PHYL 141 CH 12, 13 - Nervous System & Spinal Cord

Parts of the Central Nervous System

• Brain

• Spinal Cord

Parts of the Peripheral Nervous System

• 12 pairs of cranial nerves

• 31 pairs of Spinal nerves

• Ganglion

• Sensory Receptors

Parts of the brain (3)

• cerebrun

• cerebellum

• brainstem

Ganglion:

small masses of tissue, clusters of nerve cell bodies

2 Types of PNS

• Somatic

• Autonomic

Somatic

Voluntary control of the skeletal muscle

Autonomic

involuntary control of glands and smooth muscle

Subdivisions of Autonomic PNS

• Sympathetic

• Parasympathetic

• Enteric

Sympathetic

Fight or flight, Increases heart rate, supports exercise, emergency situations

Parasympathetic

Rest and digest, slows heart rate;

Enteric

Regulates GI fucntion/ mobility

Functions of the Nervous system

• sensory

• integration function of information

• motor function (response to stimuli)

Homeostasis - Coordination between CNS & PNS

sensory function

detection of stimuli, sense changes through sensory receptors

Integration function

decision making: analyzing incoming sensory information

determines the response to stimuli

motor function

initiating action

from the CNS motor output signals target effectors (muscles/glands)

Afferent > Efferent

Sensory, sends signals towards the CNS.

• sensory receptors initiate signals sending to the brain/spinal cord

• includes sensory input or sensation

Integration takes place in the CNS

Motor (efferent) neurons must send signals away from the CNS to target organs in the PNS

Result: contraction of muscles and/or secretion from glands

Type of Nerve cells (2)

1. Neurons

2. Neuroglia

How many neurons in brain

85 billion approx

Neuron

• functional cell of the nervous system

• Has the property of electrical excitability

• cellular structures

Parts of the neuron (7)

• cell body

• nissl bodies

• neruofibrils

• dendrites

• axon

• axon hillock

• synapse

Cell body

contains nucleus, lysosomes, mitochondria, golgi complex, nissl bodies, neurofibrils.

Nissl Bodies

Rough ER; constantly replaces the cell membrane (normal process of growth and repair). Protein synthesis occurs here!

Neruofibrils

form Cytoskeleton; give structure to the cell.

Dendrites

 receiving or input of information

Axon

conducts the nerve impulses from neuron to dendrites or to an effector organ (muscle /gland).

Axon Hillock

connects the cell body with the  axon: initiation of electrical impulse (action potential).

Synapse

site of functional contact between two neurons. (or neuron and effector organ)

Classification of Neurons (2)

• Structural classification

• functional classification

Structural classification

• on the number of processes extending from the cell body

• unipolar, bipolar, multipolar

Functional classification

sensory, association, or motor

Unipolar neuron

• single process extending from the cell body

• Several dendrites; One axon fused together (pseudounipolar neuron)

• Sensory receptors

Bipolar Neuron

• two processes attached to the cell body

• One dendrite; One axon

• the retina of the eye and the olfactory epithelium in the nose.

Multipolar Neuron

• Most common type!

• Several dendrites extending from the cell body, one axon

• motor and interneurons, which make up most of the  brain and spinal cord.

Neuroglia

• Not electrically excitable

• Also known as “glial cells”

• specialized tissue cells that support the neuron

• attach neurons to blood vessels; "acts as glue"

• produce myelin sheath (insulates) around the axon

• carries out phagocytosis

Types of glial cells found in the CNS(4)

1. astrocytes

2. oligodendrocyte

3. microglial cells

4. ependymal cells

Astrocytes

• have numerous processes (extensions) making them appear like a star; 

• Extension wrap around capillaries in brain to regulate blood entering CNS. 

• Removal of excess neurotransmitters.

Oligodendroctyes

creates myelin sheath

Microglial cells

Phagocytosis

Ependymal cells

• creates & circulates CSF

• cushions and nourishes the brain and spinal cord

Types of glial cells in PNS (2)

1. Schwann Cells

2. Satellite cells

Schwann cells

wrap around neuron axons in the PNS forming myelin sheaths

Satellite cells

wrap around the neuron cell body

What produces the myelin sheath?

Oligodendrocytes (CNS) and Schwann cells (PNS)

What is myelin?

a multilayered lipid and protein covering

Myelination

process of wrapping around axons to form layers of myelin. 

Myelin sheath increases from ….

birth to maturity

Myelination (insulation) allow neural signals to be transmitted

more quickly.

The myelin sheath is not

continuous along the axon. 

There are gaps between sheaths where

the neuron plasma membrane is exposed. 

nodes of Ranvier (aka neurofibril nodes). 

the gaps in the myelin sheath along the axon that facilitate the rapid conduction of nerve impulses.

Some axons are heavily myelinated, others are unmyelinated

True

What cell assists in regeneration of the myelin sheath in PNS?

Schwann cells

• Neurolemma: outer nucleated cytoplasmic layer of the myelin sheath 

• Neurolemma is also called ‘sheath of Schwann’

• Neurolemma aids in regeneration of an injured axon by forming a regeneration tube.

• Regeneration tube guides and stimulates regrowth of the axon.

What cell assists in regeneration of the myelin sheath in CNS?

Oligodendrocytes

• no neurolemme is formed

• no regrowth after injury

Grey matter in the spinal cord

• unmyelinated axons

• H- shaped

White matter in the spinal cord

• myelinated

The brain has a thin outer layer of gray matter covering both cerebral hemisphere of the brain called…

• cerebral cortex

• cerebellar coretx

Multiple sclerosis

• condition with progressive demyelination of neurons in the CNS (loss of oligodendrocytes). 

• Disruption of conduction of nervous signals causing impairment of sensory and motor function. 

• progressive disorder (it gets worse over time)

• Causes: unknown, considered an autoimmune disorder (body attacks itself)

Guillain-Barre Syndrome

• loss of myelin on neurons in the PNS

• characterized by impaired sensation and muscle weakness. 

• Unlike multiple sclerosis, this disorder often resolves spontaneously.

Typically caused by an infection (respiratory, gastrointestinal bacterial or viral infection)

Resting Membrane Potential

The difference in electrical potential across the membrane of a resting cell, typically negative inside relative to outside, largely maintained by the unequal distribution of ions (especially K+K+) and the action of the sodium-potassium pump.

Graded potential

In neurons, small localized change in membrane potential that can either be excitatory (depolarizing) or inhibitory (hyper polarizing)

Action potential

a sequence of rapidly occurring events that decrease and eventually REVERSE the membrane potential (depolarization) and eventually restore it to the resting state (repolarization).

Leakage channels

Always open; allowing a slow but continuous diffusion of ions

Ligand-gated channels

open only upon neurotransmitter binding

Voltage-gated channels

open when the voltage of the membrane changes

Mechanically-gated ion channels

open when a physical force is applied

Epineurium

outermost layer (connective tissue) of peripheral nerve

Perineurium

sheath of connective tissue surrounding nerve fascicle

Endoneurium

sheath of connective tissue that surrounds the axons.

Propagation

process by which an electrical impulse travels along the membrane of a nerve cell.

Factors that affect propagation

• axon diameter

• amount of myelination

• Temperature

Types of nerve fibers

1. Type A

2. Type B

3. Type C

Type A

• Motor neurons that innervate skeletal muscles

• large diameters = highly myelinated

• Action potential: very fast conduction

Type B

fibers are lightly myelinated; intermediate diameters.

Type C

unmyelinated; smallest diameters: slowest conduction

Synapse structures (3)

1. the axon terminal region

2. the adjacent membrane of the postsynaptic neuron 

3. the cleft in between

Synapse

The area where two neurons come together

Electrical synapse

 gap junctions connect cells and allow transfer of information to synchronize activity

Chemical synapse

gap junctions connect cells and allow transfer of information to synchronize activity

Postsynaptic potentials

can receive many signals at once.

The axon hillock is where summation of all

postsynaptic potentials occurs, determining whether an action potential is initiated.

Integration

When neurotransmitters bind a receptor, ion gates open causing the cell to depolarize or hyperpolarize.

Depolarization signals may generate action potentials called

EPSPSs or IPSPs

EPSPs

excitatory postsynaptic potentials

IPSPs

inhibitory postsynaptic potentials

A neural circuit

a functional group of neurons that process specific types of information.

Converging circuits

involve many sources of input which then act upon a single output neuron.  For example, the rate and depth of breathing is influenced by many sensory inputs such as blood pH, emotions, and pain among others.

Diverging circuits

• single nucleus could send signals to many output targets. 

• Example, the “fight or flight” sympathetic nervous system nuclei can send signals to increase heart and breathing rate and dilate the airways, pupils of the eye, and blood vessels.

Reverberating circuits

• rhythmic functions that will continue until there is an inhibitory signal. 

• Example: maintaining breathing patterns even while sleeping.

Parallel-after-discharge circuits

• combination of converging and diverging circuits 

• a signal can diverge from a nucleus with multiple pathways but then converge onto another nucleus. 

• Example: more complex functions (such as higher order thinking)

Parkinsons Disease

• Neurons in CNS breakdown/die

Alzheimers disease

• breakdown of energy production within the cells

Guillain-Barre syndrome

•  demyelination PNS

multiple sclerosis

• demyelination CNS

Stroke

• decrease of oxygen to neuron; neuron dies. 

ALS - Amyotrophic Lateral Sclerosis

• decrease motor neurons from brain/spinal cord

medulla oblongata

The lower half of the brainstem, responsible for autonomic functions like breathing and heart rate.

spinal cord is the continuation of the

medulla oblongata

spinal cord terminates at the

L1-L2 (adults)

Conus Medullaris

tapered portion of the spinal cord

cauda equina

horse’s tail; where the spinal roots hang off the end of the spinal cord