Physiology 210 (CNS)

Central Nervous System (CNS)

Includes the Brain (Cerebrum, Cerebellum, and Brainstem) and the Spinal Cord

Peripheral Nervous System (PNS)

Includes the Afferent Division and the Efferent Division

Afferent Division

Includes the Somatic, Visceral, and Special Sensory areas

Efferent Division

Includes the Somatic and Autonomic areas

Spinal Column

- Encases the Spinal Cord

- Sensory nerves enter the Dorsal Horn via the Dorsal Roots and exit the Ventral Horn via the Ventral Roots

- Horn refers to the horn of the grey matter inside the spine; the Root refers to the axons bundled together

- Nerve quickly piled together into a Peripherial Nerve following the entry/exit of the spinal cord

Cartiligenous Disks separate different vertebra, allowing for the spine to be flexible and to move without pain

(When cartilage breaks down the disks can slip, normally impedes on the nerves)

Q. What allows for the flexibility of the Spinal Column?

Spinal Cord

- Is encased by the Spinal Column

- A flattened cylinder, 16-18 inches in length and 3/4 inch in diameter (NOTE: this is not constant)

- Has Cervical and Lumbar enlargements

- In adult ends at L2, in newborns it ends at L4 (cord and column have different growth rates)

Vertebrae

- The bones of the Spinal Cord themselves

- Has a large, central opening that accommodates the spinal canal, which encloses and protects the spinal cord

- There are 7 Cervical, 12 Thoracic, 5 Lumbar, and 5 Sacral Vertebrae

Epidural Space

- Surrounds the Vertebrae, outside the Dura Mater

- Filled with Fat

Dura Mater

- Means Tough Mother

- The dense, irregular CT tube

- A thick membrane that is the outermost of the three layers of the meninges that surround the brain and spinal cord

- It is derived from mesoderm

Subdural Space

- Between the Dura Mater and the Arachnoid Mater

- Filled with Interstitial Fluid

Arachnoid Mater

- A spider web of Collagen fibers

- The middle of the three meningeal layers

Subarachnoid Space

- Between the Arachnoid Mater and the Pia Mater

- Filled with Cerebrospinal Fluid

Pia Mater

- Thin, third spinal meningeal layer that covers the BV

- Dentriculate ligs hold in place

Spinal Meninges

- The three layers that make up the Meninges are the Dura Mater, the Arachnoid Mater, and the Pia Mater

Meningitis

- Infection / Inflammation of the Meninges

- Primarily caused by viral infection, but can be caused by bacterial and fungal infection as well

More neural entries means more space needed to innervate the nerves

Occurs both higher up (Cervical) and for a high concentration of entry (arms and legs = Cervical & Lumbar)

Q. Why are the Cervical & Lumbar Enlargements where they are?

Spinal Tap / Lumbar Puncture

- The insertion of a long needle into the subarachnoid space in order to sample the Cerebrospinal Fluid that resides there

- Also used for injection of antibiotics, anesthetic (epidural), or chemotherapy

- Inserted between L3 and L5, as the spinal cord ends at L2 and thus needle can weave between the dorsal and ventral roots more easily

Cranial Nerves (of the PNS)

- 12 Cranial Nerves, + the Spinal Nerve

- Motor and Sensory axons in them control head & neck sensation and movement

- Malfunction on the same side as injury, ex. loss of sensation or paralysis of throat or tongue

Dermatomes

- Mapped regions of the body that pertain to specific regions of the spinal cord

- Regions are distinguished by patterns of numbness when injury or local anesthetics are implemented

Spinal Cord Transection

Injury that severs the cord, resulting in the loss of sensation and motor control below the injury in concordance with dermatomes

Neuritis

- Inflammation of nerves

- Caused by injury, vitamin deficiency, or poison

Shingles

- Infection of a peripheral nerve by the chicken pox virus

- Causes pain, skin discoloration, lines of skin blisters

Poliomyelitis

- Viral infection caused by motor neuron death and possible (real) death from cardiac failiure or respiratory arrest

Functions of the Spinal Cord

- Motoneuron (controls muscles; communication)

- Reflexes (spinal cord reflexes)

- Integration (summation of impulses)

- Highway (for sensory and motor information travel)

- Locomotor (rhythm generating circuits)

Dorsal Horn Neurons

First order sensory neurons

Ventral Horn Neurons

First order motoneurons

- Final output neuron of the Central Nervous System, and innervate the muscle

- At each segment, axons from the descending pathway travel into ventral horn and synapse on motoneurons

Interneurons

Involved in reflex behaviours and rhythmic motor behaviours

Also receive descending inputs (from the brain)

Function of Tracts of the Spinal Cord

- Highway for sensory and motor information

- Sensory tracts ascent the SC

- Motor tracts descend the SC

Naming of Tracts of the Spinal Cord

- Indicates position and direction of signal

ex. Anterior Spinothalamic Tract

- impulses from spinal cord to thalamus in brain

- found in anterior section of the SC

Spinothalamic Tract

- Is an Afferent / Sensory Tract

- Carries information on pain, temperature, deep pressure, and crude touch

- From the spinal cord to the thalamus

Dorsal Columns (Tracts)

- Is an Afferent / Sensory Tract

- Carries information on proprioception, discriminative touch, two point discrimination, pressure, and vibration

Corticospinal and Corticobulbar Tracts

- Are Direct, Efferent / Motor Tracts

- Carry information on precise, voluntary movements

Rubrospinal and Vestibulospinal Tracts

- Are Indirect, Efferent / Motor Tracts

- Carry information on programming movements, posture & muscle tone, equilibrium, and the coordination of visual reflexes

Anterior

Rostral, Ventral, Forward

Posterior

Caudal, Dorsal, Behind

Cerebrospinal Fluid (CSF)

- Originates from the Choroid Plexus

- Functions for mechanical protection, chemical protection (optimized environment for signalling and transduction), and circulation (exchange of nutrients and waste between the blood and tissue, and their transport)

Flow of CSF

Choroid Plexus > Ventricles > Subarachnoid Space > Arachnoid Villi > Venous Blood > Hear & Lungs > Arterial Blood > Choroid Plexus

Choroid Plexus

- Origin of CSF

- Capillaries covered by ependymal cells, found in;

- 2 lateral ventricles (each within each cerebral hemisphere)

- Roof of the 3rd ventricle

- Fourth ventricle

Drainage of CSF

- One median aperture & two lateral apertures allow CSF to exit from the interior of the brain into the arachnoid space

Hydrocephalus

The blockage of the pathway of drainage of CSF from the Ventricles

Blood Brain Barrier

Tight Junctions between the endothelial cells of capillaries and basement membrane keeps many molecules in blood from entering the brain

(protective by nature, but can be very problematic, ex. pharmacology)

The Common Plan

The organization of the Sensory Systems typically follows the Common Plan of

Receptors > Neural Encoding > Transmission to CNS > Organization > Central Control

Receptors

- Transducers that transform physical energy into electrical energy (also called stimulus transduction)

- Transduce stimulus features into neural codes

- First step; Formation of a receptor / generator potential. There are many different types of receptors involved in somatic sensation

Specialized Receptors

- Functional similarity is stimulus transduction

Examples; Mechanoreceptors, Chemoreceptors, and Photoreceptors

Somatosensory Processing

The transmission of information from the periphery to the CNS

Hierarchical Processing = Anterolateral System and Dorsal System

Parallel Processing = Peripheral Systems

Association cortices receive input from primary cortices and further interpret this input

The level of association can extend beyond the association cortex specific to that sensory input (due to parallel processing)

Primary Somatosensory Cortex

Conscious Perception of sensory information

Neural Encoding

- Four features of sensory stimuli are coded by the nervous system, regardless of sensory modality; somatosensory coding accounts for...

i. Stimulus Type (modality, receptor type)

ii. Intensity (strength, recruitment)

iii. Location (2point discrimination)

iv. Timing and Duration

Labelled Line Codes

- Different neurons in the brain are specific for different sensory regions

- A receptor project along specific pathways to a specific region of the cortex, involved with interpreting that specific stimulus modality

Sensory Stimulus Type

- A feature of sensory stimuli that is coded for by the nervous system

- Specialized Receptors that respond to only a specific type of physical stimulus

- Many different types of receptors involved in sensation, but serve the 4 distinct Somatic Modalities

Touch

Proprioception

Pain

Thermal Sensation

- The four facets of sensation subserved by the many different receptors;

4 Somatic Modalities

Sensory Intensity

- A feature of sensory stimuli that is coded for by the nervous system

- The higher/greater the stimulus intensity, the higher the FREQUENCY of Action Potential sent down the neuron

(No increase in AP intensity itself though, and not delivered any faster)

Recruitment

As stimulus intensity increases, the number of receptors activated will also increase

Pain and Temperature

Q. Bare nerve endings convey....

Pacinian Corpuscle

- One of the four major types of mechanoreceptor

- Nerve endings in the skin responsible for sensitivity to vibration and pressure

- Is the example of a Rapidly Adapting Sensor (to change of stimuli, not stimuli itself)

Sensory Location

- A feature of sensory stimuli that is coded for by the nervous system

- Some locations (especially on skin) have a huge discrepancy between the areas of reception for each receptor neuron

- This leads to a difference in Two-Point reception, or the body's ability to tell the difference between one and two stimuli over an area of space

Lateral Inhibition

- When a neuron on one path fires, it can inhibit the neuron next to it from firing

- This is due to overlapping receptive fields

- Result is that if there is not a certain amount of stimulation, the neuron will not fire at all

Sensory Timing and Duration

- A feature of sensory stimuli that is coded for by the nervous system

- Different receptors provide information about steady state conditions while others are more sensitive to change in stimuli (Rapidly Adapting vs. Slowly Adapting)

Anterolateral System

- Afferent neuron carries information specifically on pain and temperature

- Synapses at Dorsal Horn, and an Interneuron

- Crosses Spinal Cord and travels up the Anterolateral Column to the CNS

- Arrives at Thalamus

- Synapses onto a Third Neuron, connects to Cerebral Cortex here

The Dorsal Horn

Q. Where does cross-over occur in the Anterolateral System?

The Dorsal Horn (The Spinal Cord)

Q. Where is the 1st synapse in the Anterolateral System?

Dorsal System

- Afferent Neuron carries information specifically on touch and sensation

- Enters dorsal horn and travels up the Dorsal Column to the CNS

- Arrives at Medulla, synapses and crosses over

- Arrives at Thalamus

- Synapses onto a Third Neuron, connects to Cerebral Cortex here

The Medulla

Q. Where does the cross-over occur in the Dorsal System?

The Medulla

Q. Where is the 1st synapse in the Dorsal System?

The Thalamus

Q. Where is the 2nd synapse in both the Anterolateral and Dorsal Systems?

Thalamus

- Relay nuclei for sensory inputs

- Receives sensory information from the spinal cord and sends it to the appropriate region of the cerebral cortex

- "Post Office"

Hierarchical Organization

- Information is processed at each level providing opportunity for modification (attenuation or blockage), enhancement, and through divergence to other areas, enrichment via activation of other regions of the brain and association cortices

- Higher centers can give relatively generalized commands without having to specify details of motor action

Ex. both the Dorsal Column and the Anterolateral System

Parallel Organization

- When pathways convey only partially overlapping modalities (multiple things code for multiple things), it enriches the content of information transmitted into the CNS; it also provides a measure of redundancy in case of damage to one pathway

- An overlap of different functional components and is only important in recovery of function after local lesions

Ex. Somatosensory feedback from the periphery

Sensory Homunculus

Somatotopic representation of body surface on the somatosensory and motor cortexes

Central Sulcus

Divides the Motor (front) and Somatosensory (back) cortexes

Central Descending Control

Q. How does the brain control what information it receives?

Central Descending Control

- The method by which the brain has some control over the ascending sensory information it receives

- The wiring of afferent pathways alters the sensory information as it moves from the periphery into the CNS; lateral inhibition and negative-feedback (pre and post)synaptic inhibition

Ex. Inhibition of ascending pain information

Analgesic Presynaptic Inhibition

- A typical example of presynaptic inhibition

- Inhibition command by higher brain regions release Endorphines from a descending neuron onto the Afferent Pain Fiber's Axon, where there is an Opiate Receptor that the Endorphines bind to

- Endorphine Binding results in Substance P release being blocked

Substance P

Neurotransmitter that induces Pain (prostaglandins and bradykinin)

(there are many)

Somatic Motor Systems (SMS)

- The systems that provide the information necessary to guide our movements through the environment (balance, posture, eye movement, body and limb movement, communication)

- Systems capable of generating movements divided into 1 of 3 classes; Reflexes, Rhythmic Motor Patterns, and Voluntary

1. Convey accurately timed commands to many different muscles

2. Make postural adjustments

3. Consider the motor plant

Q. What must the Somatic Motor Systems do to control behaviour (3)?

Motor Plant

- The mechanical arrangement of muscles, bones, and joints

- The brain must know the physical restraints and what the body is capable of in order to know HOW to produce the movement

Reflex Motor Patterns

- Rapid, stereotyped, and involuntary responses that are controlled by magnitude of eliciting stimulus

- Stretch Reflex, Flexion & Withdrawal Reflexes

Rhythmic Motor Patterns

Initiation and termination generally under voluntary control but once activated these patterns continue almost automatically in an almost reflex like manner

Voluntary Motor Patterns

most complicated

1. Continuous flow of sensory information

2. Components organized hierarchically

Q. What organizational principles do Somatic Motor Systems rely on (2)?

1. Spinal Cord

2. Brainstem

3. Cortex and Subcortical Loops

Q. What makes up the hierarchical organization of the Somatic Motor System?

Spinal Cord (SMS)

- Contains Interneuronal Circuits which contain building blocks for Reflexes and Circuits for locomotion

- Contains Motoneurons which are the final common output path

Polysynaptic Withdrawal Reflex

- Reflex loop induced due to painful stimuli in the leg/foot, causing it to lift

- Inhibitory interneuron inhibits ipsilateral extensor motoneuron

- Excitatory interneuron excited ipsilateral flexor motoneuron

- Secondary interneuron projects controalaterally, resulting in postural adjustments (via the thalamus, somatosensory cortext, and then motor cortex)

- The flex/extension result is also called a Crossed Extension

Within the spinal cord and turned on from the descending commands, or sensory input (ie. stimuli)

Q. Where are the neural patterns responsible for locomotion generated?

Extrafusal Muscle Fibers

- Muscle fibers responsible for muscle contraction

- Interspersed with Intrafusal Muscle Fibers

- Have no sensors or anything for muscle length themselves

- Work generated by alpha motor neurons

Intrafusal Muscle Fibres

- Muscle Spindle Fibers dispersed between the Extrafusal Muscle Fibers

- Responsible for sending feedback to the brain (and then the motoneuron) regarding muscle length / tension

- Signals sent via gamma motor neurons

Intrafusal Muscle Fiber Activation (Gamma Motor Neuron Activation)

- When a muscle contracts the intrafusal fiber contracts, and sends information via gamma motor neurons to the brain

- Keeps track of the both the length of the muscle and how long the muscle is SUPPOSED to be

1. Sensory Input

2. Gamma Motor Neurons

Q. Where does the brain receive information from regarding muscles?

Alpha & Gamma Motor Neuron Coactivation

- Keeps the muscle fiber in sensory understanding, and the spinal cord can continue obtaining information form them about their lengths

Golgi Tendon Organs (GTOs)

- Mechanoreceptors in muscle tendons, responsible for Tension information feedback

- Arranged in Series within the muscle

- An inhibitory reflex to prevent damage (stops the muscle from further action/contraction; is protective)

- Activation of GTO inhibits the motoneuron

Muscle Tendons

The connective tissue between a muscle and bone

Brainstem (SMS)

- Multiple parallel pathways descend from the brainstem in tracks to influence segmental interneurons and motoneurons to help coordinate motor movement

- Indirect Pathways (or Medial)

Direct Pathways (SMS)

- Involved in fine motor control

- Includes the Corticospinal, and a Brainstem Pathway

Indirect Patways (SMS)

- Activate locomotor and other spinal networks

- Provide input from visual and vestibular systems to coordinate movement with visual and vestibular information

Cortex & Subcortical Loops (SMS)

Three Cortical Motor Areas

- Primary Motor Cortex

- Premotor Cortex

- Supplementary Motor Area

Two Subcortical Loops

- Cerebellum Loop

- Basal Ganglia Loop

Three Cortical Motor Areas

- Responsible for Voluntary Movement

- Goal Directed Behaviour

- Project to the brainstem and spinal cord motor areas via Corticobulbar and Corticospinal Tracts

Primary Motor Cortex

- One of Three Cortical Motor Areas

- Somatotopically Organized

- Lesions here result in weakness in affected area (motor homunculus)

- Neurons in this region involved with the execution of movement, and...

- Give rise to the Corticospinal Tract

- Synapse directly onto motoneurons

- Direct stimulation causes muscle to twitch

They are very similar

This allows for much faster processing, and shorter distances between sensory and motor processing

Q. Why are / aren't the Motor and Sensory Homunculus' similar to each other?

Premotor Cortex & Supplementary Motor Area

- Two of Three Cortical Motor Areas

- Somatotopically Organized

- Lesions here impair ability to develop strategies for movement

- Formulate the plan of action

- Direct stimulation typically evokes coordinated contractions of muscles at more than one joint (complex movements)