Nervous System, Neurons, Receptors, and Motor Control
Nervous System Divisions
Central Nervous System (CNS)
Comprises the brain and spinal cord.
Peripheral Nervous System (PNS)
Autonomic Nervous System (ANS)
Communicates with internal organs and glands.
Sympathetic division: Responsible for arousing effects ("fight-or-flight" response).
Parasympathetic division: Responsible for calming effects ("rest-and-digest" response).
Somatic Nervous System (SNS)
Communicates with sense organs and voluntary muscles.
Sensory (afferent) nervous system: Transmits sensory input from receptors to the CNS.
Motor (efferent) nervous system: Transmits motor output from the CNS to effector muscles.
Neuron Structure and Function
Structure of a Neuron
Cell body (Soma): Contains the nucleus and other organelles; integrates electrical signals.
Dendrites: Branching extensions that conduct electrical impulses toward the cell body.
Axon:
A single, long projection that carries electrical impulses away from the cell body.
May be covered by Schwann cells in the PNS, forming a discontinuous myelin sheath.
The myelin sheath insulates the axon and increases the speed of impulse conduction.
Synapse: The specialized junction or contact point between the axon terminal of one neuron and the dendrite (or cell body) of another neuron or an effector cell.
Key anatomical features (as per diagram): Includes Dendrites, Axon, Myelin, Nissl bodies, Nucleus, Nucleolus, Axonal hillock, Nodes of Ranvier, Nucleus of Schwann cell, Cell body, Neurofibrils, and collateral branches.
Types of Neurons
Motor neurons (Efferent neurons): Carry signals from the CNS to muscles and glands.
Sensory neurons (Afferent neurons): Carry signals from sensory receptors to the CNS.
Interneurons: Connect motor and sensory neurons within the CNS, facilitating communication.
Other Nervous System Cells (Neuroglia/Glial Cells)
These support cells assist neurons in various functions:
Astrocytes: Provide structural support, regulate the chemical environment, and facilitate nutrient transfer.
Microglia: Immune cells of the CNS, acting as phagocytes.
Satellite cells: Support neurons in the PNS (similar to astrocytes in CNS).
Oligodendrocytes: Form myelin sheaths around axons in the CNS.
Ependymal cells: Line the ventricles of the brain and central canal of the spinal cord, producing cerebrospinal fluid.
Schwann cells: Form myelin sheaths around axons in the PNS.
Membrane Potentials
Resting Membrane Potential (RMP)
Definition: The electrical potential difference across the neural cell membrane when the neuron is at rest.
Charge: The inside of the cell is negatively charged relative to the outside.
Typical values: Ranges from -5 ext{ to } -100 ext{ mV} in various cells; specifically, for neurons, it is typically between -40 ext{ to } -75 ext{ mV}, often around -70 ext{ mV}.
Determinants:
Permeability of the plasma membrane to various ions: The membrane is much more permeable to ext{K}^+ than to ext{Na}^+ at rest.
Difference in ion concentrations across the membrane: Specific concentrations of ions like ext{Na}^+, ext{K}^+, ext{Cl}^-, ext{and } ext{Ca}^{+2} are actively maintained.
Ion Concentrations (approximate values for typical neuron):
Cell Exterior:
ext{Na}^+: 150 ext{ mM}
ext{K}^+: 5 ext{ mM}
ext{Cl}^-: 120 ext{ mM}
Anions ( ext{A}^-): 10 ext{ mM}
Cell Interior:
ext{Na}^+: 15 ext{ mM}
ext{K}^+: 150 ext{ mM}
ext{Cl}^-: (variable, usually lower than exterior)
Anions ( ext{A}^-): 100 ext{ mM}
Maintenance: Primarily maintained by the ext{Na}^+/ ext{K}^+ pump (also known as ext{Na}^+/ ext{K}^+ ext{-ATPase}).
The pump actively transports 2 ext{ K}^+ ions into the cell and 3 ext{ Na}^+ ions out of the cell against their concentration gradients.
Potassium ( ext{K}^+) ions tend to diffuse out of the cell through leak channels due to their concentration gradient, contributing significantly to the negative resting potential.
Action Potential (Nerve Impulse)
Initiation: Occurs when a stimulus of sufficient strength causes the membrane to depolarize to a critical level known as the threshold potential (typically around -55 ext{ mV}).
Depolarization phase (Rising phase):
Threshold depolarization causes voltage-gated ext{Na}^+ channels to rapidly open.
ext{Na}^+ ions rush into the cell, causing the inside of the membrane to become rapidly more positive (e.g., up to +30 ext{ mV}).
Slow voltage-gated ext{K}^+ channels begin to open.
Repolarization phase (Falling phase):
ext{Na}^+ channels inactivate and close.
ext{K}^+ channels are fully open, and ext{K}^+ ions rapidly leave the cell.
The efflux of positive ext{K}^+ ions causes the membrane potential to return to a negative state.
After-hyperpolarization:
ext{K}^+ channels remain open for a brief period after repolarization, allowing additional ext{K}^+ to exit.
This causes the membrane potential to become even more negative than the resting potential (e.g., -90 ext{ mV}) before returning to baseline.
All-or-None Law: Once a nerve impulse (action potential) is initiated by a threshold stimulus, it will propagate down the entire length of the neuron's axon with uniform amplitude, regardless of the strength of the initiating stimulus (as long as it's at or above threshold).
Sequence of Events in an Action Potential:
Resting membrane potential: Stable state (e.g., -70 ext{ mV}).
Depolarizing stimulus: An input causes the membrane potential to become less negative.
Threshold reached: Membrane depolarizes to threshold, voltage-gated ext{Na}^+ channels open, and ext{Na}^+ enters the cell. Voltage-gated ext{K}^+ channels begin to open slowly.
Rapid ext{Na}^+ entry: Rapid influx of ext{Na}^+ depolarizes the cell further, reaching the peak of the action potential.
ext{Na}^+ channels close, ext{K}^+ channels open: The inactivation gates of ext{Na}^+ channels close, halting ext{Na}^+ influx. Delayed voltage-gated ext{K}^+ channels become fully open.
ext{K}^+ moves out: Exit of ext{K}^+ ions from the cell to the extracellular fluid repolarizes the membrane.
K^+ channels remain open: Additional ext{K}^+ leaves the cell, causing after-hyperpolarization (the membrane becomes more negative than the resting potential).
Voltage-gated ext{K}^+ channels close: As ext{K}^+ channels close, less ext{K}^+ leaks out.
Return to resting state: The cell returns to resting ion permeability and the resting membrane potential is re-established.
Sensory Receptors
Types of Sensory Receptors
Chemoreceptors: Detect changes in chemical concentrations (e.g., taste, smell, blood gases).
Mechanoreceptors: Respond to physical stimuli like pressure, touch, vibration, stretch, and sound (e.g., touch receptors in skin, hair cells in ear).
Thermoreceptors: Sensitive to changes in temperature (hot and cold).
Osmoreceptors: Detect changes in solute concentration (osmolarity) of body fluids.
Photoreceptors: Respond to changes in light and color (e.g., rods and cones in the retina).
Nociceptors: Specialized receptors that respond to painful stimuli, indicating tissue damage.
Anatomical Examples of Sensory Receptors (primarily for somatosensation)
Free nerve endings: Detect pain, heat, and cold.
Merkel disks: Detect sustained touch and pressure.
Krause end bulbs: Involved in touch sensation.
Root hair plexus: Detects hair movement and light touch.
Meissner corpuscles: Detect light touch and vibration.
Pacinian corpuscles: Detect deep pressure and vibration.
Ruffini endings: Detect stretch and sustained pressure.
Locations: These receptors are primarily located in the epidermis and dermis of the skin.
General Mechanism of Sensory Transduction
A specific stimulus (chemical, pressure, light, etc.) acts on the receptor.
The receptor activates a receptor protein or directly opens an ion channel.
This initiates a signal transduction pathway within the cell.
The pathway leads to a change in membrane potential (receptor potential).
If the receptor potential reaches threshold, it generates an action potential that sends a signal to the integrating center (CNS).
Mechanoreceptors (Proprioceptors)
Function: Provide the CNS with crucial information about body position, limb movement, and joint angle.
Types of Proprioceptors:
Free nerve endings
Golgi-type receptors
Pacinian corpuscles (also found in joints)
Joint kinesthetic receptors: These are sensitive to both joint angles and the rate of change in joint angles, allowing for the sense of joint position and movement.
Specialized Proprioceptors
Muscle Spindle:
Structure: Consists of several modified muscle fibers (intrafusal fibers) enclosed within a connective tissue sheath.
Location: Embedded within the belly of skeletal muscles.
Function: Detects muscle stretch and the rate of stretch.
Mechanism (Stretch Reflex): When a muscle is stretched, the deformation of the muscle spindle activates its sensory neuron. This neuron sends an impulse to the spinal cord, where it synapses directly with an alpha motor neuron, causing the stretched muscle to contract (e.g., patellar reflex).
Golgi Tendon Organ (GTO):
Location: Situated in the tendons, near the myotendinous junction (where muscle fibers merge into the tendon).
Structure: Occur in series (attached end-to-end) with the extrafusal muscle fibers (the main contractile fibers).
Function: Monitors muscle tension and force production.
Mechanism (Autogenic Inhibition): When an extremely heavy load or excessive tension is placed on the muscle, the GTO discharges. Its sensory neuron activates an inhibitory interneuron in the spinal cord. This interneuron, in turn, synapses with and inhibits the alpha motor neuron serving the same muscle, leading to relaxation of the muscle and protection from injury.
Muscle Chemoreceptors
Sensitivity: Sensitive to changes in the chemical environment surrounding a muscle.
Detected substances: Primarily detect ext{H}^+ ions, ext{CO}_2, and ext{K}^+ (byproducts of muscle metabolism).
Function: Provide the CNS with information about the metabolic rate of muscular activity.
Importance: These receptors are crucial in the regulation of cardiovascular and pulmonary responses during exercise, helping to match ventilation and cardiac output to metabolic demands.
Reflexes
Definition: Rapid, automatic, and unconscious means of reacting to specific stimuli without direct involvement of higher brain centers.
Order of Events (General Reflex Arc):
A sensory nerve (afferent neuron) receives a stimulus and sends an impulse to the spinal column.
Within the spinal cord, interneurons (association neurons) process the signal and activate appropriate motor neurons.
Motor neurons (efferent neurons) transmit impulses to effector muscles, controlling their movement.
Reciprocal Inhibition:
During a withdrawal reflex, excitatory postsynaptic potentials (EPSPs) are sent to the agonist muscles responsible for withdrawing the limb from the stimulus.
Simultaneously, inhibitory postsynaptic potentials (IPSPs) are sent to the antagonistic muscles (muscles that oppose the movement), causing them to relax. This ensures efficient withdrawal.
Crossed-Extensor Reflex:
A polysynaptic spinal reflex that occurs in conjunction with the withdrawal reflex, especially when the withdrawal involves a weight-bearing limb.
It ensures that the opposite limb supports the body during the withdrawal of the injured or stimulated limb.
Mechanism (Example: stepping on a pin with the left foot):
Left arm (Withdrawal side): Flexors are stimulated to contract, and extensors are inhibited.
Right arm (Support side): Conversely, flexors are inhibited, and extensors are stimulated to contract, thereby extending the limb to bear weight and maintain balance.
Motor Units
Definition: A single alpha motor neuron and all the muscle fibers it innervates.
Innervation: Each individual muscle fiber typically receives input from only one motor neuron.
Innervation Ratio: The number of muscle fibers that are part of a single motor unit. This ratio can vary widely, from as low as ~10 muscle fibers (for fine motor control, e.g., eye muscles) to more than 1000 muscle fibers (for gross motor control, e.g., large leg muscles).
Motor Neuron Pool (Motor Nucleus):
The entire group or collection of alpha motor neurons located within the spinal cord that innervate a single, specific muscle.
Muscles requiring finer, more precise movements generally have a larger motor neuron pool (meaning more motor units are available for recruitment and grading of force, even if individual units have small innervation ratios).
Force Generation:
The total force generated by a given muscle is directly dependent on the firing rate of its alpha motor neurons.
A higher firing rate (frequency of action potentials) from the motor neuron leads to increased muscle fiber stimulation and consequently, greater force production through mechanisms like summation and tetanus.
Force can also be increased by recruiting more motor units (known as motor unit recruitment).