Ch. 12

Expanded segments in spinal cord form 2 enlargements

Cervical Enlargements Lumbar Enlargements

Ventral rami form four major plexuses

Cervical plexus Brachial plexus Lumbar plexus Sacral plexus

Convergence

If several neurons synapse on a single postsynaptic neuron this is termed as convergence type of neural circuit.

Perineurium

Middle layer of nerve. Divides nerve into series of compartments (called as fascicles) which contain bundles of axons.

Arachnoid mater (arachne, spider + mater, mother)

Middle meningeal layer of spinal cord. Subarachnoid space: Subarachnoid space - extends between arachnoid membrane and outer surface of the pia mater.

Withdrawal reflexes

Move affected body parts away from a stimulus Strongest withdrawal reflexes - triggered by painful stimuli.

Conus medullaris

Tapered, cone-shaped part of the spinal cord inferior to the lumbar enlargement.

Cervical enlargement

Supplies nerves to shoulder and upper limbs.

Neural circuit

"Wiring diagram" of neuron interaction. Common patterns of neural circuit include: Divergence Parallel processing Serial processing Convergence Reverberation The brain - site of the most complex neural processing.

Steps in a simple reflex arc: 5 steps

1. Activation of a receptor by a stimulus Receptor - specialized cell or dendrites of a sensory neuron is activated Ex. Leaning on a tack stimulates pain receptors in the hand. 2. Activation of a sensory neuron. Information reaches spinal cord through dorsal root. 3. Information processing in the CNS Information - integrated with other stimuli arriving at the same moment. 4. Activation of a motor neuron. 5. Response of a peripheral effector. Release of neurotransmitters at axon terminals leads to a response of peripheral effector (muscle). Ex. Skeletal muscle contraction to pull hand away from the tack.

Steps in the patellar reflex: 5 steps

1. Activation of a receptor by a stimulus. 2. Activation of a sensory neuron. Distortion of the stretch receptors then stimulates sensory neurons. 3. Information processing in the CNS: Occurs at the cell body of the motor neuron. 4. Activation of a motor neuron. 5. Response of a peripheral effector. Response - extends knee in a brief kick.

Types of reflexes based on their development (2 types)

A: Innate reflexes Result from connections formed during development B: Acquired reflexes or conditioned reflexes - learned, not pre-established.

Types of reflexes based on the complexity of the circuit

A: Polysynaptic reflexes (poly = more than one): Involve at least one interneuron in addition to a sensory and a motor neuron. Can activate several muscle groups at once. B: Monosynaptic reflexes (mono = one): Involves only one synapse (between sensory neuron and motor neuron).

Types of reflexes based on the nature of motor response

A: Somatic reflexes: Provide for involuntary control of skeletal muscles Ex. Withdrawal reflex B: Visceral reflexes or autonomic reflexes: Control or adjust activities of smooth muscle, cardiac muscle, glands, and adipose tissue.

Types of reflexes based on the processing site (2 types)

A: Spinal reflexes: Interconnections and processing events occur in nuclei of the spinal cord. B: Cranial reflexes: Interconnections and processing events occur in nuclei of the brain.

Spinal cord structure

Adult spinal cord dimensions: Shorter than the vertebral column. Ends between L1 and L2. Spinal cord is divided into 5 regions, and 31 segments: 8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal Amount of gray matter - greatest in spinal cord segments & is dedicated to sensory and motor control of the limbs.

Lumbar and sacral plexuses

Arise from lumbar and sacral segments of spinal cord. Innervate the pelvic girdle and lower limbs.

Stretch reflex or myotatic reflex

Best known monosynaptic reflex. Automatically regulates skeletal muscle length. Ex. Patellar reflex

Epidural space

Between dura mater and walls of vertebral canal.

Stretch reflexes (Reflexes used in diagnostic testing)

Biceps reflex, triceps reflex, ankle-jerk reflex - stretch reflexes tested during a physical exam. Each controlled by specific segments of the spinal cord.

CNS (Central nervous system) & PNS (peripheral nervous system)

Brain (CNS) and cranial nerves (PNS) Spinal cord (CNS) and spinal nerves (PNS) Spinal cord - simplest part of the CNS Both brain and spinal cord: Receive sensory input from receptors Contain reflex centers Send motor output to effectors

Brain influence in reflexes

Brain can facilitate or inhibit motor neurons or interneurons involved in a reflex. Reinforcement: if brain facilitates neurons involved in reflex it is called as reinforcement.

Tracts (Functional organization of white matter)

Bundles of axons in the CNS. All axons in a tract relay the same type of information in the same direction. 2 types of tracts: 1. Ascending tracts 2. Descending tracts

Descending tracts

Carry motor commands. Tend to be located in anterior columns.

Ascending tracts

Carry sensory information. Tend to be located in posterior columns.

Postural reflexes

Category of stretch reflexes. Help us maintain a normal, upright position. Ex. Standing

Sensory information

Collected from peripheral structures and delivered to sensory nuclei in thoracic and lumbar spinal cord. Arrives over dorsal and ventral rami. Visceral sensory information - reaches dorsal root through dorsal, ventral and white rami.

Cauda equina (cauda, tail + equinus, horse)

Collection of dorsal and ventral roots of spinal segments L2 to S5 along with filum terminale.

Nerve plexus (plexus, braid)

Complex interwoven network of nerves. Formed during development. Ventral rami of adjacent spinal nerves converge and blend their fibers & produce plexuses.

Cervical plexus

Consists of ventral rami of spinal nerves C1-C5 Phrenic nerve: Formed by branches from C3-C5. Provides nerve supply to diaphragm.

Ventral root

Contains axons of motor neurons.

Spinal nerve

Contains axons of sensory (dorsal root) and motor neurons (ventral root). 3 components of spinal nerve: Ventral root, dorsal root & dorsal root ganglion 31 pairs of spinal nerves from 31 spinal cord segments. Identified by association with adjacent vertebrae. Spinal nerves in cervical region: Seven cervical vertebrae but eight cervical spinal nerves C1 spinal nerve emerges between skull and first cervical vertebra.

Dorsal root

Contains axons of sensory neurons whose cell bodies are in the dorsal root ganglion. Dorsal root ganglion (DRG): Contains cell bodies of sensory neurons. Each spinal cord segment has pair of dorsal root ganglia (one on each side).

Gray commissure (commissura, a joining)

Contains axons that cross from one side of the cord to other.

Superficial layer of spinal cord (w/ white matter)

Contains white matter with large numbers of myelinated and unmyelinatad axons.

Anterior median fissure

Deep groove along the anterior surface of the spinal cord.

Deep layer of spinal cord (w/ gray matter)

Dominated by cell bodies of neurons, neuroglia, and unmyelinated axons. Surrounds small central canal Forms the shape of H or butterfly .

3 connective tissues layers of spinal nerve

Epineurium, Perineurium Endoneurium

Connective tissue layers of a spinal nerve

Every segment of spinal cord - connected to a pair of spinal nerves. Each spinal nerve - surrounded by a series of connective tissue layers - comparable with connective tissue layers associated with skeletal muscles.

Subarachnoid space

Filled with cerebrospinal fluid (CSF) CSF - acts as shock absorber and diffusion medium for gases, nutrients, chemical messengers & wastes.

Sacral plexus

Formed from spinal nerves L4-S4. Contains the sciatic nerve. Sciatic nerve - largest and longest nerve in the body.

Neuronal pools

Functional groups of interconnected neurons. Pattern of interaction gives clues to the function of the pool.

Negative Babinski reflex or plantar reflex

In normal adults - stroking foot on lateral side of the sole produces a curling of toes - plantar or negative Babinski reflex Reflex often tested when CNS injury is suspected.

Endoneurium

Innermost layer of nerve.

Pia mater (pia, delicate + mater, mother)

Innermost meningeal layer of spinal cord. Firmly bound to underlying neural tissue (spinal cord and brain).

Dorsal ramus

Innervates muscles, joints, and skin of the back.

Brachial plexus

Innervates pectoral girdle and upper limbs. Originate from: Trunks: Large bundles of axons contributed by several spinal nerves. Cords: Smaller branches that originate at trunks. Clinical importance of brachial plexus: Nerve damage can be precisely localized by testing sensory function of the hand.

Ventral ramus

Innervates structures in the upper and lower limbs and the lateral and anterior trunk.

Structures surrounding the spinal cord

Intervertebral foramen: Spinal nerve - passes through the intervertebral foramen. Dorsal root ganglion: Lies between pedicles of adjacent vertebrae.

Reverberation

Involves collateral branches of axons along the circuit.

Crossed extensor reflexes

Ipsilateral reflex arcs (ipsi, same) - Stretch reflexes and withdrawal reflexes involve sensory stimulus and motor responses on the same side of the body - ipsilateral reflex. Contralateral reflex arc (contra, opposite) - Crossed extensor reflexes involve an additional motor response on the side opposite the stimulus - contralateral reflex arc (contra, opposite) Ex. Stepping on a tack

Supporting ligaments of the spinal cord

Ligaments prevent lateral movement of spinal cord Denticulate ligaments: Extend from pia mater through arachnoid mater to dura mater. Ligaments prevent superior-inferior movements of the spinal cord.

Abdominal reflex

Light stroking of skin produces reflexive twitch in abdominal muscles moving the navel toward the stimulus is termed. Absence of the reflex may indicate damage to descending tracts.

Functional organization of gray matter

Nuclei: Functional groups of neuron cell bodies in the gray matter of the spinal cord. 2 types of nuclei: Sensory nuclei: Receive and relay sensory information. Motor nuclei: Issue motor commands to peripheral effectors.

Epineurium

Outermost covering of the nerve.

Dura mater (dura, hard + mater, mother)

Outermost covering of the spinal cord. Tough and fibrous.

3 types of horns in spinal cord based on location

Posterior/dorsal gray horn: Posterior horn is located on posterior /dorsal side & contains somatic and visceral sensory nuclei. Lateral gray horn: Found laterally but only in thoracic and lumbar segments. Anterior/ventral gray horn: Located on anterior/ventral side and contains somatic motor nuclei.

Communicating rami

Present in the thoracic and superior lumbar segments of spinal cord.

Spinal tap (Lumbar puncture)

Procedure for withdrawing sample of cerebrospinal fluid. In lumbar puncture - needle is inserted into the subarachnoid space in lumbar region inferior to the tip of conus medullaris.

Spinal nerve divisions

Rami: Branches of spinal nerves - rami (singular ramus) Spinal nerves of thoracic and lumbar segments - carry motor output of the sympathetic nervous system - responsible for "fight-or-flight" response. Different rami based on location: Dorsal ramus, ventral ramus and communicating rami.

Reflexes

Rapid, automatic responses to specific stimuli. Preserve homeostasis by making rapid adjustments in organ or organ system function. Overall process of reflexes - reflex arc Ex. Pulling hand away from painful stimulus.

Majority of nervous system functions are outside of our awareness

Reflex: Rapid, automatic response triggered by specific stimuli. Spinal reflexes: Reflexes controlled in the spinal cord - spinal reflexes Spinal reflexes - can function without input from the brain.

Babinski sign (or positive Babinski reflex)

Stroking an infant's foot on the lateral side of the sole - produces an extension of hallux and fanning out of other toe. Occurs due to lack of inhibition by descending motor fibers. Response disappears as descending motor pathways develop.

Polysynaptic reflexes

Responsible for automatic actions involved in complex movements. Ex. Walking and running

Muscle spindles

Sensory receptors involved in the stretch reflex. Intrafusal muscle fibers. Consist of bundles of small, specialized skeletal muscle fibers - intrafusal muscle fibers. Gamma motor neuron innervates each muscle spindle.

Spinal meninges

Series of specialized membranes surrounding the spinal cord. Spinal meninges are continuous with cranial meninges (meninges around the brain) Three layers of meninges: 1: Dura mater 2: Arachnoid mater 3: Pia mater

Posterior median sulcus

Shallow, longitudinal groove on the posterior surface of spinal cord.

Filum terminale ("terminal thread")

Slender strand of fibrous tissue extending from inferior tip of conus medullaris to second sacral vertebra. Provides longitudinal support to spinal cord.

2 types of spaces between meningeal layers

Subarachnoid Space Epidural Space

Dermatome

Specific bilateral region of skin surface monitored by a single pair of spinal nerves. Clinically important to determine extent of damage or infection of spinal nerve or dorsal root ganglion. Loss of sensation or characteristic signs appear on skin of that dermatome.

Structural organization of gray matter

Spinal cord has horn like structures (gray matter) which project toward outer surface of spinal cord.

Structural organization of white matter

Spinal cord white matter - divided into three columns on each side of the spinal cord. Posterior, lateral and anterior white columns Posterior white column: Bbetween posterior gray horns and posterior median. sulcus Lateral white column: Between the anterior and posterior columns. Anterior white column: Between the anterior gray horns. Anterior white commissure: Commissure which interconnects anterior white columns is called as anterior white commissure.

Motor commands

Spinal nerve distribute motor commands - originate in the motor nuclei of thoracic or superior lumbar segment of spinal cord. 1. Ventral root of spinal nerve - with axons of somatic motor and visceral motor neurons 2.Ventral root and dorsal root join to form the spinal nerve 3. White ramus communicans - carries commands to a nearby sympathetic ganglion 4.Gray ramus communicans - contains postganglionic fibers that innervate glands and smooth muscles in body wall or limbs The white and gray rami together - known as the rami communicantes or communicating branches.

Spinal nerves in thoracic and lumbar regions

Spinal nerves emerge inferior to vertebra & they're named after thoracic or lumbar vertebrae.

Divergence

Spread of information from one neuron to several neurons (or from one pool to several pools). Ex. Sensory information to the CNS distributed to neuronal pools throughout spinal cord and brain.

Lumbar enlargement

Supplies nerves to pelvis and lower limbs.

Flexor reflex

Type of withdrawal reflex. Affects muscles of a limb. Ex. Painful stimulus of accidentally grabbing a hot pan on the stove.

Shingles

Viral infection of dorsal root ganglia. Caused by varicella-zoster virus (VZV). Same herpes virus that causes chickenpox. Produce painful rash and blisters.

Serial processing

When information relayed in a stepwise fashion, from one neuron to one neuron. Ex. Sensory information relayed from one part of the brain to another part.

Parallel processing

When several neurons or neuronal pools process the same information simultaneously.