Introduction to Human Physiology - 2 Nervous System, Senses, Somatic NS

Source: My personal notes from Introduction to Human Physiology | Coursera

we consider the nervous system starting with how individual cells (neurons) function and then how they interact as integrative system. The nervous system provides rapid communication throughout the body coordinating the actions of trillions of cells. It responds to internal changes to the body as well as to changes in our external environment.

In this module, we consider two types of cells: one that relays information to the central nervous system (brain) for interpretation and a second set, motor neurons which relay information away from the central nervous system to govern voluntary movement. The input pathway to the brain is mediated by specific cells called senses. The senses convert energy (such as light or heat) into an energy form (electrical potentials) recognized by neurons in the brain. The brain, in turn, interprets this information (as vision or pain) and then sends out a motor response via the motor neurons of the somatic nervous system to effector cells in the body. The motor neurons activate skeletal muscle to control breathing and the movement of the limbs.

The Nervous System

Cell types

There are neurons and glial cells

Nervous system response needs are quick (very fast compared to endocrine system)

Cell body = nucleus and organelles

Dendrites = receive information

Axon = takes info Away (a like axon) cell body

Initial segment = where axon meets cell body

Axon terminals = branches

Glial cells

A type of glial cell: Schwann cells

Schwann cells wrap around axon, provide electrical insulation to axons

Type of glial cell: Oligodendrocytes

Types of glial cells

• Peripheral nervous system: Schwann cells = one cell myelinates a portion of an axon

• Central nervous system (CNS):

  • oligodendrocytes = one cell myelinates many different axons

  • Astrocytes - support neurons (e.g. metabolically)

  • Microglial cells - immune cells of CNS

Key Concepts

Neurons and glia are the principal cells of the nervous system. Neurons transmit information by ion conduction. Glia provide structural and metabolic support for neurons.

 

Membrane Potentials

Membrane Potentials = difference in charge between membranes

 

Electrochemical Gradients

ATP sets up electrochemical gradients.

Na+ K+ gradients are set up

Negative membrane potential = more negative charges inside cell, than outside

 

For a cell, there is more K on inside the cell and more Na on outside of cell.

Ion channels = specific (they only transport an ion, e.g. K = potassium). They are gated since ATP has done work to create the electrochemical gradient.

 

• No electrical potential = 0 mV

• Chemical gradient with ion channel open, K+ moves outside

• More a electrical gradient, so some K+ moving back into cell, though most are moving out

• Electrical gradient is in place, opposing chemical gradient. Potential = negative number mV

 

Equilibrium potential will be same as membrane potential when ion channels are open.

At rest there is a negative potential (more -ve inside cell)

 

 

For Na+, resting membrane potential is -ve and chemical gradient will drive Na inside if channels are open. Equilibrium potential is ve 60mV = many Na are inside cell due to chemical gradient, but electrical gradient is +ve.

Ena = 60mV

 

For K+, resting membrane potential is -ve and chemical gradient will drive K+ outside of cell if channels are open.

If K+ channels are open, chemical gradient will drive equilibrium potential to be negative.

EK = -90mV. At -90mV the electrical gradient is opposing the chemical gradient, causing equilibrium when K+ channels are open.

 

Notice for K+ and Na+ equilibrium potentials, the chemical gradient dictates whether the equilibrium potential will be more negative or positive than the cell membrane potential. If chemical gradient predicts movement of positive ions inside, equilibrium will be more positive than resting and vice-versa.

 

So if both K+ and Na+ channels are open, the equilibrium potential will settle somewhere between 60 to -90 mV.

There are leak channels where K+ and Na+ leak through cell membranes. K+ leak channels > Na+ leak channels

e.g. if EK = -50mV and Ena = +50mV, assuming equal concentrations, the equilibrium potential when both channels are open is 0mV

 

Polarization

Polarization can be caused by different channels opening.

 

Graded Potential

There are various grades (levels) of potentials. Greater stimulus = greater potentials

e.g. ligand gate channel from a neurotransmitter, channel opens longer for larger concentration of neurotransmitter.

 

Key Concepts

• The equilibrium potential of an ion is when the chemical gradient and the electrical gradient are equal in magnitude but opposite in direction.

• The Na/K+ ATPase re-equilibrates (through pump - active transport = solute is pumped against its concentration gradient) the resting membrane potential. Inactivation of the Na/K+ ATPase (active transport) will lead to depolarization over time as the K+ leaks out and Na+ enters dissipating the resting the membrane potential (more negative).

• Graded potentials are electrical signals that are local, vary with intensity of the stimulus, can summate (spatial and temporal), can be stimulatory or inhibitory, and have no refractory period. Potentials will decay over time and space as stimulus decays.

Action Potentials

Major way that neurons can communicate.

Action potential vs. graded potentials - Action potentials are due to voltage gated channels. Membrane can become more positive or negative.

 

Voltage Gated Na+ & K+ channels

In Step 3, Na gate becomes inactive and K+ becomes activated. Gate change is due to voltage change. Gate inactivation means it cannot be activated (different from closed). Inactivation due to at rest. Refractory period is minimum time period before an inactivated channel can be activated again.

Action potentials in stages

• At rest

• Depolarization: Stimuli (neurotransmitters create graded potentials). Enough neurotransmitters are required to break a threshold. Threshold creates an action potential. Na+ channels open. Membrane potential becomes positive. Head toward EquilibriumNa

• Repolarization: Na channel inactive, K+ channel open.

• Na~~, K~~ open, move towards EquilibriumK

• Move to at rest. Voltage gradients are varying, though concentrations are relatively similar.

Unidirectional Propagation of AP

Action potentials are propagated across membrane.

Voltage gated channels are opening down the axon as stimuli comes.

Na+ rushes in areas with action potential.

 

Action potential always move in one way along the axon due to refractory period when Na channels cannot be activated.

 

 

Integration of Signals

Dendrites and cell body mostly have ligand gated ion channels, resulting in graded potentials.

Axons: voltage gated ion channels, resulting in action potential activated when thresholds are reached.

 

• Some Na+ entering - synapse fires a message (presynaptic neurons fire messages)

• Some Na+ entering - synapse fires a message

• Hyper polarization

• 1+2 synapses fires at once

• 1+3 depolarization and hyperpolarization cancel out

• 1, 2, 2, additive stimuli cause action potential

Saltatory Conduction

The production of the myelin sheath is called myelination. Myelin is a dielectric = electrically insulating.

Na+ spreads thanks to myelin sheath since voltage is conserved within axon.

Inactivation of voltage gated Na+ channels means Na+ cannot flow back and can only flow in one direction.

 

Think an axon is like a wire. A large diameter (wide) axon / wire has more paths for electric current/ions to travel, so it can have a faster current than a small diameter axon/wire.

 

Key Concepts

An action potential is a wave of depolarization followed immediately by a wave of repolarization. During an action potential, depolarization is due movement of Na+ into the nerve cell. Repolarization is due to the movement of K+ out of the cell.

 

Action potentials are electrical signals that propagate without decrement along axons, are “all or none”. Have refractory periods, and unidirectional propagation in neurons.

 

Nervous System Methods of Communication

Neurons can be very long cells. Action potentials have to travel far (e.g. from spine to foot).

 

The Chemical Synapse

• End of an neuron synapses (verb) with another neuron, another cell (e.g. muscle)

• Synapse = come really close to another cell = called the postsynaptic cell.

• Ca (calcium) is pushed out -> vesicles containing neurotransmitters are released into synaptic cleft.

• The cleft is small. Lots of neurotransmitters are released. Receptors are usually ligand gated channels.

• Synapses can be:

  • Excitatory postsynaptic potential = graded depolarization due to influx of ions

  • Inhibitory postsynaptic potential = graded hyperpolization due to net influx of negative ions (e.g. Cl-) or net efflux of positive ions (e.g. K+)

Neuronal Networks

1 to many or many to 1 communication models for neurons

 

 

Lateral Inhibition: Applied neuronal networks and postsynaptic potential

 

Neuron 2 is the most excited. 1 and 3 are somewhat excited.

Neuron 2 is firing action potentials frequently. This sense allows us to determine it is a point of a pencil, rather than an eraser.

 

Bottom part of the diagram: All the neurons inhibit each other; however, since 2 is firing so frequently, 1 and 3 are more inhibited than 1 or 3 inhibiting 2.

 

Cholinergic Synapses

Acetylchoine (ACh) = neurotransmitter

 

Has 2 types for receptors:

• Nicotinic - ligand gate ion channel (skeletal, muscle, brain).

• Muscarinic - G protein coupled (heart, smooth muscle, glands, brain). It binds the ACh and starts a signal transduction cascade (Signal transduction occurs when an extracellular signaling molecule activates a specific receptor located on the cell surface or inside the cell. In turn, this receptor triggers a biochemical chain of events inside the cell, creating a response).

Adrenergic Synapses

• Catecholamines = neurotransmitters (tyrosine based)

• Norepinephrine

• Epinephrine = adrenaline

 

There are alpha and beta adrenergic receptors: G protein coupled that act via second messenger system (heart, smooth muscle, glands).

 

Key Concepts

• Neurons process incoming information in dendrites and the soma by summing the post-synaptic membrane potential changes (graded potentials).

• Spatial summation is the addition of inputs occurring simultaneously within neighboring synapses.

• Temporal summation is the addition of inputs occurring very close to one another in time within the same synapse.

• Neurotransmitters transmit signals within milliseconds by binding receptors on the postsynaptic cell.

Nervous System Organization

• Afferent neurons (starts with “A”) = things start with the afferent neurons, they are sensory and transmit information to the CNS.

• Central nervous system (CNS) (starts with “C” middle between A and E) integration is at the spinal cord or brain.

• Efferent neurons (starts with “E” later in alphabet) = they cause change and transmit information away from the CNS.

• The peripheral nervous system (PNS) is the part of the nervous system that consists of the nerves and ganglia outside of the brain and spinal cord.

 

PNS Sensory Input

Sensory systems: vision, hearing, taste, equilibrium, olfaction, somatosensation (inside the body)

 

Somatosensation receptors in the skin, muscle, and bone & visceral receptors (in orgrans) detect pain, temperature, touch, pressure, and proprioception (let’s you know where in space you are within vision - joint capsule, tendon, and muscle stretch)

 

PNS Input and Output

Looking at the efferent parts of the CNS

Somatic is for skeletal muscle

Autonomic - other muscles, organs

 

Efferent part from CNS messaging

• Notice all 4 start with a release of ACh and nicotinic AChR (acetylcholine receptors).

• For parasympathetic, AChR causes release to muscarinic AChR (signal transduction cascade).

• For the sympathetic NS, AChR in adrenal gland medula releases epinephrine is released in the blood and bind to adrenergic receptors or AChR causes norepinephrine to be released for adrenergic receptors

 

Many organs are enervated by sympathetic and parasympathetic nervous systems.

• Sympathetic = fight or flight systems, high adrenaline/dangerous situations ~ like a gas pedal

• Parasympathetic = resting and digestion, relaxing ~ like a brake

• Both systems allow fine/exquisite control.

Key Concepts

• The nervous system is composed of the brain, spinal cord, cranial nerves, and spinal nerves. The first two make up the CNS, the latter two constitute the PNS.

• The CNS and PNS constitute a reflex arc. The CNS (brain and spinal cord) integrates sensory input (PNS) from afferent neurons and provides output to effectors (PNS).

• The efferent portion of the PNS is divided into somatic and autonomic nervous systems. Somatic innervates skeletal muscle to cause contraction. Autonomic nervous system (ANS) is divided into sympathetic, parasympathetic, and enteric. Sympathetic (SNS) and parasympathetic (PNS) act in opposition (accelerator and brake). Enteric division acts independently in the gut, but can be modulated by the other divisions of the ANS. SNS is the “fight or flight”; ParaSNS is the “rest or digestion”.

The Senses: Introduction and Vision

The Senses in General

• Somatosensation = sensors inside the body, somato = internal to body

  • Receptors in skin, muscle, bones, and visceral receptors (in organs). In anatomy, a viscus ˈvɪskəs is an internal organ, and viscera is the plural form. The adjective visceral, also splanchnic, is used for anything pertaining to the internal organs

 

These are special sense - receptors confined to a specific organ and are associated with cranial nerves. The receptors have a specific organ and are confined to the head.

• Vision

• Hearing

• Vestibular system (equilibrium)

• Chemical senses:

  • Smell

  • Taste

Also visceral stimuli like pH and O2 content of blood, osmolarity, blood glucose

 

Principles

Sensation = sensory info reaching the brain (what the stimulus is, where, how strong (action potential frequency, more neurons stimulated. Neurons are selective to stimuli to isolate the exact stimulus like hot or cold)

 

Perception = how we interpret the sensation

 

Adaption = decrease in sensitivity - decreased action potential frequency with the same stimulus. Some systems are more sensitive to adaption like vision and hearing. We filter out most sensations to focus on what is important.

 

PNS Sensory Input

Afferent neurons

Neurons are in ganglia, outside the CNS

 

In the example

• Skin has an afferent neuron. The single axon take information from PNS to the CNS in the spinal cord. Axon is dorsal root.

• CNS assesses stimulus and sends out a response in efferent pathways.

Somatosensations

1. Touch and Pressure

   • Afferent neurons have mechanoreceptors that sense touch and pressure.

   • Mechanoreceptors are encapsulated and are free nerve endings (e.g. end of axon).

   • An example, encapsulated receptors sense a certain vibration.

2. Proprioception

   • Muscle stretch receptors, mechanoreceptors in skin, joints, tendons, ligaments, vision, vestibular system

3. Temperature

   Different types of thermoreceptors (ion channel activated at certain temperatures). They also respond to chemicals

   • Cold thermoreceptors respond (bind) to menthol

   • Hot thermoreceptors respond to capsaicin and ethanol

    

4. Pain

   Free nerve endings expressing nociceptors that sense mechanical deformation, chemicals released by damaged cells or immune cells responding to damaged cells.

    

Visual system

• A special sense

• Used to determine shape and colour of objects and their movement.

• Light particles (photons) have wavelengths and energies associated with different colours. Photons are detected.

 

The eye is like a camera that focuses light on the retina using a lens and an aperture (pupil) whose size can be adjusted to change the amount of entering light.

 

Vision: process by which light reflected from external objects are translated into a mental image. The process has 3 steps:

• Light enters the eye and focused by a lends on to the retina.

• Retinal photoceptors transduce light energy into electrical signal

• Processing of the electrical signals through neural pathways

 

• Light hits Cornea, most light focusing is occurring there since light is going from air to through tissue containing water.

• Pupil

• Lens. Focuses light, tuning of image (near vs. far focusing). Lens changes shape to focus

• Retina with photoreceptor cells.

Len shape changes

Ciliary muscles contract and relax to change lens shape.

Contracted = lens is rounded since ring is smaller - near focus

Relaxed = lens is flatter since ring is bigger - distant focus

 

Phototransduction

Photoreceptors receive photon energy and convert it to electrical energy.

 

Retinal (a G-coupled protein) changes when hit by photons.

 

This slide is counterintuitive since target cells cGMP gated cation channel closed - cell becomes hyperpolarized and reduces neurotransmitter secretion when the stimulus (photon) comes in.

 

Transduction of Signal

Receptors are at back of retina

Note 6 rods are feeding into 1 ganglion cell

• low resolution since 1 ganglion = 1 signal

• Better sensitivity since there are 6 rods the photons could hit per 1 ganglion

• Good lighting uses cones. There is 1 ganglion and 1 bipolar cell for every 1 cone providing high resolution

Light, Cones, and Rods

Degrees of vision quality based on amount of light.

 

No/low resolution -> little to no colour > good vision -> saturation

 

Cones and Colour Vision

Colours are perceived by activation of cones that are receptive to certain colours (blue, green, red cones). The CNS looks at the relative activation of blue, green, and red cones.

 

The diagram shows activation of blue, green, and red cones by their own colour. The black line shows no colour.

Example colours with relative activation of each type of cone is shown and circled.

 

Key Concepts

• Basic function of a sensory receptor is to detect change in the environment (stimulus) and convert the energy (heat, light, pressure, etc.) of the stimulus into electrical signals (action potentials) in the nervous system. This conversion is called transduction.

• Somatosensory pathways carry information about pain, temperature, touch, pressure, vibration, position of body parts, and their movements.

• The visual system detects the shape and colour of objects and their movement in the external environment. Light is detected by different cells (rods, cones) in the eye and sent via the optic nerve to the visual cortex for processing.

 

Hearing and the Vestibular System

Auditory system

• Detects sounds = sound waves = compression and expansion of air molecules in form of pressure waves

• Amplitude (height) of wave = volume

• Frequency of wave = pitch

• Auditory system detects complex sounds and breaks them into basic sound frequencies. Sound frequencies are converted to action potentials by the ear and related to the brain (auditory areas) for interpretation.

Anatomy of the Ear

• Sounds are focused on the tympanic membrane (ear drum)

• Auditory canal forcuses sound

• Malleus, incus, and stapes bones rock and cause vibrations of cochlea. The bones also amplify the vibrations.

• Cochlea is like a rolled up tube that fluids flow through

• Scala vestibuli = the upper fluid path shown in the diagram, it is behind the oval window

• Scala tympani = the lower fluid path

1. Organ of Corti (zoomed in)

   Stereocilia of hair cells are embedded in the tectorial membrane. As the basilar membrane bounces up and down the stereocilia bend. Bending in one direction depolarizes the cell; in the other direction hyperpolarizes the cell.

   Fluid flows around the Organ of Corti (u shaped)

    

    

2. Sound transduction

   After a hair cell (=receptor) activates the afferent neuron, axons from the these neurons join to form the cochlear nerve.

   Cochlear nerve is made of these afferent neurons.

    

   The region of the basilar membrane that vibrates the most correlates with the frequency of the sound. The louder the sound, the more vibration and the greater the frequency of action potentials produced in the afferent neurons.

    

Sense of Balance: Vestibular System

Two receptor organs in the inner ear sense movement of the head.

They detect:

• Angular acceleration (shake or nod of head)

• Linear acceleration (elevator drops, or body leans to one side)

Semicircular canals: respond to changes in head rotation

Otolith organs: saccule detect vertical movement, utricle detects horizontal movement

 

There are 3 semicircular canals at 90 degrees angles to each other, along detection along three perpendicular axes.

 

1. Semicircular Canals

   Fluid is stationary, hair cells moves to detect movement

Demonstration of Semicircular canal

Canal rocks back and forth, while fluid is stationary

 

1. Otolith Organs

   Involve hair cells

    

   In this animation, green crystals represent otoliths that pull on hair cells

    

   Otoliths (green) are heavier and have more inertia.

    

Key Concepts

 

The auditory system detects complex sounds and breaks them into their basic sound frequencies. The fundamental frequency components are transduced into action potentials and transmitted through the auditory pathways to the brain for interpretation.

 

The vestibular system aids in maintaining the body’s balance by detecting the position and motion of the head in space. The sensory cells are in the ampullae of the semicircular canals and in the utricle and saccule.

 

Chemical Senses

Chemoreceptors (bind chemicals > graded > action potentials:

• Taste buds (tongue)

• Receptors in carotids (O2, H+, CO2)

• Neurons in brain (osmolarity)

• Odor receptors (nose)

Chemoreception & Taste

Taste cells recessed inside mouth. Requires it to be in liquid for chemical to be detected (ie. Dissolved in Saliva)

Taste Modalities (flavours)

Taste ligands dissolved in saliva bind to chemoceptors (proteins) on the taste buds. Receptor binding raises intracellular Ca++ causing the release of neurotransmitters (graded potentials). This leads to the initiation of action potentials in the postsynaptic neuron (primary neuron).

 

GPCRs are G protein–coupled receptors (GPCRs), also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein–linked receptors (GPLR), constitute a large protein family of receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. They are called seven-transmembrane receptors because they pass through the cell membrane seven times.

 

Five sensations of taste:

• Sweet (sugars) - GPCRs

• Sour (H+) - ion channels - protons are detected

• Salt (Na+) - ion channels - protons are detected

• Bitter (plant alkaloids) - GPCRs

• Umami (glutamate = an amino acid) - GPCRs

 

GPCRs will sense the external molecules, activate a signal transduction pathway to increase intracellular Ca++ and cause release of neurotransmitters.

 

Smell

• Smell is unique in that it is the primary afferent neurons themselves that express olfactory receptors and binding to the odorants result in signals sent to the central nervous system (CNS)

• In the nasal epithelium cavity, odorants bind to GPCRs.

• Different neurons can bind to a single molecule. Binding and nature of bind allows detection of many smells.

 

Key Concepts

Taste receptor cells in the tongue detect chemicals associated with the five basic taste qualities: umami (meaty, broth flavour), salt, sour, sweet, and bitter. Their areas of distribution overlap and sensation of taste result from combination of taste receptor activation.

 

Smell result from the activation of the olfactory receptors in the nose.

 

The Somatic Nervous System - Introduction and Structure

Somatic NS = Efferent pathways

 

Pathways review

 

 

 

Somatic, efferent pathway is a single neuron stretching to skeletal muscle. Skeletal muscle performs voluntary and involuntary actions (e.g. diaphragm that controls our breathing is skeletal muscle).

 

 

Somatic motor neurons in contexts of the afferent neurons and CNS. Somatic neurons start in the ventral horn of the spinal cord.

 

Muscle Cells and Neurons

Muscle cells = muscle fibers

In the diagram below, blue fibres are being innervated.

Motor unit = motor neuron + muscle fibres controlled by neuron

Somatic neuron decides whether muscle contracts

 

1. How do muscles get innervated? See below

   Neuron fires > action potential > release of Ca++ due to voltage gated ion channels > action potential in muscle due to voltage gated ion channels in the post-synaptic cell (the muscle).

    

Key Concepts

Somatic nervous system controls locomotion, fine movements, body posture, and equilibrium by acting on motor neurons in the spinal cord that innervate skeletal muscles.

 

A motor neuron and the muscle fibers that it innervates constitute a motor unit.

 

More neurons have cell bodies located in grey matter of the ventral horn of the spinal cord. The spinal cord contains interneurons which play a role in coordinating the responses of antagonistic and synergistic muscles to carry out intended movements as well as reflexive movements initiated by sensory receptors.

 

Somatic Nervous System - Control of Movement

Controlled much by spinal cord, rather than brain

 

Two Types of lower motor neurons

• Alpha motor neurons - innervate extrafusal fibers

• Gamma motor neurons - innervate infrafusal fibers

• The muscle spindle consists of intrafusal fibers, though they do not contribute to force production. The intrafusal fibers are used for monitoring contraction/stretching (length of the muscle) since it is like a parallel system to the extrafusal fibers for force production.

 

 

Alpha and gamma neurons are coactivated

 

The muscle spindle senses stretch (muscle length & speed of stretch). Think spindle starts with “S” and so does stretch.

 

Gamma neurons are coiled around the spindle. They are sensors. High stretch causes gamma neurons to fire to counteract and cause contraction. Afferent neurons act in reciprocal innervation causing contraction of stretched muscle and relaxation of the antagonistic muscle. E.g. Bicep contracts with relaxation of tricep.

 

Gamma motor neurons acts in a negative feedback loop to keep muscle fibers at a desired length.

 

Golgi tendon organs afferent neurons sense tension (think tendon and tension) and fire frequently as tension increases when the muscle contraction increases. So at maximum muscle contraction, the golgi tendon organs fire very frequently to indicate tension - a negative feedback resulting in relaxation of the muscle.

 

Muscle Stretch and Reflex

 

High action potential frequency = muscle contraction

Low action potential frequency (e.g. by inhibitory interneuron) = muscle relaxation

 

In the knee jerk reflex, the afferent neuron around the muscle spindle of the quadricep senses stretch. It act in a reciprocal innervation causing contraction of the quadricep (top of diagram) and relaxation of the antagonistic muscle which is the hamstring (bottom of diagram).

 

A voluntary application of reflex is maintaining an active stretch/contraction (isometric) in the presence of a force exertion on the muscle. e.g. someone is pouring a drink for you and you are holding the cup. You maintain the level of the cup by contracting your bicep muscles, preventing any stretch due to the force of the drink entering the cup.

 

Golgi tendon Reflex

In this example, the bicep tendon is over contracted activating the Golgi tendon afferent neurons to fire. With reciprocal innervation, the tricep (antagonistic muscle) is contracted and the bicep is relaxed. The negative feedback servers to prevent muscle and muscle-tendon junction damage.

 

Withdrawal Reflexes

Other afferent neurons (e.g. pain from skin) can cause somatic motor neurons to fire.

 

• This example is more complex. The leg where the pain is has the quadricep relaxed and hamstring contracted in response to the pain; the opposite leg also reacts by extending the leg to balance the body. Notice the extended leg has an opposite reaction in that the quadricep is contracted and the hamstring is relaxed.

• Without that extension, the withdrawal reflex would cause a person to fall over.

• Both legs are innervated, though the reactions are different and opposite.

 

Central Pattern Generators (CPG)

Neural circuits produce timing and coordination of complex patterns of movement independent of sensory input and adjust them in response to sensory feedback.

 

e.g. walking moving from swing phase (swinging foot forward and back) to stance phase (ready to push off ground).

• First step, left leg in stance phase, right leg in swing phase.

• Next step right leg in stance phase, left leg in swing phase. They are opposites.

 

CPG characterized by:

• Oscillartoy

• Flexible due to control by brain (change from walking to running by reducing stance phase and sequence of limb movements in quadrupeds). Reduction of walking stance phase increases speed to running.

 

Locomotion

Forward flexion (swing phase) of a limb and then backward extension (stance phase) that is repeated. Reciprocal innervation of limbs (similar to withdrawal reflex)

 

Key Concepts

Specific sensory system provide input to the motor system.

Sensory receptors in muscle detect and regular muscle length and velocity of shortening (spindle) as well as tension (Golgi tendon).

 

Reflex actions are initiated when sensory information is sent from muscle, tendon, or skin receptors to the spinal cord, causing movement in the appropriate muscles.

 

Walking movements are initiated by central pattern generator in the spinal cord. Interneurons coordinately drive antagonistic muscles in withdrawal and crossed-extensor reflexes and can be modified by sensory feedback.