Neurophysiology

The Central Nervous System

Nervous System

Figure 11.1

• Functions– Sensory input – monitoring stimuli occurring inside & outside the

body – Integration – interpretation of sensory input– Motor output – response to stimuli by activating effector organs

Organization of the Nervous System

• Central nervous system (CNS) – Brain and spinal cord– Integration and

command center

• Peripheral nervous system (PNS)– Paired spinal and

cranial nerves– Carries messages to

and from the spinal cord and brain

• Afferent (sensory) division – transmits impulses from receptors to the CNS.

– Somatic afferent fibers – carry impulses from skin, skeletal muscles, and joints

– Visceral afferent fibers – transmit impulses from visceral organs

Peripheral Nervous System: Afferent Division

• Motor (efferent) division – transmits impulses from the CNS to effector organs. Two subdivisions:

– Somatic nervous system – provides conscious control of skeletal muscles

– Autonomic nervous system – regulates smooth muscle, cardiac muscle, and glands

Peripheral Nervous System: Efferent Division

Sensory• General somatic senses – receptors are widely spread

• Touch• Pain• Vibration• Pressure• Temperature

• Proprioceptive senses – detect stretch in tendons and muscle • Body sense – position and movement of body in space

• Special somatic senses • Hearing• Balance• Vision• Smell

• Visceral sensory• General visceral senses – stretch, pain, temperature, nausea, and hunger

• Widely felt in digestive and urinary tracts, and reproductive organs • Special visceral senses - taste

Motor

• General somatic motor – signals contraction of skeletal muscles

• Under our voluntary control

• Often called “voluntary nervous system”

• Visceral motor• Regulates the contraction of smooth and cardiac muscle

• Makes up autonomic nervous system

• Controls function of visceral organs

• Often called “involuntary nervous system”

• Autonomic nervous system

• Brain

• Spinal cord

Central Nervous System:

Organization of the Nervous System

Figure 8-1: Organization of the nervous system

• The two principal cell types of the nervous system are:– Neurons – excitable cells that transmit

electrical signals– Neuroglia - supporting cells

Nerve Tissue

• Functional: – Sensory (afferent) — transmit impulses toward the CNS– Motor (efferent) — carry impulses away from the CNS– Interneurons (association neurons) — shuttle signals through CNS

pathways

Neuron Classification

Neuroglia

Figure 12.6

• Hair, skin, cranium• Meninges• Cerebrospinal fluid• Blood brain barrier

CNS Protection

Meningeal Layers

Figure 9-4b

• Meningeal layer of the brain cushion and protect delicate neural tissue

Cerebrospinal Fluid

• Shock absorbing medium

• Provides a optimum and stable environment for generating nerve impulses

• Provides a medium for the exchange of nutrients and wastes between blood and nervous tissue

Cerebrospinal Fluid• Formed by selective transport across ependymal cells• Volume 125-150 ml and is replaced > 3 times/day, flow

maintained by 10 mmHg pressure gradient• Path: ventricles subarachnoid space, reabsorbed into blood

in dural sinuses through arachnoid villi

• Extensive capillaries & sinuses• Tight junctions promoted by astrocyte• Limits permeability for most molecules except O2, CO2, alcohol,

steroids, H2O• Protects brain: hormones & circulating chemicals

– Protects CNS from chemical fluctuations– Prevents entry of harmful substances– Prevents entry of molecules that could act as neurotransmitters

• Brain receives 15% of blood pumped by heart• Brain responsible for about half of body’s glucose consumption• Membrane transporters move glucose from plasma into the brain

interstitial fluid

Blood Brain Barrier

Figure 9-6: The blood-brain barrier

• Trillion interneurons fill the brain

• Up to 200,000 synapses each

• Levels of complexity– Cerebral cortex– Basal nuclei– Thalamus– Hypothalamus– Cerebellum– Brain stem

Brain Organization

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 FunctionsBrain component

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. Regulation 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 patterns

1. Maintenance of balance2. Enhancement of muscle tone3. Coordination and planning of skilled voluntary muscle activity

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

Cerebral cortex

Basal nuclei

Thalamus

Hypothalamus

Cerebellum

Brain stem(midbrain, pons,and medulla)

Brain Overview

Cerebrum• Highly developed• Makes up about 80% of total brain weight

(largest portion of brain)• Inner core houses basal nuclei• Outer surface is highly convoluted cerebral

cortex– Highest, most complex integrating area of the brain– Plays key role in most sophisticated neural functions

Cerebral Cortex• Three specializations

– Sensory areas - sensory input translated into perception– Motor areas - direct skeletal muscle movement– Association areas - integrate information from sensory and

motor areas, can direct voluntary behaviors

Cerebral Cortex• Each half of cortex divided into four major lobes

– Occipital lobe - carries out initial processing of visual input– Temporal lobe - initial reception of sound sensation, taste, smell– Parietal lobe - somatosensory processing– Frontal lobe responsible for

• Voluntary motor activity• Speaking ability• Elaboration of thought

Primary Somatosensory Cortex

• Located in the postcentral gyrus, this area:– Receives information from

the skin and skeletal muscles

– Exhibits spatial discrimination

• Somatosensory homunculus – caricature of relative amounts of cortical tissue devoted to each sensory function

Primary Motor Cortex

• Located in the precentral gyrus

• Composed of pyramidal cells whose axons make up the corticospinal tracts

• Allows conscious control of precise, skilled, voluntary movements

• Motor homunculus – caricature of relative amounts of cortical tissue devoted to each motor function

Language

Figure 9-23: Cerebral processing of spoken and visual language

•Primary areas of cortical specialization for language–Broca’s area governs speaking ability

–Wernicke’s area•Concerned with language comprehension•Responsible for formulating coherent patterns of speech

Functional Areas of the Cerebral Cortex

Figure 9-15

Brain Function: Cerebral Lateralization• Each lobe has special functions

Figure 9-16

Cerebral Cortex

• Schematic Linking of Various Regions of the Cortex

Basal Nuclei

• Act by modifying ongoing activity in motor pathways

• Primary functions– Regulates muscle tone

throughout the body– Selecting and maintaining

purposeful motor activity while suppressing useless or unwanted patterns of movement

– Helping monitor and coordinate slow, sustained contractions, especially those related to posture and support

– Controls large automatic movement

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Thalamus• Final relay point for

ascending sensory information

• Coordinates the activities of the cerebral cortex and basal nuclei

• Domain-specific Domain-specific information information processingprocessing

Hypothalamus

• Receives indirect sensory inputs from all sensory systems

• Sends neural outputs to various motor control nuclei

• Sends neural outputs to sympathetic and parasympathetic nervous systems

• Sends both neural and hormonal outputs to pituitary

Hypothalamus • Controls somatic motor activities at the subconscious

level• Controls autonomic function• Coordinates activities of the endocrine and nervous

systems• Secretes hormones• Produces emotions and behavioral drives• Coordinates voluntary and autonomic functions• Regulates body temperature• Coordinates circadian cycles of activity• 4Fs: feeding, fighting, fleeing, and reproductive behavior

Limbic System• Cingulated gyrus

– Coordinates sensory input with emotions

– Emotional responses to pain – Basic, inborn behavioral patterns

related to survival and perpetuation of the species

– Regulates aggressive behavior • Hippocampus - sends memories out

to the appropriate part of the cerebral hemisphere for long-term storage and retrieving them when necessary, Plays important role in motivation and learning

• Amygdala - involved in emotional responses, hormonal secretions, and memory,

Cerebellum• Basic functions: coordination, balance, motor learning, etc.

• Vestibulocerebellum – balance and control of eye movement

• Spinocerebellum – enhances muscle tone and coordinates skilled voluntary movement – important in synchronization and timing

– Receives input concerning desired action from motor cortex– Receives feedback concerning actual action from proprioceptors, vestibular

apparatus, eyes– Compares inputs and sends adjustments or corrective signals to motor tracts

• Cerebrocerebellum – planning and initiation of voluntary activity by providing input to the cortical motor areas also involved in procedural memories

Brain Stem: Midbrain, Pons & Medulla• An important link between spinal cord and higher brain levels, relays motor and

sensory impulses between other “higher” parts of the brain and spinal cord• Midbrain – eye movement control• Pons/Medulla

– Signal relay– Involuntary functions

• Many cranial nerves enter• Pyramids – nerve tracts crossover

Cranial Nerves

Table 9-1: The Cranial Nerves

Pons• Sensory and motor nuclei for four cranial nerves• Nuclei that help control respiration• Nuclei and tracts linking the cerebellum with the

brain stem, cerebrum and spinal cord

Medulla oblongata • Contains relay stations and reflex centers

– Cardiovascular and respiratory rhythmicity centers• Cardiovascular center - regulates rate and force of heartbeat

and vasoconstriction/dilation• Respiratory center - regulates basic breathing rhythm

• Reticular formation begins in the medulla oblongata and extends into more superior portions of the brainstem

Reticular Activating System

• Network in brain stem• Arousal, sleep, pain, &

muscle tone• Ascending fiber sends

signals upward• Arouses and activates

cerebral cortex• Controls overall degree

of cortical alertness or level of consciousness:– maximum alertness– wakefulness– sleep– coma

ElectroEncephaloGram -EEG

• Records electrical activity within cerebral cortex from EPSP, IPSPs

• Used for– Diagnose cerebral dysfunction– Brain death– Sleep Patterns

Consciousness - Sleep

Functions of Sleep

• “Catch up” time – restore biochemical and physiological processes

• Role of adenosine– Increased levels while awake– Inactivates RAS– Caffeine blocks adenosine receptors

• Shift gears – long term structural and chemical changes required to consolidate memory and learning

Learning and Memory• Learning has two broad types

– Associative – conditioning, linking two events together

– Nonassociative• Habituation• Sensitization

• Memory has several types– Short-term and long-term– Reflexive and declarative

Memory • Storage of acquired knowledge for later recall• Memory trace - Neural change responsible for retention or storage of

knowledge• Short-term memory - Lasts for seconds to hours• Long-term memory - Retained for days to years• Consolidation - Process of transferring and fixing short-term memory

traces into long-term memory stores• Working memory - Temporarily holds and interrelates various pieces of

information relevant to a current mental task

Long-Term vs Short-Term Memory

Short Term Memory• Seconds to hours

• Limited capacity

• Rapid retrieval

• Synaptic alterations– Changes in ion channels– Presynaptic facilitation cAMP– Long term potentiation (LTP)

Basic Learning - Behavior

• Habituation – decreased responsiveness to stimulus1. Closure of Ca++ channels leads2. Reduced neurotransmitter release3. Decrease EPSP

• Sensitization – increase responsiveness1. Release of serotonin from interneuron2. Increases cAMP in presynaptic neuron3. Blocks K+ channels and prolongs AP 4. Ca++ channels are open longer5. Increasing neurotransmitter output

LTP• Glutamate acts as NT, binds to:

– AMPA receptors elicit EPSPs – NMDA receptors open Ca++ channels > 2nd messenger system

• Increases # of AMPA receptors / releases nitric oxide (NO)• NO causes presynaptic neuron to increase neurotransmitter release

Long Term Memory

• Last days to years

• Unlimited capacity

• Permanent structural changes– Formation of new synapses between

existing neurons– Increased dendritic surface area– Increase in neurotransmitter receptors– Changes in neurotransmitter synthesis

Long Term Memory• Consolidation is affected by:

– Amount of rehearsal– Association of new & old data– Level of excitement/importance of information– CREB – regulatory proteins that activate genes important in

long term memory storage

Memory Processing• Hippocampus (limbic system)

– Short term memory and consolidation– Declarative memory (facts), specific objects– Requires conscious recall

• Cerebellum – “how to” memories– Motor skills (procedural)– Subconscious recall

• Prefrontal cortex - working memory– Processes new and retrieved information– Temporary storage– Problem solving, planning, organizing

• Cervical

• Thoracic

• Lumbar

• Sacral

Spinal Cord Regions

Figure 9-4a: ANATOMY SUMMARY: The Central Nervous System

• Gray matter: mostly cell bodies– Dendrites & terminals– Spinal reflex integrating center

• White matter– Bundles of myelinated axons

• Ascending tracts – sensory• Descending tracts – motor

– Dorsal roots– Ventral roots

Spinal Cord Organization

Spinal Cord Organization

Figure 9-7: Specialization in the spinal cord

Spinal Cord: Integrating Center

Figure 9-8

• Stimulus

• Sensory receptor

• Sensory (afferent) neuron

• CNS integration

• Efferent (motor) neuron

• Effector (target tissue)

• Response (movement)

• Feedback to CNS

Neural Reflexes: Overview

Neural Reflex

Figure 13-1: Monosynaptic and polysynaptic somatic motor reflexes

Patellar Tendon Reflex: Stretch & Reciprocal Inhibition

Figure 13-7: The knee jerk reflex

Cross Extensor Reflex/Withdrawal Reflex

Figure 13-8: Flexion reflex and the crossed extensor reflex