The Nervous System

CLS 224Deemah Dabbagh

Nervous system•Composed of:• Brain (CNS)• Spinal cord (CNS)• Nerves to body parts (PNS)

• Function:•Maintain homeostatis by

detecting stimuli in the environment

Structure

•The nervous system is composed of two general types of cells:

1. Neurons (nerve cells): building blocks of the NS

2. Neuroglia cells (supporting cells)

The Neuron

•Also known as the nerve cell•Basic unit of the nervous system•Composed of:• A cell body• Dendrites• An axon

•Responsible for receiving and transmitting nerve impulses and forming long fibers by linking together

Important physiological properties of a Neuron

• Excitability: respond to a stimuli (e.g. change in environment)

• Conductivity: Ability to transmit a signal (pass it on to other cells)

Neuron Cell body• Also called soma or

perikaryon

• Contains:• Nucleus , nucleolus• cytoplasm• organelles responsible for

cell maintenance

Neuron Cell Body

• Nissl Bodies• A unique structure

to neurons

• Large granular bodies , clusters of rough ER

• Gives the cell body its gray color (gray matter)

• Synthesize and release proteins

Dendrites• Extensions of the cell

body

• Multi-branched portions which receive impulses and bring them towards cell body

• Increase the surface area for connection with other neurons

Axon• “The tail of the neuron”

• A long cell process arises from a slight elevation of the cell body (Axonal Hillock)

• Propagates the signal down the neuron and then to other cells

• May have branches called collaterals

Axonal Transport• Occurs through microtubules

• Materials can move up the axon towards the cell body (retrograde transport)

• Materials can move down the axon away from the cell body (anterograde transport)

• Transport can:• Fast (mitochondria, pathogens, synaptic vesicles)• Slow (enzymes and other substances)

Classification of Neurons

1. Based on their anatomy

2. Based on what they do

Anatomical Classification

•Unipolar

•Bipolar

•Multipolar

Unipolar Neuron • A single process• called “pseudounipolar” in

humans

• One process protruding from cell body

• At a short distance from the cell body the process divides into two branches (central and peripheral)

• It is a sensory (touch, pain) neuron in the peripheral nervous system

Bipolar Neuron• Two processes protruding from cell body; axon and dendrite

•Dendrites receive information and axon gives information

• Sensory neuron e.g. retinal neuron, olfactory neuron

Multipolar Neuron• Neuron with a cell body and

3 or more processes

• One process is the axon, the others are dendrites

• Most abundant in the brain and spinal cord

Functional Classification of Neurons

•Sensory

•Motor

Sensory Neurons

• Afferent: carry impulses from peripheral body toward the CNS

•Most sensory neurons are unipolar. Some are bipolar.

Motor Neurons

• Efferent: carry information from the CNS to the periphery

•Multipolar

• Send CNS signals down to body parts, like glands and muscles

Neurotransmission

•Neurotransmission depends on action potential:

• Short lasting electrical event on the plasma

membrane of neurons.

Action Potential (AP)• All cells, including neurons, have a maintained polarity on both sides of

their membrane

• the polarity is maintained by stable concentrations of Na+ outside the cell and K+ inside the cell

• Stimulus causes the polarity to change

• This creates an electrical potential

• Electrical potential propagates along the axon until it reaches the axonal terminal branches

• Signal is the transmitted to the next neuron

Transmission of signal from presynaptic neuron to post-synaptic neuron

Some important notes:• Clusters of cell bodies are named differently when

they are in the CNS and PNS

• In the CNS,• Clusters of cell bodies are called nuclei

• In the PNS, • Collections of cell bodies are called ganglia

• White matter: myelinated regions of CNS containing mostly nerve fibers

• Gray matter: unmyelinated regions of CNS containing mostly cell bodies

Neuroglial Cell

• Also known as• glia cell• Glia

• General Functions:• Support• Nutrition• Help maintain homeostasis• Form myelin• Help in signal transmission within

the nervous system

Types of Neuroglial Cells

•Astrocytes

•Oligodendrocytes

•Microglial cells

•Ependyma

Astrocytes

• Structural support

• Most abundant of all the glial cells

• Found between blood vessels and neuron cell bodies

• Play an important role in the blood brain barrier

Oligodendrocytes• Found in the CNS

• Produce myelin• Myelin sheath is a fatty

insulator protecting nerve fibers (axons)

• They can myelinate multiple neurons

In the PNS myelination of neurons is done by Schwann cells. (one cell per schwann)

Microglia cells

•Fewer in number than all other glia cells

•Have a phagocytic function (engulf pathogens, damaged neurons ect…)

•They increase in number during infections

Epyndema Cells•cuboidal epithelial cells

•Line the cavities of the brain and spinal cord

•Ciliated, helps in circulating CSF that fills those cavities

Divisions of the Nervous system

Divisions of the Nervous System

•Central nervous system (CNS)• Brain (coordinates all bodily activities)• Spinal cord (connects brain to body)

•Peripheral nervous system (PNS)•Made up of nerves outside of the CNS:• Somatic• Autonomic

Central Nervous System

Central Nervous System CNS

•The Brain:•Cerebrum•Diencephalon •Cerebellum•Brain stem

The Central Nervous System

•Cerebrum:

• The largest part of the brain• Two lateral hemispheres (left and

right)• Left side is logical, right side is

creative• Connecting the 2 hemispheres is a

layer of neurons called the corpus collosum.• Cerebrum is divided into 4 lobes

The Central Nervous System•Cerebrum:

• Functions:

• Provides higher brain functions (deep thinking,

intelligence,learning)

• memory

• Sensory information (touch, smell, taste, visual, hear)

• Coordinates skeletal muscles

• Language, comprehension, speech

• Personality development (sense of humor, competitiveness

The Central Nervous System

•Cerebrum:• Frontal lobe

• Primary motor area: conscious movement of all skeletal muscles• Higher intellectual reasoning

• Parietal lobe:• Somatic sensory cortex area: Touch, pain, temperature..etc)

• Temporal lobe:• Auditory area, olfactory area

• Occipital lobe:• Visual area

The Central Nervous System

•Diencephalon:

• Composed of:1. Thalamus2. Hypothalamus3. Epithalamus

Diencephalon cont:• Thalamus:

• Relay station for sensory impulses• Cerebrum Sensory signals thalamus brain stem spinal cord PNS

• Hypothalamus:• Regulates body temp, water balance, metabolism (TSH)• Stress management: by adrenocorticotropic hormone (ACTH) adrenal

glands• Controls pituitary gland:

• GH, prolactin, oxytocin…etc.• Important part of the “limbic system” or emotional visceral brain (thirst,

appetite, pleasure, pain..etc)• Influences controls of the medulla oblongata

• Epithalamus:• Contains pineal gland melatonin (sleep hormone)• Choroid plexux forms CSF

The Central Nervous System

•Brain Stem:• Midbrain

• Bulges in front called cerebral peduncles: coordinate fine motor movements (e.g. grasping an object between a thumb and a finger)

• Dorsally: protrusions called corpora-quadrigemina: involved in vision and hearing

• Pons• Involved in the control of breathing

• Medulla oblongata• Cardioregulatory centers (heartbeat, blood

pressure, vasoconstriction…)• Breathing, swallowing, vomiting

The Central Nervous System

•Cerebellum:

• Located under the cerebrum

• Functions:• Controls balance and

movement coordination • By receiving information from

the body and sending information to the body

The Central Nervous System

•Spinal Cord:

• Receives signals from

the brain

• Passes signals to the

PNS (which take them

to the rest of the body)

Peripheral Nervous System is Subdivided into:

•Somatic nervous system

•Autonomic nervous system

Somatic Nervous System

•Connects CNS to skeletal muscles and skin

•Conscious control

•voluntary

Autonomic Nervous System

• Connects CNS to viscera and glands

• Unconscious, involuntary control (breathing, heartbeat, digestive system..etc)

• Further subdivided to:• Sympathetic• parasympathetic

Sympathetic VS Parasympathetic• Sympathetic:

• Also called thoracolumbar division

• Fight or flight (scared, excited, thriving for survival..etc)

• Dialation of pupils

• Inhibit salivation

• Relax bronchi (to get more air in)

• Accelerate heartbeat

• Inhibit peristalsis and secretion (can survive longer without food)

• Stimulate glucose production and release

Sympathetic VS Parasympathetic• Parasympathetic:

• The “craniosacral division”

• Relaxed mode

• Constrict pupils

• Stimulate flow of saliva

• Constrict bronchi

• Slow heartbeat

• Stimulate peristalsis and secrestion

• Stimulate bile release

Autonomic Nervous system

• Two primary neurotransmitters:

• Acetylcholine• Major neurotransmitter• Used by parasympathetic and sympathetic nervous

system

• Norepinephrine• Used by sympathetic nervous system

Protection of CNS

•Meninges

•Blood Brain barrier

Meninges

•Three C.T. membranes covering structures of the brain:

1.Dura matter (outer most)

2.Arachnoid matter (middle)

3.Pia matter (inner most)

Blood Brain Barrier

• Protects brain from blood borne substances

• Composed of the least permeable capillaries in the body (brain capillaries).

• Only water, glucose and essential a.a can pass through its capillaries walls.

• Metabolic wastes; urea, toxins..etc are prevented