cell body, one axon leads away from cell body

Bipolar: Rare, sight, smell, & hearing) two extension, one fused dendrite leads toward cell body, one axon leads away from cell body

Unipolar: Most PNS sensory neurons. One process from cell body, forms central and peripheral processes, only distal ends are dendrites

8.3 Describe the events involved in the generation and propagation of an action potential

Resting Nerve cells: resting neuron's cell membrane is polarized (electrically charged)

Potential Difference: difference in electrical charge between 2 points (i.e. across a cell membrane)

K+: High inside, Na+: High outside, Cl-: high outside

Resting Membrane Potential (RMP) of a nerve cell is -70mV or millivolts)

Membrane Potential

RMP can be disrupted in two ways

  1. Hyperpolarization: more negative
  1. depolarization: less negative

cell membrane of a neuron must be depolarized to -55mV in order to start a nerve impulse

Action Potential: When RMP is depolarized to -55mV

Threshold potential is -55mV, threshold stimulus is +15mV

After threshold is reached, rapid opening of Na+ channels result is rapid depolarization (reversal of the membrane potential to +30mV)

K+ channels open (while Na+ close)and repolarization occurs and recovery of the RMP to -70mV

Occurs in 1/1000 seconds

8.4 At synapses, communication occurs among neurons or between neurons and other cells

Nerve Impulse Transmission

Nerve Impulse (NI): the progregation of action potentials (AP) alnong a nerve fiber (length of neuron)

NI is an electrical impulse, impulse begins on a dendrite, runs toward the cell body, through the cell body, and down the axon

Characteristics of a NI

Refractory Period: period following a NI when a threshold stimulus cannot produce another NI

All or Nothing Response: if a nerve cell responds at all, it responds completely

Summation: many subthreshold stimuli received one after another may allow threshold potential to be reached, trigger an AP and begin a NI on a neuron

Conduction: The manner in which the NI runs down the nerve/neuron fiber

Unmyleinated nerve fibers: Ni must travel the length of the nerve fiber; slow

myelinated nerve fiber (Salutatory Conduction): NI jumps from Node of Ranvier to Node of Ranvier. Very fast

Synaptic (Chemical) Transmission

Synapse: junction between two neurons where a nerve impulse is transmitted

Occurs between the axon of one neuron and dendrite or cell body of a second neuron

The two neurons do not touch, There is a gap between them which is the synaptic cleft

Scheme of Synaptic Transmission

  1. NI reaches axonal terminal of pre-synaptic neuron causing depolarization of synaptic knob
  1. Ca+ channels open and calcium ions rush into axonal terminal
  1. Causes synaptic vesicles (Filled with Neurotransmitter) to release NT via exocytosis (outside of cell) into the synaptic cleft
  1. NT diffuses across synaptic cleft and depolarizes the post-synaptic neuron's membrane
  1. An AP is triggered and a NI begins in the post-synaptic neuron

Neurotransmitters (NT)

At least 30 produced by CNS

Most common is Acetylcholine(ACh), released by all motor neurons and some CNS neurons

Monoamines (modified amino acids): widely distributed in the brain where they play a role in emotional behavior and circadian rhythm

Includes: Epinephrine, Norepinephrine, Dopamine, Serotonin, Histamine

Unmodified AA Include: Glutamate, aspartate, GABA (gamma aminobutyric acid, glycine

Fate of Neurotransmitter in Synaptic Cleft

Destruction of NT: Enzymes that are present in the synaptic cleft destroys NT

Reuptake of NT: Transported back into pre-synaptic knob

8.5 The brain and spinal cord are surrounded by three layers of membranes called the meninges

Meninges

The Dura Mater

Outermost layer, tough and fibrous, attached to the inner peritoneum of the skull, contains many blood vessels and nerves, DM splits into two layers where it encloses the dural sinuses (collects venous blood from brain).

Arachnoid

Middle layer, thin net-like membrane, beneath the arachnoid mater lies a wide space called sub-arachnoid space. Space is filled with cerebrospinal fluid and serves as a cushion for the brain

Pia Mater

Inner layer that clings to brain surface, very thin delicate CT, many nerves and blood vessels=nourishment, dips into grooves and contours

Spinal Cord

DM is not attached to bone of the vertebra, space between the DM and bone is called the epidural

Space between PM and AN is the subarachnoid

8.6 Discuss the roles of gray matter and white matter in the spinal cord

Gross structure of Spinal Cord

Length: 17 inches, start=foramen magnum, end=tapers to point (conus medullaris) and terminates near the intervertebral disc that separates 1st and 2nd lumbar vertebra

Contains 31 segments, cauda equina: the lower lumbar and sacral nerves travel downward

Cross-Sectional Anatomy

Gray Matter: butterfly, bundles of interneuron cell bodies, posterior dorsal horns, lateral horns, and anterior horns

located in central canal, gray commissure, anterior median fissure, posterior median sulcus

White Matter: myelinated (interneuron) axons

Loation: Posterior funiculi or white column, lateral funiculi, anterior funiculi

Ascending tracts: Located in posterior dorsal columns and conduct sensory (afferent) impulses from body parts

Descending tracts: located in anterior columns and conduct motor (efferent) impulses from the bran to effectors