The Nervous System

How does the endocrine system communicate?

through hormones secreted into the blood

How does the nervous sysetm communicate?

by sending electrical and chemical signals via nerves and neurons

3 basic steps to carry out a task in the nervous system

1. sensory neurons recieve information and sends coded messages to the CNS

2. CNS processes information and decides what response is appropriate

3. CNS commands motor neurons to carry out a response signal

What is the CNS?

Central Nervous System - brain and spinal cord

What is the PNS?

Peripheral Nervous System - nerves and ganglia

What is a nerve? and how many neurons does it have?

bundle of nerve fibers wrapped in fibrous CT
1 nerve has a bunch of neurons functioning within it

Sensory (afferent) division of PNS

carries signals from stimuli to CNS

somatic vs. visceral sensory divisions of the PNS?

somatic - carries signals from receptors in skin, muscles, bones, and joints

visceral - carries signals from organ receptors

motor (efferent) division of the PNS

carries signals from CNS to the body

Sympathetic vs. Parasympathetic divisions of the visceral motor division

sympathetic - fight or flight response
parasympathetic - rest and digest

excitability or irritability

ability to respond to stimuli

conductivity

ability to produce electrical signals that can be conducted to other cells

secretion

ability to secrete a neurotransmitter that stimulates the next cell

sensory neurons

specialized to detect stimuli

interneurons

90% of body’s neurons

interconnect sensory pathways and CNS so that reflexes can happen quickly

motor neurons

send signals to muscles and glands

what is the soma and what does it contain?

it is the control center or body of the neuron
has cytoplasm that contains the mitochondria, lysosomes, Golgi complex, rough ER, etc.

what are dendrites?

branches coming from soma which receive stimuli

What is an axon and what are they covered with?

conducts nerve impulses from soma
covered in myelin sheaths

axon hillock

where the axon starts and the soma ends

axon collaterals and terminal arborization

axon collaterals - branches off of the axon

terminal arborization - branches off of the distal end of axon

Multipolar neuron

has one soma with many dendrites and 1 axon coming off of it
found in CNS and somatic motor neurons

bipolar neuron

has 1 soma with 1 dendrite and 1 axon coming off of it

found in olfactory cells, retina, and inner ear

unipolar neurons

dendrites immediately connect to axon which immediately connects to soma

sensory neurons

2 passages of axonal transport and what motor protein each uses

anterograde - movement away from soma uses Kinesin

retrograde - movement toward soma, uses Dynein

how do motor proteins work?

they crawl along microtubules with the materials on their back

Fast axonal transport

can be anterograde or retrograde

quickly transmits materials like a direct flight

slow axonal transport

like taking multiple connecting flight

always anterograde

stops frequently to repair nerve fibers

Neuroglial cells

Make up a large number of the neurons in the nervous system

build framework for nervous tissue, support and protect neurons, guide neurons

oligodendrocytes

plasma membrane extends to myelinate the axons

ensures neurons don’t touch and improperly transmit signals

ependymal cells

make cerebral spinal fluid

microglia

specialized macrophages

like vacuum cleaners of the brain

astrocytes

align neurons and create synapses

most abundant

Schwann cells

only in the PNS
produce a myelin sheath and assist in the regeneration of damaged nerves

satellite cells

only in the PNS
surround soma and provide electrical insulation

meningioma

brain tumor of the meninges (layer of CT covering brain)

brain tumors can arise from

meninges, metastasis from non-neuronal tumors, and from glial cells that are mitotically active

gliomas

grow rapidly and are highly malignant

myelin sheath

insulating layer around nerve fiber
formed by oligodendrocytes (CNS) and Schwann cells (PNS)

myelination

begins in the 14th week of fetal development
myelination must be maintained throughout life by eating omega 3s

Myelination in PNS

Schwann cells spiral around nerve fibers
neurilemma - thick outermost coil of myelin sheath

Myelination in CNS

oligodendrocytes reach out and myelinate all nerve fibers in its vicinity

What makes repair of nerves possible?

Schwann cells and endoneurium
only occurs in PNS

nodes of Ranvier

gaps between myelin segments

internodes

myelin covered segments from one gap to the next

initial segment

short section of nerve fiber between the axon hillock and first glial cell

trigger zone

axon hillock and initial segment
important for initiating nerve signal

multiple sclerosis

autoimmune disorder in which oligodendrocytes and myelin sheaths deteriorate
nerve conduction can become disrupted causing double vision, tremors, numbness, etc.

Tay-Sachs disease

a hereditary disorder of infants of Eastern European Jewish ancestry
digestive enzymes in lysosome are missing so as material is sent to be broken down lipids accumulate

unmyelinated axons

neurons lay down in membrane of Schwann cells
mesaxon - neurilemma wrapping of unmyelinated nerve fibers

what is the conduction speed of nerve fibers determined by?

size of the fiber and presence/absence of myelin

ex.) big and myelinated is fastest

small unmyelinated fibers conduction speed

0.5-2.0 m/sec

small myelinated fibers conduction speed

3-15 m/sec

large, myelinated fibers conduction speed

120 m/sec

Regeneration of nerves

soma must be intact
injured fiber degenerates, soma swells, regeneration tube forms, nerves sprout and move throughout the regeneration tubes

regeneration tube

formed by Schwann cells, basal lamina, and the neurilemma near injury
responsible for guiding sprouts to original target for healing

What is the resting membrane potential?

-70 mV

what is the threshold (in mV) for Na+ voltage channels?

-55mV

axodendritic synapse

most common synapse
between synaptic bulb and dendrite

axosomatic synapse

2nd most common synapse
between the synaptic bulb and soma

axoaxonic synapse

rare, specialized synapse
between 2 synaptic bulbs

presynaptic

communicates and sends signals through synaptic bulb

postsynaptic

communicates and sends signals through synaptic bulb, dendrites, and soma

what makes a weak or strong synapse?

The distance of the synapse from the trigger zone

A synapse located farther from the trigger zone leads to a weaker signal, as the change in charge may diminish and fail to reach the threshold required for triggering an action potential.

at what voltage do the Na+ voltage channels close?

+30/35 mV