The Nervous System

What is the nervous system?

• sensory

• communication

• control

The nervous system continuously monitors and processes sensory information both…

from the external environment and form within the body

Three overlapping functions of the nervous system

• sensory receptors monitors changes inside(stimuli) and outside of the body→ sensory input

• process and interprets sensory input(integration) makes decision

• dictates a response by activating effector organs(motor output)

Central nervous system

• is the integrating and command center

• it consists of the brain and spinal cord

Peripheral nervous system

• consists of nerves extending from the brain(cranial nerves) and spinal cord(spinal nerves)

• links all regions of the body to the CNS

Sensory(afferent)

going towards the CNS

Motor(efferent)

going away from the CNS

Both the sensory inputs and motor outputs are further subdivided…

according to the body regions they serve

Somatic body region

• structures external to the ventral body cavity

• skin, skeletal musculature, bone

Visceral body region

• the viscera within the ventral body cavity

• digestive tube, lungs, heart, bladder, etc.

Regions result in four main subdivisions…

• somatic sensory

• visceral sensory

• somatic(voluntary) motor

• visceral(involuntary) motor

Somatic sensory

• the sensory innervation of the outer part of the body

• general somatic senses

• proprioceptive senses

• special somatic senses

General somatic senses

• senses whose receptors are widely spread throughout the outer part of the body

• touch, pain, vibration, pressure, and temp.

Proprioceptive senses

• detect stretch in tendons and muscles

• give body sense of position and movement of body in space

Special somatic senses

hearing, balance, vision, and smell

Visceral sensory

• the sensory innervation of the viscera

• general visceral senses

• special visceral senses

General visceral senses

• stretch, pain, temp, nausea, and hunger

• widely felt in digestive and urinary tracts

• reproductive organs and other viscera

Special visceral senses

taste

Somatic(voluntary) motor

• the motor innervation of the outer tube; specifically skeletal muscles

• general somatic motor

General somatic motor

signals contraction of skeletal muscles; under our voluntary control; often called “voluntary nervous system”

Visceral(involuntary) motor

• the motor innervation of the inner tube; specifically smooth muscle, cardiac muscle, and glands

• general visceral motor

General visceral motor

• regulates the contraction of smooth and cardiac muscle and secretion from glands

• makes up autonomic nervous system(ANS)

• controls function of visceral organs often called involuntary nervous system

Nervous tissue

• cells are densely packed and intertwined

• composed of two main cell types

• neurons

• support cells(glial cells)

Neurons

excitable nerve cells that transmit electrical signals

Support cells(glial cells)

non-excitable cells that surround and wrap the neurons

Neurons(nerve cells)…

• are the basic structural units of the nervous system

• they are specialized cells that conduct electrical impulses long their plasma membrane

• send and receive signals

Other special characteristics of neurons…

• longevity

• do not divide

• high metabolic rate

Cell body(soma or perikaryon)

• all neurons have one

• cytoplasm contains a single nucleus and the usual organelles

• chromatophilic or nissl bodies

• neurofibrils

• ganglia

Most neuronal cell bodies are located within the CNS…

and protected by bones of the skull and vertebral column

Chromatophilic or nissl bodies

clusters of rough ER and free ribosomes that stain darkly and renew membranes of the cell

Neurofibrils

bundles of intermediate filaments; form a network between chromatophilic bodies

Ganglia

clusters of cell bodies not in CNS; lie along nerves in the PNS

Most neuronal cell bodies are located…

within the CNS

Two types of processes

• processes extend from the cell bodies of all neurons

• axons

• dendrites(dendro=”tree”)

Dendrites

• branch from the cell body

• all organelles that occur in the cell body also occur here

• bear receptors for neurotransmitters released by other neurons

• transmit electrical signals toward the cell body

Axon

• transmits impulses away from the cell body; no protein synthesis in axon

• neurofilaments, actin microfilaments, and microtubules provide structural strength along the length of the axon

• aids in the transportation of substances to and from the cell body

Branches(axon collaterals) along…

length are infrequent

Terminal branches(telodendria)

multiple branches at the end of an axon

Terminal branches end in knobs called…

axon terminals

Stored in axon terminals…

neurotransmitter chemical in tiny vesicles

Neurotransmitters(NT)

chemical messenger molecules that excite or inhibit neurons

Some neurotransmitters

• acetylcholine

• norepinephrine

• dopamine

• serotonin

• GABA

• glycine

• neuromodulators

Acetylcholine

voluntary movement of the muscles

Norepinephrine

wakefulness or arousal

Dopamine

• voluntary movement and motivation

• “wanting”

• pleasure

• associated with addiction and love

Serotonin

• memory

• emotions

• wakefulness

• sleep

• temp. regulation

GABA

inhibition of motor neurons

Glycine

spinal reflexes and motor behaviour

Neuromodulators

sensory transmission(especially pain)

Synapse

• site at which neurons communicate

• signals pass across it in one direction(diffusion→ high to low)

Synaptic cleft

• tiny gap

• each axon terminal is separated from the cell body or dendrites of the next(postsynaptic) neuron

• therefore there is no physical contact between neurons

Several types of synapses…

• axodendritic synapses

• axosomatic

• less common: axoaxonic, dendrodendritic, and dendrosomatic

Axodendritic synapses

• between axon terminals of one neuron and dendrites of another

• most common type of synapse

Axosomatic synapses

between axon terminals of one neuron and neuronal cell bodies of another

When an impulse reaches the axonal terminals, some of the synaptic vesicles rupture and release neurotransmitters into the synaptic cleft…

which diffuses across the synaptic cleft to bind to membrane receptors on the next neuron

Neurons conduct electrical signals..

along the length of their plasma membranes

Resting(unstimulated) neuron

• the membrane is polarized

• the inner cytoplasmic side is negatively charged with respect to the extracellular side

Depolarized neuron

• when a neuron is stimulated, the permeability of the plasma membrane changes at the side of the stimulus

• allowing positive ions to rush in

• as a result, the inner face of the membrane becomes less negative

Action potentials(AP) on axons

• a strong stimulus applied to the axon triggers a nerve impulse or AP

• membrane becomes negative externally; impulses travel the length of the axon

• membrane repolarizes itself

Structural classification depends on the…

number of processes off cell body

Multipolar

• possess more than two processes

• numerous dendrites

• one axon

• most numerous type(99%+)

Bipolar

• possess two processes extending from opposite sides of body

• rare neurons

• found in some special sensory organs

Unipolar(pseudounipolar)

• possess one short emerging from the cell body that divides like an inverted T into long branches

• start as bipolar neurons during development

Neurons are grouped functionally according to the…

direction the nerve impulse travels relative to CNS

Sensory(afferent) neurons transmit impulses toward the CNS..

• virtually all are unipolar neurons

• cell bodies in ganglia outside the CNS

• have a short, single process that divides into two process

• central and peripheral process

Central process

runs centrally into the CNS

Peripheral process

extends peripherally to the receptors

Motor(efferent) neurons carry impulses away from the CNS to effector organs…

• most motor neurons are multipolar

• cell bodies are within the CNS

• form junctions with effector cells

Interneurons(association neurons)…

• most are multipolar

• lie between motor and sensory neurons

• confined to the CNS

Supporting cells

• non-excitable cells that surround and wrap the neurons

• six-types

• 4 in the CNS

• 2 in the PNS

• Function: provide a supportive scaffolding for neurons and insulate the non-synaptic parts of neurons

Why do you need to insulate the neurons?

• we want that signal to perceive the entire length of the axon

• we want to make sure it breaks off

• we want to make sure it doesn’t weaken

• we want to make sure the signal is sent

Supporting cells in the CNS

neuroglia(“nerve glue”) or glial cells

Neuroglia or glial cells

• cells have branching processes and a central cell body

• outnumber neurons 10 to 1; make up half the mass of the brain

• can divide throughout life

• small size and darkly staining nuclei

• Types: astrocytes, microglia, ependymal cells(“wrapping garment”), oligodendrocytes(“few branched cells”)

Astrocytes

• most abundant glial cell type

• take up and release ions to control the environment around neurons

• recapture and recycle neurotransmitters (ex. glutamate)

• involved with synapse formation in developing neural tissue

• produce molecules necessary for neural growth

• propagate calcium signals that may be involved in memory

Microglia

• smallest and least abundant

• they are the phagocytes and macrophages of the CNS

• engulf invading microorganisms and dead neurons

• derived from blood cells called monocytes

Ependymal cells

• forms a simple epithelium that lines the central cavity of the spinal cord and brain

• have cilia to help circulate the cerebrospinal fluid

Oligodendrocytes

• have fewer branches than astrocytes

• wrap their cell processes around thicker axons in CNS

• produce myelin sheaths

Supporting cells in the PNS

• satellite cells

• schwann cells(neurolemmocytes)

Satellite cells

surround neuron cell bodies within ganglia

Schwann cells

surround all axons in the PNS and form myelin sheaths

Myelin sheaths

• segmented structures composed of the lipoprotein myelin

• forms an insulating layer

• increases the speed of impulse conduction

• produced by oligodendrocytes(CNS) and schwann cells(PNS)

Myelin sheaths in the PNS…

• formed by many individual schwann cells

• wrap in concentric layers around the axon

• creating a tightly packed coil of membranes

• discontinuous sheath separated by gaps called nodes of ranvier

Thick axons are myelinated and…

thin axons are unmyelinated

During the wrapping process, the cytoplasm is squeed form between adjacent layers of the schwann cell membranes…

so that a tight core of plasma membrane material encompasses the axon

This wrapping is called the…

myelin sheath

The schwann cell nucleus…

and the bulk of its cytoplasm ends up just beneath the outermost portion of its plasma membrane

The peripheral part of the schwann cell and its exposed plasma membrane is called…

the neurilemma(sheath of schwann)

Myelin sheaths in the CNS are formed by oligodendrocytes…

in contrast to schwann cells each oligodendrocyte has multiple processes that coil around several different axons

Nerves

• cordlike organs in the PNS

• consists of numerous axons(nerve fibers)

• arranged in parallel bundles wrapped in C.T.

Within a nerve, each axon is surrounded by schwann cells…

then a layer of loose C.T. called endoneurium

Nerve fascicles

groups of axons bound into bundles

Perineurium

C.T. wrapping surrounding a nerve fascicle

Epineurium

whole nerve is surrounded by tough fibrous sheath

Reflex arcs

• simple chains of neurons that explain reflex behaviors and determine the basic structural plan of the nervous system

• responsible for reflexes…rapid, automatic motor responses to stimuli

Somatic reflexes

resulting in the contraction of skeletal muscles

Visceral reflexes

reflexes activating smooth muscle, cardiac, or glands

Every reflex arc has five essential components…

• receptor

• sensory neuron

• integration center

• motor neuron

• effector

Receptor

site where stimulus acts

Sensory neuron

transmits afferent impulses to the CNS

Integration center

consists of one or more synapses in the CNS