The Central and Peripheral Nervous Systems Flashcards
(26 cards)
Functions of the Nervous System
- Receiving sensory input. Monitor
internal and external stimuli - Integrating information. Brain and
spinal cord process sensory input
and initiate responses - Controlling muscles and glands
- Maintaining homeostasis
Regulate and coordinate physiology - Establishing and maintaining
mental activity. Consciousness,
thinking, memory, emotion
Flow of Information in the Nervous System
Sensory receptor -> brain and spinal cord -> effector (motor output)
Nervous System Organisation
Nervous system:
- Central nervous system - Brain, spinal cord
- Peripheral nervous system - sensory and motor division - somatic nervous system and autonomic nervous system - Sympathetic NS, Parasympathetic NS, Enteric NS
Neuron Structure: Cell Body and Dendrites
- Nissl bodies
- Dendrites
- Nucleus
- Cell body
- Axon hillock
- Neurofilaments
- 86 billion neurons
- 100 trillion connections
Neuron Structure: Axon
- Axon
- Myelin (cut)
- Synaptic knob of axon terminal
- Schwann cell
- Portion of a collateral
- Neurilemma
- Myelin sheath
- Neurofibrils
- Schwann cell nucleus
Interaction Between the CNS and PNS
- PNS collects sensory info, transmits it to CNS for processing.
- CNS sends motor commands through PNS to initiate actions (e.g., muscle movement).
Glia cells – Functions: Protection, Insulation,
Support
- 86 billion neurons
- 86 billion glia cells
- 100 trillion connections
Stimulation of the Action Potential: Resting
Membrane Potential
Resting Membrane Potential:
- Inside of neuron is negatively charged (~ -70 mV)
- Maintained by Na⁺/K⁺ pump: pumps 3 Na⁺ out, 2 K⁺ in
- Also due to more K⁺ leak channels (K⁺ diffuses out)
Stimulation of Action Potential:
- Stimulus causes Na⁺ channels to open
- Na⁺ rushes in, making inside less negative (depolarization)
- If threshold (~ -55 mV) is reached, action potential fires
- Followed by repolarization (K⁺ exits) and restoration of resting state
Depolarisation and Repolarisation Phases
Depolarization:
- Stimulus opens Na⁺ channels
- Na⁺ enters the neuron
- Membrane potential becomes more positive
- If threshold is reached, action potential occurs
Repolarization:
- Na⁺ channels close, K⁺ channels open
- K⁺ exits the neuron
- Membrane potential returns to negative (resting state)
Action Potential Generation
1) Resting State:
- Neuron at ~-70 mV, Na⁺ and K⁺ channels closed
2) Depolarization:
- Na⁺ channels open, Na⁺ rushes in
- Membrane becomes positive inside
3) Repolarization:
- Na⁺ channels close, K⁺ channels open
- K⁺ flows out, restoring negative charge
4) Hyperpolarization:
- K⁺ channels stay open slightly too long
- Membrane becomes more negative than resting
5) Return to Resting Potential:
- Na⁺/K⁺ pump restores original ion balance
Synapses Connect Neurons
- A synapse is the junction between two neurons.
- Presynaptic neuron sends the signal; postsynaptic neuron receives it.
- Signal travels as an electrical impulse to the synapse, triggering release of neurotransmitters (e.g., dopamine, acetylcholine).
- Neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic neuron, continuing the signal.
Synaptic Transmission of Electrochemical Signal
1) Action potential reaches the axon terminal of the presynaptic neuron
2) Ca²⁺ channels open, Ca²⁺ enters the terminal
3) Causes neurotransmitters to be released into the synaptic cleft
4) Neurotransmitters bind to receptors on the postsynaptic neuron
5) Triggers an electrical response in the postsynaptic neuron (new action potential if threshold is reached)
Bundle of Nerve Fibres, Forming a Fascicle:
Many Fascicles Bundled Together Form a Nerve
- Nerve fibers (axons) are grouped into bundles called fascicles
- Multiple fascicles are bundled together to form a nerve
Surrounded by protective layers:
- Endoneurium (around each fiber)
- Perineurium (around fascicles)
- Epineurium (around entire nerve)
A Reflex Arc Is the Simplest Neural Pathway
- A reflex arc is a fast, automatic response to a stimulus
Involves 5 basic components:
1) Receptor – detects stimulus
2) Sensory neuron – carries signal to CNS
3) Integration center – usually in spinal cord
4) Motor neuron – sends signal to effector
5) Effector – muscle or gland that responds
- No brain involvement, so response is quick
The Patellar Reflex Involves a Sensory Neuron
and a Motor Neuron
A simple reflex involving just two neurons:
1) Sensory neuron detects tap on patellar tendon
2) Sends signal to spinal cord, directly activates
3) Motor neuron, which triggers quadriceps muscle to contract
- No interneuron involved — very fast response
A Withdrawal Reflex Involves a Sensory Neuron an Interneuron and a Motor Neuron
- A protective reflex that pulls body away from harm (e.g., touching something hot)
Involves three neurons:
1) Sensory neuron detects pain
2) Interneuron in spinal cord processes the signal
3) Motor neuron activates muscles to withdraw the body part
- Faster than conscious reaction, but more complex than patellar reflex
Spinal Cord
- Part of the central nervous system (CNS)
- Runs through the vertebral column
Functions:
- Transmits signals between brain and body
- Controls reflexes through reflex arcs
Composed of:
- Gray matter (inner) – neuron cell bodies
- White matter (outer) – myelinated nerve fibers
Ascending and Descending Sensory Tracts
Ascending Sensory Tracts:
- Carry sensory information from the body to the brain
Examples:
- Spinothalamic tract – carries pain and temperature signals
- Dorsal column tract – carries touch and proprioception signals
Descending Motor Tracts:
- Carry motor commands from the brain to the body
Examples:
- Corticospinal tract – controls voluntary muscle movement
- Reticulospinal tract – regulates posture and balance
The Brain
- Part of the central nervous system (CNS), located within the skull
Divided into major parts:
1) Cerebrum – largest part; controls sensory, motor, and cognitive functions
2) Cerebellum – coordinates movement and balance
3) Brainstem – connects brain to spinal cord; regulates vital functions (heart rate, breathing)
4) Diencephalon – includes thalamus (sensory relay) and hypothalamus (controls homeostasis)
- Protection: Surrounded by meninges and cerebrospinal fluid (CSF)
Spinal Nerves
- 31 pairs of nerves that emerge from the spinal cord
- Divided into regions: Cervical, Thoracic, Lumbar, Sacral, and Coccygeal
Each pair is formed by:
1) Dorsal (sensory) root – carries sensory information to the spinal cord
2) Ventral (motor) root – carries motor commands from the spinal cord to muscles
- These nerves serve specific areas of the body (dermatomes and myotomes) and allow sensory and motor functions.
Dermatomal Map
- Dermatomes are regions of skin innervated by sensory fibers from a specific spinal nerve.
- Each spinal nerve (except C1) serves a particular dermatome on the skin.
- Dermatomal maps show the distribution of these sensory areas, helping to pinpoint nerve damage or injury.
Common example:
C5 – skin over the shoulder
L3 – skin over the thigh
Motor Pathways: Autonomic & Somatic
1) Somatic Nervous System (SNS):
- Controls voluntary movement of skeletal muscles
- One motor neuron extends from the CNS to the muscle
- Example: Movement of arm muscles
2) Autonomic Nervous System (ANS):
- Controls involuntary functions (e.g., heart rate, digestion)
Two-neuron chain:
1) Preganglionic neuron from CNS to ganglion
2) Postganglionic neuron from ganglion to target organ
Divided into:
Sympathetic (fight or flight)
Parasympathetic (rest and digest)
Sympathetic Division of the Autonomic Nervous System (ANS)
- Function: Prepares the body for “fight or flight” responses (stressful situations).
Effects:
- Increases heart rate and blood pressure
- Dilates bronchioles for easier breathing
- Inhibits digestion
- Stimulates pupillary dilation (pupils get bigger)
- Releases glucose from the liver for quick energy
Neurotransmitters:
- Preganglionic neurons release acetylcholine (ACh)
- Postganglionic neurons release norepinephrine (noradrenaline)
Parasympathetic Division of the Autonomic
Nervous System (ANS)
- Function: Promotes the “rest and digest” response, conserving energy and supporting normal bodily functions.
Effects:
- Decreases heart rate and blood pressure
- Stimulates digestion and nutrient absorption
- Constricts pupils
- Promotes salivation and urination
- Bronchoconstriction (narrowing of airways)
Neurotransmitters:
- Both preganglionic and postganglionic neurons release acetylcholine (ACh)
