The Central Nervous System

Nerve and muscle cells can develop several electrical states at their membranes.

• Polarization - This is any state, positive or negative, other than 0 mV.

• Depolarization - This change makes the membrane potential less negative than the resting potential.

• Repolarization - The membrane returns to resting potential after depolarization.

• Hyperpolarization - This makes the membrane more polarized, more negative inside.

• Electrical signals are produced by changes in ion movement across the plasma membrane.

• Changes in ion movement are brought about by changes in membrane permeability in response to triggering events.

• Water-soluble ions cannot penetrate plasma membrane it need a carrier or a channel.

• Membrane channels are either leak or gated channels.

There are four kinds of gated channels:

• Voltage-gated channels: Open or close in response to membrane potential.

• Chemically gated channels: Change shape in response to binding of a specific extracellular chemical messenger.

• Mechanically gated channels: Respond to stretch.

• Thermally gated channels: Respond to temperature.

Action Potentials

• Brief, rapid, large, changes in membrane potential during which the excitable cell transiently becomes more positive than outside.

• The duration of action potential last for only 1 msec.

• If the initial trigger does not reach threshold potential, no action potential takes place.

• All or None phenomena.

Voltage-Gated Sodium Channel

(ECF)

Plasmamembrane

(ICF)

Inactivationgate Activation

gate Rapidopeningtriggeredat threshold

Slowclosingtriggeredat threshold

At resting potential(–70 mV)

From threshold to peak potential(–50 mV to +30 mV)

From peak to resting potential(+30 mV to –70 mV)

Closed but capableof opening Open (activated)

Closed and not capableof opening (inactivated)

Voltage-Gated Potassium Channel

(ECF)

Plasmamembrane

(ICF)

Closed Open

At resting potential; delayedopening triggered at threshold;

remains closed to peak potential(–70 mV to +30 mV)

From peak potential throughafter hyperpolarization

(+30 mV to –80 mV)

Delayedopeningtriggeredat threshold

Falling phaseRisi

ng p

hase

Threshold potential

Resting potentialCaused by K

+ effluxCa

used

by

Na+

influ

x

Triggering event

Depolarization(decreased membrane

potential)

Positive-feedback cycle

Influx of Na+

(which further decreases

membrane potential)

Opening of somevoltage-gatedNa+ channels

Nucleus

Input ZoneDendritesandCell body

Trigger ZoneAxon hillock

Conducting ZoneAxon (may be from 1mmto more than 1m long)

Output ZoneAxon terminals

Arrows indicate the direction in which nerve signals are conveyed.

Brain

Centralnervoussystem(CNS)

Spinalcord

Peripheralnervoussystem(PNS)

Afferentdivision

Efferentdivision

Sensorystimuli

Visceralstimuli

Somaticnervous system

Autonomicnervous system

Motor neurons

Sympatheticnervous system

Parasympatheticnervous system

Skeletal muscle

Smooth muscleCardiac muscleGlands

Effector organs(made up of muscle and gland tissue)

(Input to CNSfrom periphery)

(Output from CNSto periphery)

• There are three classes of neurons:

1. Afferent neuron sends signals toward the CNS. It generates action potentials from sensory receptors at its peripheral end. It has a long axon and is found mainly in the PNS.

2. Efferent neuron sends signals away from the CNS to an effector organ. It has a long peripheral axon in the PNS.

3. Interneuron is found entirely within the CNS. It lies between afferent and efferent neurons.

Centralnervous system(spinal cord)

Peripheralnervous system

Axonterminals

Cellbody

Afferent neuron

Centralaxon

Peripheral axon(afferent fiber) Receptor

Interneuron

Efferent neuron*Effector organ(muscle or gland)

Axon(efferent fiber)

Axonterminals

* Efferent autonomic nerve pathways consist of a two-neuron chain between the CNS and the effector organ.

Cellbody

• Glial cells:

• About 90% of the CNS cells are glial cells.

• Glial cells do not send signals. They support interneurons physically, metabolically, and functionally.

• There are four main kinds:

1. Astrocytes

2. Oligodendrocytes

3. Microglia

4. Ependymal cells

The astrocyte has many functions:

• Holding neurons together

• Guiding neurons during development

• Establishing a blood-brain barrier (BBB)

• Repairing brain injuries

• Playing a role in neurotransmitter activity

• Taking up excess K+ from the brain ECF

The oligodendrocyte forms myelin sheaths around axons in the CNS.

• In fetal life it secretes nerve-growth-inhibiting proteins.

Microglia are the immune defense of the CNS.

• Activated microglia

releases destructive

chemicals against

targets.

Ependymal cells line the internal cavities of the CNS.

• The ependymal cells lining the ventricles help form

cerebrospinal fluid.

• They serve as neural stem cells with the potential of

forming glial cells and neurons.

The CNS is protected several ways:

• The cranium encloses the brain. The vertebral column encloses the spinal cord.

• It is wrapped by several meninges: the outer dura mater, the middle arachnoid mate, and the innermost pia mater.

• The brain is surrounded by the cerebrospinal fluid (CSF).

• The blood-brain barrier limits access of blood-borne substances to the brain.

The CSF is formed and circulates.

• It is produced by the choroid plexuses inside the ventricles.

• It circulates through the ventricles.

• From the fourth ventricle it enters the subarachnoid space, between the arachnoid mater and pia mater.

• Arachnoid villi is this space drain the CSF into the blood.

• The blood brain barrier is highly selective.

• It is a series of capillaries that regulate the exchange

between the blood and the brain.

• These capillaries allow a limited number of substances to

pass from the blood to brain cells.

• The brain needs a constant input of oxygen and glucose

from the blood.

The CNS consists of the brain and spinal cord.

• The outline for brain anatomy is:

1. Brain stem

2. Cerebellum

3. Forebrain

A. Diencephalon

• Hypothalamus Thalamus

B. Cerebrum

• Basal nuclei Cerebral cortex

Hypothalamus

Brain stem

Cerebral cortex

Thalamus(medial)

Basal nuclei(lateral to thalamus)

Cerebellum

Spinal cord

Midbrain

Pons

Medulla

Brain component

Cerebral cortex

Basal nuclei

Thalamus

Hypothalamus

Cerebellum

Brain stem(midbrain, pons,and medulla)

Major Functions

1. Sensory perception2. Voluntary control of movement3. Language4. Personality traits5. Sophisticated mental events, such as thinking memory, decision making, creativity, and self-consciousness

1. Inhibition of muscle tone2. Coordination of slow, sustained movements3. Suppression of useless patterns of movements

1. Relay station for all synaptic input2. Crude awareness of sensation3. Some degree of consciousness4. Role in motor control

1. Regualtion of many homeostatic functions, such as temperature control, thirst, urine output, and food intake2. Important link between nervous and endocrine systems3. Extensive involvement with emotion and basic behavioral patterns1. Maintenance of balance2. Enhancement of muscle tone3. Coordination and planning of skilled voluntary muscle activity

1. Origin of majority of peripheral cranial nerves2. Cardiovascular, repiratory, and digestive control centers3. Regulation of muscle reflexes involved with equilibrium and posture4. Reception and intergration of all synaptic input from spinal cord; arousal and activation of cerebral cortex5. Role in sleep-wake cycle

Brain component

Cerebral cortex

Basal nuclei

Thalamus

Hypothalamus

Cerebellum

Brain stem(midbrain, pons,and medulla)

• The cerebrum is on top of the lower brain regions. It is highly developed in humans.

• The cerebral cortex is its highly convoluted, outer layer of gray matter. It covers an inner core of white matter.

• The cerebrum has an inner core of basal nuclei located

deep within the white matter.

• The cerebral cortex has four lobes. Each is specialized for different activities:

• Occipital - initial processing of visual input.

• Temporal - integration of all sensory input.

• Parietal - somatosensory processing; Each region of its cortex receives somesthetic and proprioceptive input from a specific body area. This part of the cortex receives most input from the opposite body side.

• Frontal - voluntary motor activity, speaking ability, and elaboration of thought; Stimulation of different areas of its primary motor cortex moves different body regions.

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Frontallobe

Central sulcus

Parietallobe

Parietooccipitalnotch

Occipitallobe

Preoccipitalnotch

CerebellumBrain stem

Temporallobe

Lateralfissure

• The mapping of somatotopic areas varies slightly between individuals and is in a dynamic steady state.

• These areas are influenced by use-dependent competition.

• They are modified by experience.

• The plasticity of the brain can be remodeled in response to varying demands.

FrontLefthemisphere

Primarymotorcortex

Topview

Somato-sensorycortex

Righthemisphere

Frontallobe

Centralsulcus

Parietallobe

Occipital lobe

Back

Lefthemisphere

Cross-sectional view

Temporal lobe

Sensory homunculus

FrontLefthemisphere

Primarymotorcortex

Topview

Somato-sensorycortex

Righthemisphere

Frontallobe

Centralsulcus

Parietallobe

Occipital lobe

Back

Lefthemisphere

Cross-sectional view

Temporal lobe

Motor homunculus